Pathways for Venus Exploration
Venus Exploration Analysis Group (VEXAG)
October 2009
VEXAG is NASA’s community-based forum that provides science and technical assessment of
Venus exploration for the next few decades. VEXAG is chartered by NASA Headquarters
Science Mission Directorate’s Planetary Science Division and reports its findings to both the
division and to the Planetary Science Subcommittee of the NASA Advisory Council. VEXAG,
which is open to all interested scientists and engineers, regularly evaluates Venus exploration
goals, objectives, investigations, and priorities on the basis of the widest possible community
outreach.
http://www.lpi.usra.edu/vexag
Front cover is a collage showing Venus at radar wavelength, the Magellan spacecraft, and artists’ concepts for a Venus Balloon, the Venus In situ Explorer, and the Venus Mobile Explorer. (Collage prepared by Tibor Balint) Pathways for Venus Exploration
Venus Exploration Analysis Group (VEXAG)
October 21, 2009
VEXAG Executive Committee:
VEXAG Co-chairs: Sanjay Limaye, University of Wisconsin, Madison, Wisconsin (sanjayl@ssec.wisc.edu) and
Suzanne Smrekar, Jet Propulsion Laboratory (JPL), California Institute of Technology, Pasadena, California
(Suzanne.E.Smrekar@jpl.nasa.gov)
Mark Allen, Focus Group Lead for Laboratory Measurements, JPL, Pasadena, California
(Mark.A.Allen@jpl.nasa.gov)
Kevin Baines, Focus Group Lead for Atmospheric Evolution, JPL, Pasadena, California
(blueskies4321@yahoo.com)
James Cutts, Focus Group Lead for Venus Exploration Technologies, JPL, Pasadena, California
(James.A.Cutts@jpl.nasa.gov)
Lori Glaze, Focus Group Lead for Planetary Formation and Evolution, Goddard Space Flight Center, Greenbelt,
Maryland (lori.s.glaze@nasa.gov)
David Grinspoon, Focus Group Lead for Venus-Earth Climate Connections, Denver Museum of Nature and
Science, Denver, Colorado (david.grinspoon@dmns.org)
Adriana Ocampo, Ex Officio, Venus Program Executive, NASA Headquarters, Washington D.C.
(adriana.c.ocampo@nasa.gov)
Supporting members of the VEXAG Executive Committee:
Tibor Balint, JPL, Pasadena, California (tibor.balint@jpl.nasa.gov)
Mark Bullock, Southwest Research Institute, Boulder, Colorado (bullock@boulder.swri.edu)
Larry Esposito, Laboratory for Atmospheric and Space Physics, University of Colorado
(larry.esposito@lasp.colorado.edu)
Jim Garvin, Goddard Space Flight Center, Greenbelt, Maryland (james.b.garvin@nasa.gov)
Ronna Hurd, Lunar and Planetary Institute, Houston, Texas (hurd@lpi.usra.edu)
Natasha Johnson, Goddard Space Flight Center, Greenbelt, Maryland (natasha.m.johnson@nasa.gov)
David Senske, JPL, Pasadena, California (david.senske@jpl.nasa.gov)
Tommy Thompson, JPL, Pasadena, California (thomas.w.thompson@jpl.nasa.gov)
Allan Treiman, Lunar and Planetary Institute, Houston, Texas (treiman@lpi.usra.edu)
Past VEXAG Co-Chairs (in chronological order):
Janet Luhmann, University of California, Berkeley, California (jgluhmann@ssl.berkeley.edu); Sushil Atreya,
University of Michigan, Ann Arbor, Michigan (atreya@umich.edu); Ellen Stofan, Proxemy, Inc.
(ellen@proxemy.com)
Steve Mackwell served as Focus Group Lead for Planetary Formation and Evolution since VEXAG was formed
in 2005 until spring 2009. Janet Luhmann, and Sushil Atreya, served as VEXAG Co-Chairs since VEXAG was
formed in 2005 until November 2007. Ellen Stofan served as VEXAG Chair from November 2007 until spring
2009.
Pathways for Venus Exploration: 2009
TABLE OF CONTENTS
Foreword ........................................................................................................................................ iii Findings and Proposed Actions ...................................................................................................... v Venus New Frontiers and Discovery Missions........................................................................... v Venus Science Laboratory Measurements ................................................................................. vi Venus as a Future Earth/Comparative Planetology .................................................................. vii Technology Development ........................................................................................................ viii Technology Development for a Venus Flagship Mission.......................................................... ix 1. Context for Venus Exploration ................................................................................................. 10 1.1 Key Themes and Questions for Exploring Venus .......................................................... 10 1.2 The Role of VEXAG ...................................................................................................... 11 1.3 Background: Recent Reports Addressing Venus Exploration ....................................... 12 1.4 Current Status of NASA’s Support of Venus Research ................................................. 17 1.5 Fifty Years of Venus Exploration .................................................................................. 18 2. Venus Exploration Missions ..................................................................................................... 23 2.1 Discovery, New Frontiers, and Flagship Missions ........................................................ 23 2.2 Venus Flagship-Class Missions ..................................................................................... 25 2.3 Traceability of VEXAG Science Objectives to Future Flagship Missions .................... 30 2.4 ESA Cosmic Vision—European Venus Explorer .......................................................... 33 2.5 Russian Space Agency Venera-D .................................................................................. 33 3. Venus Laboratory Measurements ............................................................................................. 34 4. References and White Papers for Next Decadal Survey ........................................................... 37 5. Acronyms and Abbreviations ................................................................................................... 40 Appendix A. Venus Goals, Objectives, and Investigations .......................................................... 41 Appendix B. Comparative Climatology Overview ....................................................................... 47 Appendix C. Enhancing and Enabling Technologies for Venus Exploration .............................. 48 NASA's Venus Flagship Mission Fact Sheet ................................................................................ 54 Vignette 1: Magellan......................................................................................................................14
Vignette 2: Venus Express: Revealing the Mysteries of a Neighboring World ............................16
Vignette 3: Recent Venus Express VIRTIS Results ................................................................20–21
Vignette 4: Lessons Learned from Pioneer Venus Orbiter and Huygens ......................................22
Vignette 5: Experiencing Venus by Air: The Advantages of Balloon-Borne
In Situ Exploration .....................................................................................................24
Figure 2-1. Artist’s Concept of Elements of the Venus Design Reference Mission.....................26
Figure 2-2. Overviews of Venus Missions Endorsed by Solar System Exploration Roadmap ....28
Table 1-1. Summary of Past, Present, and Future Venus Missions .............................................19
Table 2-1. Traceability Matrix of Objectives Met with Venus Flagship Missions .....................31
Table 3-1. New Laboratory Studies to Support Future Venus Missions .....................................34
Recommended bibliographic citation: VEXAG (2009), Pathways for Venus Exploration
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Pathways for Venus Exploration: 2009
VEXAG Charter. The Venus Exploration Analysis Group is NASA's community‐based forum designed to provide scientific input and technology development plans for planning and prioritizing the exploration of Venus over the next several decades, including a Venus surface sample return. VEXAG is chartered by NASA's Solar System Exploration Division and reports its findings to NASA. Open to all interested scientists, VEXAG regularly evaluates Venus exploration goals, scientific objectives, investigations, and critical measurement requirements, including especially recommendations in the NRC Decadal Survey and the Solar System Exploration Strategic Roadmap. Perspective view of Ishtar Terra, one of two main highland regions on Venus. The smaller of the two, Ishtar Terra, is located near the north pole and rises over 11 km above the mean surface level. Courtesy NASA/JPL–
Caltech. ii
Pathways for Venus Exploration: 2009
FOREWORD
Since the previous release of the VEXAG report in May 2008, several events have occurred
that necessitate an update to Venus exploration goals, objectives, investigations, and priorities
(now this document, Pathways for Venus Exploration). NASA Headquarters has accomplished
some of the findings and proposed actions that were originally formulated at the February 2007
VEXAG meeting. A significant accomplishment was the formation of a Venus Science and
Technology Definition Team (STDT), which was established in January 2007 to develop a
credible Venus flagship mission in the 2020s that addressed the VEXAG goals, objectives, and
investigations. The Venus STDT completed its final report in April 2009, with a presentation to
NASA Headquarters on April 6, 2009. In addition, the European Space Agency (ESA) Venus
Express Mission, which arrived in April 2006, has continued to observe Venus on a daily basis,
providing key information about the Venus atmosphere, surface, and near-space environment.
Also, the Japanese Aerospace Exploration Agency (JAXA) is on schedule for its 2010 launch of
the Venus Climate Orbiter, a multiyear mission to study the clouds in winds of Venus in
unprecedented detail. In parallel with this, the National Research Council (NRC) and NASA
initiated a new Planetary Sciences Decadal Survey in early 2009 to reexamine NASA’s planetary
missions as a follow-on to the 2003 Decadal Survey. To support the 2009 Decadal Survey, there
are a number of Venus community white papers, which are posted on the VEXAG web site.
Through its interactions with the community, VEXAG developed an overarching goal—
Understanding Venus and the Implications for the Formation of Habitable Worlds—
supported by a set of three scientific goals:

Origin and Evolution: How did Venus originate and evolve, and what are the
implications for the characteristic lifetimes and conditions of habitable environments on
Venus and similar extrasolar systems?

Venus as a Terrestrial Planet: What are the processes that have shaped and still shape
the planet?
Climate Change and the Future of Earth: What does Venus tell us about the fate of
Earth’s environment?

VEXAG findings and proposed actions, which were originally the outcome of the January
2007 VEXAG meeting, have been updated to reflect current circumstances and are presented
below as an executive summary. Section 1 provides a context for Venus exploration with
vignettes providing additional information on Magellan, Venus Express, recent Venus Express
VIRTIS results, and lessons learned from Pioneer Venus Orbiter and Huygens. Possible missions
to accomplish these goals are discussed in Section 2, with a vignette providing additional
information on balloon-borne in situ exploration. New laboratory measurements needed to
maximize the science return from current and future Venus missions are identified in Section 3.
The appendices are brief overviews of white papers submitted to the Planetary Sciences Decadal
Survey: Appendix A. Venus Goals, Objectives, and Investigations; Appendix B. Comparative
Climatology Overview; and Appendix C. Enhancing and Enabling Technologies for Venus
Exploration. The fact sheet for NASA’s Flagship Mission to Venus follows the appendices.
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Pathways for Venus Exploration: 2009
Artist’s concept of Mariner 2, the first spacecraft to visit Venus (1962) Artist concept of Magellan spacecraft at Venus (1990–1994) iv
Pathways for Venus Exploration: 2009
FINDINGS AND PROPOSED ACTIONS
As noted above, the first set of VEXAG findings and proposed actions was developed at the
January 2007 VEXAG meeting and remained unchanged until the February 2009 meeting.
NASA has acted on some of the proposed actions, particularly the formation of a Venus STDT in
January 2007. The current set of VEXAG findings and proposed actions, discussed at the
February 2009 meeting, is presented here.
Key unanswered questions about Venus relevant to our understanding of the Earth—as well
as other terrestrial planet systems—include the following:
 Was there ever an ocean on Venus and, if so, when did it exist and how did it disappear?
 Was Venus ever habitable?
 Was the early Venus atmosphere like the early atmosphere of Earth, and at what point did
it diverge in character so greatly and why?
 Why does Venus rotate so slowly and is the lack of a planetary dynamo a consequence?
What was the impact on the evolution of Venus?
 Why does the Venus atmosphere rotate 60-times faster than its solid body? How are
atmospheric heat and momentum transferred from equator to poles?
 What caused the extensive resurfacing of Venus during the last 500 million to one billion
years? Is Venus still an active planet? Are the resurfacing and climate change somehow
related?
To address these questions and the recommendations of recent reports described in Section 1,
VEXAG, through interaction with the community, developed an overarching goal—
Understanding Venus and the Implications for the Formation of Habitable Worlds—
supported by a set of three overarching scientific goals:
 Origin and Evolution: How did Venus originate and evolve, and what are the
implications for the characteristic lifetimes and conditions of habitable environments on
Venus and similar extrasolar systems?

Venus as a Terrestrial Planet: What are the processes that have shaped and still shape
the planet?
 Climate Change and the Future of Earth: What does Venus tell us about the fate of
Earth’s environment?
A prioritized set of objectives and investigations to support these goals is given in
Appendix A.
VEXAG discussions and deliberations to date, together with these Venus goals, lead to the
following findings and proposed actions for the next steps toward Venus exploration. VEXAG
strongly suggests that the outstanding proposed actions be enacted as soon as possible.
Venus New Frontiers and Discovery Missions
The 2003 NRC Decadal Survey endorsed the Venus In Situ Explorer (VISE) as one of four
New Frontiers mission candidates. In the 2009 New Frontiers AO, NASA identified a number of
key science objectives and solicited a mission that would address all or a subset of these.
Specifically, a New Frontiers Venus mission should be designed to:
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Pathways for Venus Exploration: 2009

Understand the physics and chemistry of Venus’ atmosphere, especially the abundances
of its trace gases, sulfur, light stable isotopes, and noble gas isotopes.
 Constrain the coupling of thermochemical, photochemical, and dynamic processes in
Venus’ atmosphere and between the surface and atmosphere to understand radiative
balance, climate, dynamics, and chemical cycles.
 Understand the physics and chemistry of Venus’ crust.
 Understand the properties of Venus’ atmosphere down to the surface and improve our
understanding of Venus’ zonal cloud-level winds.
 Understand the weathering environment of the crust of Venus in the context of the
dynamics of the atmosphere and the composition and texture of its surface materials.
 Search for planetary scale evidence of past hydrological cycles, oceans, and life and for
constraints on the evolution of the atmosphere of Venus.
Finding: VEXAG considers Venus In Situ Explorer (VISE) to be a vital New Frontiers
mission as it has extremely high science value in the exploration of Venus. The 2003
NRC Decadal Survey recommended VISE as one of four 2003 New Frontiers AO
mission candidates. The 2009 New Frontiers AO has included VISE as one of eight
endorsed missions.
Proposed Action: NASA should make Venus a priority for a future New Frontiers
mission.
The Discovery program, which began in the early 1990s, consists of PI-led missions that
provide opportunities for targeted investigations with relatively rapid flight missions. Ten full
missions and four missions of opportunity (instruments and investigations flown on a non-NASA
spacecraft as well as extended missions for NASA spacecraft) have been selected. The Discovery
program is open to proposals for scientific investigations that address any area embraced by
NASA’s Solar System Exploration program, including the search for planetary systems around
other stars.
Finding: The Discovery program provides an excellent means for tapping the creativity
of the planetary science community for Venus exploration. Although Venus missions
have been proposed at every Discovery opportunity, none has been selected for
implementation. Thus, VEXAG finds that Venus Discovery-class missions are viable as
they address fundamental solar system exploration goals.
Proposed Action: NASA should make Venus a priority for a future Discovery-class
mission.
Venus Science Laboratory Measurements
As described in Section 3, new laboratory measurements are needed to maximize the science
return from current and future Venus missions. These measurements would characterize
fundamental Venus processes based on newly revealed Venus system variables for (1) the
atmosphere above the clouds, in which the temperature and pressure conditions are similar to
those in the terrestrial atmosphere; (2) the sulfuric-acid-laced cloud layer; (3) the atmosphere
below the clouds, in which the temperature and pressure range is unique for solar system
exploration; and (4) the super-heated surface. Many of these laboratory measurements could be
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Pathways for Venus Exploration: 2009
conducted in a Venus Environmental Test Facility, which would simulate pressure, temperature,
and atmospheric composition as a function of altitude. This enables would enable insights into
how elements behave in the Venus environment and would also enable development and testing
of new instruments and subsystems to operate under relevant conditions.
Findings: New laboratory measurements are needed to maximize the science return from
current and future Venus missions.
Proposed Actions:

NASA should support the development of a Venus Test Chamber suitable for both
scientific research and instrument testing and provide access to all interested parties as a
community facility.
 NASA should develop new research opportunities for funding Venus laboratory
measurements and field investigations that support future Venus missions.
Venus as a Future Earth/Comparative Planetology
An understanding of the evolutionary histories and current states of the Venus and Mars
climates is directly relevant for studies of the past and future climates of Earth (Appendix B).
Investigating global warming and climate change on Earth has raised consciousness about the
potential instability of terrestrial climate systems and the value in understanding the Venus
greenhouse for comparisons with Earth’s changing climate. Also, a key finding from the 2006
Chapman Conference on Venus as a Terrestrial Planet is that Venus may have had an ocean and
could have been habitable for much of its history.
Venus provides climatologists with an opportunity to test state-of-the-art models simulating
the mechanisms and processes that led to Venus’ extreme climatology. NASA and VEXAG are
pursuing an increased dialogue between Earth and Venus science communities. The thick
atmosphere of Venus—with its long radiative time constants and incomplete knowledge of the
global radiation balance—poses computational challenges. Current numerical models are
inadequate.
Findings: The study of the Venus greenhouse effect is needed to better understand
Earth’s climate stability and change. There is much to be learned about both planets as
well as extrasolar-terrestrial planets from the divergent evolutionary paths of Earth and
Venus. A key question is whether the warming Earth ultimately will become a Venus.
Also, the Venus Atmosphere General Circulation Models still lag behind the capabilities
of the terrestrial climate models. Such modeling efforts should be supported by NASA to
fully exploit the atmospheric circulation observations made from past, current, and future
missions to Venus.
Proposed Action: A research program, encouraging conferences and/or workshops,
should be initiated that brings together Earth and Venus scientists for a focused study of
the evolutionary aspects (past and future) of these terrestrial-planet twins. Areas of
mutual interest could include extreme climate scenarios, the role of volcanism and
tectonics, as well as the presence/absence of a planetary dynamo in determining the fate
of a planet and its atmosphere.
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Pathways for Venus Exploration: 2009
Technology Development
Just as landed and mobile in situ exploration of the Mars surface has answered many key
scientific questions, major advances in Venus science will require short- and long-lived landers,
eventually leading to mobile in situ surface and/or near-surface measurements, networks, and
surface sample return. In addition, a long-lived seismic network is of high priority but is
currently a technological challenge. The impediment has been the technical difficulty of
operating at the extreme pressures and at the high temperatures near and on the surface of Venus.
There are opportunities to leverage technologies already developed for operation in similar
environments encountered in aerospace (jet and rocket engine) and deep drilling applications.
Nevertheless, the hot, supercritical carbon dioxide conditions at the surface of Venus are
significant challenges for operations. Many other considerations related to the environment are
unknown due to the lack of a Venus Environmental Test Facility. There are credible technical
approaches, leveraging from technologies already developed in industry to achieve extended
operation in the Venus environment. High-temperature electronics can enable systems that could
operate for extended periods in the corrosive, high-pressure on the Venus surface. Advanced
radioisotope-power systems and active thermal-control systems could enable operation of
conventional components such as microprocessors or imaging sensors for extended periods on
the Venus surface. Further details on technology requirements to enable or enhance future Venus
missions can be found in the white paper “Technologies for Future Venus Exploration,”
submitted to the Planetary Sciences Decadal Survey, available on the VEXAG web site.
Findings: Although further work on mission architectures will be needed to define
specific performance goals, technology work can and should begin now. NASA’s
involvement is needed to apply industry experience to the specific needs of in situ and
near-surface exploration of Venus.
Proposed Action: NASA should initiate a program to develop technologies for operation
in the extreme environment of Venus, reflecting the priorities identified by the 2008–
2009 Venus Flagship Mission Study.
These technologies could be competed through an amendment to the ROSES NRA:





Sample Acquisition and Handling System: To understand the resource needs and
technology development steps needed to meet the Venus design reference mission
(VDRM) objectives.
Lander Design: For the rotating pressure vessel and rough terrain landing approach in
order to accommodate diverse terrain.
Long-lived Seismometry and Meteorology: To address questions concerning which
technical approach will be most fruitful and what performance can be achieved.
Humans-in-the-Loop Lander Missions: To quantify how much lifetime is required for
different levels of interaction. Technological advances are needed to achieve the required
lifetime, which must be long enough to accommodate meaningful humans-in-the-loop
interactions.
Near-surface Aerial Mobility: To assess both refrigerated and nonrefrigerated
implementations.
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Pathways for Venus Exploration: 2009
Technology Development for a Venus Flagship Mission
As described in Section 2, certain high-priority investigations are so challenging that they
cannot be achieved within the constraints of the Discovery and New Frontiers programs. With
costs significantly higher than those of New Frontiers missions, flagship missions represent
major national investments that must be strategically selected and implemented. Examples
include comprehensive studies of planetary bodies, such as those undertaken by Voyager,
Galileo, Cassini, and the Mars rovers. Thus, flagship missions conduct in-depth studies of solar
system bodies as well as sample return from planetary surfaces. These missions generally require
large propulsion systems and launch vehicles. In addition, flagship missions often require
significant, focused technology development prior to mission start, extended engineering
developments, and extensive pre-decisional trade studies to determine the proper balance of cost,
risk, and science return.
Finding: As part of the 2008–2009 Venus Flagship Study, the NASA-appointed Science
and Technology Definition Team reviewed the science goals and priorities identified by
the VEXAG community and, with the support from a JPL Engineering Team,
recommended a mission that resulted in the highest science return. The study also
identified technology development needs that require further assessments and
refinements.
Proposed Action: The Planetary Science Division should support a second phase of the
Venus Flagship Study, which would assess:






Key technologies, such as those for sample acquisition and handling.
Instrument development for in situ exploration elements.
Precursor scientific measurements and technology developments that might be
implemented with prior Discovery and New Frontiers missions.
Technology investments needed for a Venus flagship mission emphasizing the long-lead
time technologies needing early investments.
Requirements for a Venus Environmental Test Facility, enabling instrument and
subsystem development and testing at relevant pressure and temperature conditions.
Alternative mission architectures.
Alternative mission architectures would assess a Venus near-surface mobile explorer, a longlived surface network, a lifetime extension of the VDRM, and VDRM modifications if some
objectives were accomplished on a prior Discovery or New Frontiers mission.
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Pathways for Venus Exploration: 2009
1. CONTEXT FOR VENUS EXPLORATION
The context for Venus exploration, presented here, includes overviews of Venus as Earth’s
twin; the role of VEXAG; recent reports that address Venus exploration (such as the 2003 NRC
Decadal Survey, Solar System Roadmap, etc.); the current status of NASA’s support of Venus
research; and recounting fifty years of Venus exploration. Accompanying vignettes provide
additional information on Magellan, Venus Express, recent Venus Express VIRTIS results, and
lessons learned from the Pioneer Venus Orbiter and Huygens.
1.1
Key Themes and Questions for Exploring Venus
Venus is a unique terrestrial planet—as distinct from Earth, Mars, and Mercury as Titan and
Io are from the other Galilean and Saturnian satellites. Reasons for the divergent nature of these
orbiting bodies are as varied as the bodies themselves. However, Venus is particularly
compelling because it is so like Earth in size and bulk composition. In addition, Venus and Earth
orbits are similarly close to the Sun, as a mere 0.3 AU separates Venus from Earth in a solar
system that is >30 AU in scale. Although called “Earth’s twin” due to these similarities, Venus,
rather surprisingly, is a far cry from the Earth in terms of surface habitability as well as in
atmospheric composition, chemistry, composition, global circulation, and meteorology. Thus,
Venus can provide valuable insights into our origins and our ongoing searches for and
characterization of terrestrial planets in our galaxy. Although Mars may provide a more
hospitable environment for humans and life, Venus provides insights into possible states of
terrestrial planets as well as Earth’s future evolution.
As part of a science community’s contribution to the 2003 NRC Decadal Survey [1], Crisp
and co-authors [2] summarized what has been learned from previous missions to Venus,
including the Soviet Venera lander and Vega balloon missions, the Pioneer Venus orbiter and
probe, and Magellan radar missions. Crisp and co-authors suggested the following scientific
themes and key knowledge areas for future research and exploration of Venus:

Past: Origin of terrestrial planets in our solar system
o Noble and trace gases as evidence of early history and evolution
o Surface properties and age determination
o The history of interior volatiles

Present: What processes shape the terrestrial planets?
o The Venus greenhouse mechanism
o Atmospheric super-rotation
o Lightning
o Middle-atmosphere composition and dynamics
o Thermospheric composition and dynamics
o Ionospheric structure, composition, and dynamics

Future: What does Venus tell us about the fate of the Earth’s environment?
o Venus runaway greenhouse and the future of the Earth
o The limits of plate tectonics and future geologic processes on Earth
Several fundamental issues need to be addressed in order to explore these themes: the nature
of geologic and atmospheric processes that stabilize climate, the evolutionary effects of impacts,
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Pathways for Venus Exploration: 2009
and the ways in which surface composition, internal makeup, and geologic history can sustain
habitable environments. Factors that contributed to the uniqueness of each of the terrestrial
planets include their bulk compositions, distances from the Sun, internal structures, and impact
histories as well as the histories of their water and other volatiles. The analysis of Venus’ surface
in seven locations by the Soviet landers in the 1970s and the isotopic measurements of volatiles
by these landers and the Pioneer Venus probes provide some but not all of the information for
constraining Venus’ evolutionary history.
The 2003 NRC Decadal Survey [1] and Crisp et al. [2] concluded that further research and
analysis of existing data are also needed to understand how Venus operates as a system and how
it arrived at its present state. A renewed program focusing on Venus was called for as an
important target for NASA scientific exploration. Several Venus missions in all size ranges were
suggested in order to address the key questions in a systematic and reasonable way. These
missions included






Small noble and trace gas explorer
Small atmospheric-composition orbiter
Small- to medium-class global geological-process mapper
Medium atmospheric-dynamics explorer with an orbiter and balloons
Large-class surface and interior explorer mission, including a small network of landers
Sample-return mission
A subsequent 2006 Solar System Roadmap [3] investigated Venus missions with goals that
mirror those in the 2003 NRC Decadal Survey and the Crisp paper. Although Discovery-class
missions are left undefined, this Solar System Roadmap endorsed a New Frontiers Venus
mission to perform in situ sampling and analysis of the atmosphere and surface as well as a
subsequent sample-return flagship mission. This mission sequence has surface and lower
atmosphere remote sensing and upper atmosphere in situ measurements from orbit, followed by a
visit to the surface—including first measurements and sampling technology demonstrations—
which in turn is followed by a sample return mission. The 2006 NASA Science Plan [4],
developed by a subgroup for the NASA Advisory Council, took inputs from the 2003 NRC
Decadal Survey and Solar System Exploration Roadmap to synthesize a grand vision for NASA
science missions and supporting programs as a whole. Habitability is the guiding theme for both
the Solar System Roadmap and the 2006 NASA Science Plan.
At a Chapman Conference on Venus as a Terrestrial Planet held in February 2006 [5], the
concept was presented that Venus had an ocean and a relatively mild climate for the first few
billion years of its history. This suggests that future research should focus on the possible
existence of a past ocean and a search for fossil biosignatures. The LPI workshop on Venus
Geochemistry: Progress, Prospects, and Future Missions [6] in February 2009 provided an
assessment of the most crucial investigations for understanding the geochemistry of Venus.
1.2
The Role of VEXAG
NASA’s Science Mission Directorate established the community-based Venus Exploration
Analysis Group (VEXAG) in July 2005 to provide scientific and technical assessments for the
exploration of Venus. VEXAG reports its findings to NASA and to the Planetary Science
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Pathways for Venus Exploration: 2009
Subcommittee of the NASA Advisory Council. VEXAG is currently composed of two co-chairs
and five focus groups for Laboratory Measurements, Atmospheric Evolution, Venus Exploration
Technologies, Venus–Earth Climate Connections, and Planetary Formation and Evolution. Each
focus group includes scientists, technology experts, NASA representatives, international partner
representatives, and the VEXAG chair. Other focus groups may be constituted, as needed. The
Jet Propulsion Laboratory, operated for NASA by the California Institute of Technology,
manages VEXAG logistics on behalf of NASA's Planetary Science Division.
This Pathways for Venus Exploration and its predecessor, VEXAG Goals, Objectives,
Investigations, and Priorities, were developed to provide information for Venus exploration
needs. It is a living document, with revisions on an as-needed basis to capture the consensus
community views of the Venus community. From the first edition in November 2007 through
February 2009 (VEXAG Meetings 4–6), modest updates were made to the document. This
edition, now titled Pathways for Venus Exploration: 2009, has updates based largely on the
Venus STDT efforts and the addition of a section on laboratory measurements.
1.3
Background: Recent Reports Addressing Venus Exploration
Major studies involving Venus exploration include the 2003 NRC Decadal Survey for Solar
System Exploration [1], with the companion paper by Crisp [2]; the 2006 NASA Solar System
Exploration Roadmap [3]; and the 2006 NASA Science Plan [4]. As these studies provide a
foundation for the expectations for Venus exploration, their findings on Venus science and
missions are summarized here.
1.3.1 New Frontiers in the Solar System: An Integrated Exploration Strategy (2003)—The
NRC Decadal Survey for Solar System Exploration
The relevant section of the Venus exploration 2003 NRC Decadal Survey is Chapter 2,
“Inner Solar System: Key to Habitable Worlds.” Unifying themes for studies of the inner planets
were identified as:
 The past: Where did we come from? What led to the uniqueness of our home planet?
 The present: What is going on now? What common processes shape Earth-like planets?
 The future: Where are we going? What fate awaits Earth’s environment and that of the
other terrestrial planets?
Measurements to address these themes include:
 High-resolution imaging of the surface and radar probing of the subsurface.
 Surface mineralogical and chemical weathering investigations.
 Seismicity, heat flow, and other probing of the interior.
 Abundances of noble gases and their isotopes.
 Trace gas and atmosphere composition, density, temperature, and dynamics
measurements from the upper atmosphere (including the ionosphere) to the surface, with
a wide range of spatial and temporal sampling.
 Detection of radiation fields within the atmosphere and on the surface.
 In situ and remote imaging of the cloud layers and their dynamics (including winds) and
variability.
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Pathways for Venus Exploration: 2009


Lightning detection by electromagnetic and/or optical means.
Ionospheric and exospheric observations pertaining to atmosphere escape, and the
contextual information to interpret them.
To accomplish these measurements, the inner-planets panel of the 2003 NRC Decadal
Survey for Solar System Exploration endorsed Venus In situ Explorer (VISE) as the next Venus
New Frontiers–class mission. VISE would have measurement objectives to determine:
 Atmosphere composition, including trace gases and isotopes.
 Noble gas isotope abundances.
 Meteorological information, including cloud-level winds.
 Near-infrared surface images at 10 km and closer ranges.
 Composition and mineralogy of a core sample.
 Information on surface weathering.
These measurement objectives would be accomplished by multiple in-situ elements,
including an aeroshell entry, passive insulation, and a rapid sample-acquisition system.
Suggested instrumentation included a neutral-mass spectrometer, a meteorological package,
radio tracking of balloon(s) for wind measurement, and a landed package with a near-infrared
camera, a surface composition probe, imaging microscope, and a mineralogy analyzer. The
technologies developed for VISE would be vital to a subsequent Venus Surface Sample Return
(VSSR) mission, as rock ages, isotope ratios, and trace-element abundances can be obtained only
with sophisticated Earth-based laboratory analyses of Venus return samples.
1.3.2 NASA’s 2006 Solar System Exploration Roadmap
Subsequent to the NRC Decadal Survey, NASA commissioned a new Solar System
Roadmap Study [3] to establish planetary mission planning and priorities over the next 25 years.
While revisions to these NASA roadmaps occur from time to time in order to update the science
priorities and to build upon new discoveries and technologies, they are used as an official guide
in targeting technology development and selections of flight investigations. Planetary exploration
is carried out using both strategic missions, endorsed by these roadmaps, as well as by the
Principal Investigator–led Discovery and New Frontiers missions.
The lower-cost Discovery program solicits proposals of mission concepts with any targeted
planetary science focus, while the New Frontiers program solicits mission proposals addressing
one of several specific high-priority science goals endorsed by the 2003 NRC Decadal Survey.
Missions endorsed by the Solar System Roadmap are enabled by investments in both Science
Definition Teams as well as technology developments tailored to their measurement goals and
architectures. NASA’s 2006 Solar System Roadmap [3] had planetary habitability as its guiding
theme. The main science questions for a Venus mission were identified as:
1. How did the Sun’s family of planets and minor bodies originate?
2. How did the solar system evolve to its current diverse state?
3. What are the characteristics of the solar system that led to the origin of life?
4. How did life begin and evolve on Earth? Has it evolved elsewhere in the solar system?
13
Pathways for Venus Exploration: 2009
Vignette 1: Magellan
The Magellan spacecraft was launched May 4, 1989, and arrived at Venus on August 10, 1990. The
Magellan synthetic aperture radar (SAR) mapped 98% of the surface of Venus, with a resolution of about
100 m. Global altimetry and radiometry observations also measured surface topography and electrical
properties. A global-gravity map was obtained after Magellan’s aerobraking to a circular orbit. This
aerobraking paved the way for several future missions. The Magellan mission ended in October 1994 with
a controlled entry into the Venusian atmosphere.
Magellan SAR images confirmed that an Earth-like system of plate tectonics does not operate on
Venus, most likely due to the lack of surface water. Volcanism characterizes the surface; more than 85%
consists of volcanic plains. Two types of highland regions were identified: topographic rises with abundant
volcanism interpreted to be the result of mantle plumes, and complexly deformed highland regions called
tessera plateaus, hypothesized to have formed over mantle upwellings or downwellings. The gravity field
is highly correlated with surface topography, with some highland regions apparently supported by
isostatic compensation and others by mantle plumes. Erosion of the surface is not significant due to the
lack of water, although some surface modification by wind streaks was seen.
The biggest surprise revealed by the Magellan mission was the crater population of Venus, which is
randomly distributed and largely unmodified. Although resurfacing in the last 500 million to one billion
years has obscured the impact history of Venus (particularly when compared to the Moon, Mars, and
Mercury), the mean surface age is estimated to be ~500 million to one billion years. A debate has ensued
over whether the entire surface was resurfaced in a catastrophic event approximately 500 million years
ago, or if it was resurfaced more slowly over time. Understanding the history of the surface is not only
important for constraining the interior evolution of Venus, but also the evolution of the atmosphere. While
Magellan unveiled Venus, the data returned did not answer the question of why Venus and Earth have
followed such different evolutionary paths. However, Magellan data provide a basis for a new set of
specific scientific investigations, which will help constrain how habitable planets evolve.
Magellan Radar Mosaic. Blues and greens are the lower plains areas; whites are the rugged highlands.
14
Pathways for Venus Exploration: 2009
The first question on how the planets and minor bodies originated can be addressed by
comparative studies of the abundances and isotopic ratios of the noble gases. The second
question on the evolution of the solar system can be addressed with comparative studies of
climate evolution and Venus’ surface-atmosphere interaction as well as via the chemical and
isotopic compositions of Venus’ surface and atmosphere. The characterization of surface rocks—
particularly granitic and/or sedimentary rocks, hydrated silicates, and oxidized iron—can give
immensely valuable clues to climate change, in particular, yielding insights into the existence of
an ocean early in Venus’ history. The third and fourth questions regarding the origin and
evolution of possible past life-forms could similarly be addressed with surface samples that
might contain isotopic, chemical, or structural signatures of biological specimens or processes.
The Solar System Roadmap noted that the history of water and climate on Venus, including
the timing and fate of a possible early ocean, should be investigated with in situ measurements
prior to a sample return. In particular, it endorsed a VISE mission involving a brief visit to the
surface to obtain samples, followed by sample analysis aloft in a balloon-borne robotic
laboratory. A flagship Venus Mobile Explorer (VME) mission was proposed as the follow-on to
VISE, possibly implemented via floating platforms rather than rovers. Whereas VISE would
survive for only a few hours, VME would operate for days to weeks. VISE and VME would, in
turn, enable a subsequent Venus sample return mission.
The Solar System Roadmap recognized that both VISE and VME need technological
developments in order to carry out their measurements at the high temperatures and pressures as
well as the corrosive environment associated with Venus’ lower atmosphere and the surface. The
Roadmap identified several enabling technologies, including:


Radioisotope-power systems that can operate at the 460ºC near-surface temperatures,
Thin metal balloons that would provide buoyancy and mobility and would survive under
the harsh conditions,
 Thermal-control systems and pressure vessels to contain instruments and electronics, and
 High-temperature electronics and sample-acquisition mechanisms.
Development of instrumentation for mineral and isotopic analyses that are sophisticated and
accurate enough to produce answers to the key questions, and that can operate semiautonomously on a balloon platform, is also needed. The Solar System Roadmap noted that the
technology developments on VISE would enable a subsequent VME mission. Thus, a New
Frontiers VISE mission could be a technology demonstration mission as well as a science
mission.
1.3.3 2006 NASA Science Plan
The 2006 NASA Science Plan [4], developed by a subgroup for the NASA Advisory
Council, took inputs from the NRC Decadal Survey and Solar System Exploration Roadmap to
synthesize a grand vision for NASA science missions and supporting programs as a whole.
Reports from community advisory groups such as VEXAG were also used. As with the Solar
System Roadmap, the guiding theme for the overall vision in this 2006 NASA Science Plan was
habitability. This translated to five main science questions and four associated research
objectives.
15
Pathways for Venus Exploration: 2009
Vignette 2: Venus Express: Revealing the Mysteries of a Neighboring World
Circling the planet once per Earth day since arriving in April 2006, ESA’s Venus Express is the first
mission to comprehensively explore the entire globe of our sister world from the ground up through the
mesosphere, thermosphere, ionosphere, and into space. In particular, Venus Express is the first Venus
orbiter to utilize the new tool of nighttime near-infrared spectroscopic imaging to regularly map the
structure and movement of clouds and gases in the hostile depths of Venus below the obscuring upperlevel clouds, thereby obtaining new insights into the planet’s enigmatic circulation, dynamic meteorology,
and complex chemistry. This novel spectroscopic tool—embodied on Venus Express as the Visible and
Infrared Thermal Imaging Spectrometer (VIRTIS)—maps both (1) the structure and movement of clouds
at three different levels (~50-km altitude on the nightside, and 59- and 70-km altitude on the dayside), and
(2) the abundances of a plethora of chemically reactive species, including water (H2O), sulfur dioxide
(SO2), carbon monoxide (CO), and OCS—at a variety of altitudes in the deep atmosphere below the
clouds. It also observes the hot (~740 K) surface of Venus near 1-micron wavelength, mapping thermal
emissions from the ground, which can be used to constrain 1-micron surface emissivity and composition
as well as to search for and characterize active volcanic processes, as evidenced by locally elevated
thermal temperatures and enhanced trace-gas abundances.
Further information from the surface comes from a bistatic-radar experiment that utilizes the
spacecraft’s communication-radio system to reflect signals off the surface toward Earth. As one facet of
the Venus Radio experiment (VeRa), these echoes of Venus are then intercepted by NASA’s Deep
Space Network (DSN) to reveal characteristics of Venus’ surface texture and emissivity at cm
wavelengths. VeRa also utilizes radio-occultation techniques to measure the vertical profile of Venus’
temperature, density, and pressure down to ~36-km altitude over a large range of latitudes, thereby
providing detailed information on the planet’s 3-D temperature structure, thermal winds, and vertical wave
properties. The Venus Monitoring Camera (VMC) images the upper-level clouds in the UV and near-IR at
0.36 and 0.94 µm wavelength, thus providing high-spatial resolution imagery (better than 1-km resolution)
of the wave and cell structures of Venus’s clouds, as well as providing detailed movies of their motions.
Long exposures by this experiment of Venus’ night side can be used to search for lightning.
Venus Express also scrutinizes the upper atmosphere of Venus above the clouds. Dual UV and nearIR spectrometers, SPICAV and SOIR, regularly observe the limb of the planet in solar occultation from
close range (typically less than 1000 km), thereby producing high-resolution (~5-km) vertical profiles of a
variety of light-absorbing species, including H2O, CO, and SO2. VIRTIS observes nighttime emissions
produced by the recombination of photochemically generated oxygen atoms into oxygen molecules,
thereby revealing key day-to-night circulation flows near the 120-km level. Also, VIRTIS maps the
nighttime temperatures of the atmosphere at 5-km vertical resolution from 60 to 90 km, providing
constraints on the thermal winds in this region. Enigmatic polar features known as Polar Dipoles at the
south and north poles, possible manifestations of the Hadley circulation, can also be mapped in detail and
followed in time.
Venus Express also investigates the planet’s ionosphere and near-space environment. ASPERA
measures the solar wind as it streams around Venus, assessing the number density and speed of protons
ejected from the Sun. A magnetometer experiment (MAG) measures the local magnetic field produced by
ionization of Venus’ upper atmosphere by both intense UV sunlight and solar wind. Joint measurements
by ASPERA and MAG from a variety of positions around Venus then reveal how Venus interacts with the
Sun’s magnetosphere and solar wind. ASPERA also measures ionized atoms such as hydrogen and
oxygen ejected from the planet’s tenuous uppermost atmosphere by the solar wind, thus providing
constraints on the loss of atmospheric elements responsible for the extremely dry state of Venus today.
Venus Express has generated more than 1 Terabit of data to Earth in its first 500 days of operation.
Recent Venus Express VIRTIS results are given in Vignette 3.
16
Pathways for Venus Exploration: 2009
The science questions are:





How did the Sun’s family of planets and minor bodies originate?
How did the solar system evolve to its current diverse state?
What are the characteristics of the solar system that led to the origin of life?
How did life begin and evolve on Earth and has it evolved elsewhere in the solar system?
What are the hazards and resources in the solar-system environment that will affect the
extension of human presence in space?
The associated research objectives (some abridged) to address these questions are:
 Learn how the Sun’s family of planets and minor bodies originated and evolved.
 Understand the processes that determine the history and future habitability in the solar
system.
 Identify and investigate past and/or present habitable environments and determine if there
is or ever has been life elsewhere in the solar system.
 Explore the space environment to discover potential hazards to humans and to search for
resources.
The expected similar yet disparate natures of present-day Venus and Earth were noted, with
Venus being an excellent test bed for understanding the evolution of habitable planets.
The first mission, VISE, would spend a short time on the surface and would provide some of
the technological and scientific groundwork for sample return. A subsequent detailed surface
exploration by a Venus Mobile Explorer (a strategic mission also endorsed by the Solar System
Roadmap) would lead to a sample-return mission. The instrument developments for in situ
Venus lower atmosphere and surface exploration are technical precursors for these future Venus
missions.
1.4
Current Status of NASA’s Support of Venus Research
The Crisp paper [2], the adjunct to the 2003 NRC Decadal Survey [1], provides a summary
of Venus science status prior to ESA’s Venus Express Mission. Their assessment was that after
the observations of the Pioneer Venus Orbiter (PVO) and Magellan, vital knowledge of the
interior structure, surface composition, lower and middle atmosphere, and atmosphere-surface
interactions is still minimal to missing. Although Magellan exposed the resurfacing enigma, we
do not know why it happened, over what time period it occurred, and what the larger
implications and consequences are. Similarly, PVO found possible evidence of active volcanism,
but the results are only suggestive.
In the interest of brevity, we refer the reader to the Crisp paper [2] for the details and focus
here on programmatic issues related to our ability to expand the Venus knowledge base. More
recently, NASA has supported more than a dozen science investigations as part of its Venus
Express mission support. Some eighteen investigators distributed among three categories—
Participating Scientists, Interdisciplinary Scientists, and Instrument Scientists—have been
supported to help optimize the science return of the Venus Express Mission as well as to conduct
focused investigations on the chemistry, dynamics, evolution, magnetospheric environment, and
surface properties of Venus. As part of this effort, Venus Express data will be deposited in an
17
Pathways for Venus Exploration: 2009
open European data archive that parallels the NASA Planetary Data system. These data are
especially beneficial to atmospheric science investigators.
In the past, Venus data analysis has benefited from targeted Venus Data Analysis Programs
(VDAP) within NASA’s research and analysis (R&A) initiatives. Such programs focus on
selected topics of interest to NASA, such as exploiting new observations toward increasing
knowledge needed for informed exploration planning. It is important to exploit the data from
current, past, and future missions to answer the science questions posed above. The challenge is
how these key Venus data analyses and interpretations can be supported within NASA’s R&A
program’s budgets. Of the ~1000 NASA Planetary R&A investigations funded per year, about
thirty are currently for Venus-focused investigations. Although a few of these are for data
archiving activities, the majority are for science efforts that run the gamut from tectonism to
climate change. In summary, NASA’s investments in analysis of Venus Express results,
continued study of Venus geology and geophysics, and understanding Venus atmosphere
evolution are needed to understand Venus and to provide a foundation for future missions.
1.5
Fifty Years of Venus Exploration
To complete the context for future Venus exploration, we examined the past and current
Venus missions (Table 1-1), which have been carried out by the Russian, European, Japanese,
and American space agencies. The Russian space program in 1961 initiated an extensive
program for the exploration of Venus, which included atmospheric probes, landers, orbiters, and
balloon missions. This produced many successful missions, which provided information on how
to survive and conduct experiments in the Venus environment. The Venera 1 impactor was the
first spacecraft to land on another planet. The Venera 13 lander survived on the surface for 127
minutes, which is still unmatched by any other spacecraft at Venus. The Vega balloons
demonstrated the ability of balloons for aerial exploration.
U.S. Venus exploration commenced in 1962 with the flyby of the Mariner 2 spacecraft.
Following this, U.S. missions conducted an exploration of the atmosphere and the surface of
Venus. In the late seventies, NASA conducted the orbiter/multiprobe Pioneer–Venus mission,
with the objective of understanding the atmosphere of the planet. Magellan in the early 1990s
mapped 98% of the surface of the planet, as described in Vignette 1.
Today, Europe’s Venus Express orbiter is providing significant science contributions to the
understanding of Earth’s sister planet by measuring atmospheric dynamics and structure;
composition and chemistry; cloud layers and hazes; radiative balance; the plasma environment
and escape processes; and, to a certain extent, surface properties and geology through remote
sensing, as described in vignettes 2 and 3. Another orbiter, Japan’s Planet-C (Venus Climate
Orbiter, VCO), is under development for a mid-2010 launch. VCO investigations include surface
imaging with an infrared camera and experiments designed to detect possible lightning and
present-surface volcanism.
18
Pathways for Venus Exploration: 2009
Table 1‐1. Summary of Past, Present, and Future Venus Missions. Spacecraft
Launch
Date
Type of Mission
Venera 1
1961
Impactor; spacecraft sealed and pressurized with nitrogen
Mariner 2
1962
Flyby; first to fly by Venus (US)
Zond 1
1964
Probe and main bus; entry capsule designed to withstand 60 to 80°C / 2 to 5 bars
Venera 2 & 3
1965
Probe and main bus; entered the atmosphere of Venus; designed for 80 °C / 5 bar
Venera 4
1967
Stopped transmitting at 25 km; 93 minutes descent; first to descend through the
atmosphere; designed for 300 °C / 20 bar (Russia)
Mariner 5
1967
Flyby (US)
Venera 5
1969
Hard-lander; stopped transmitting at ~20 km (320 °C / 27 bar); 53 min descent
(Russia)
Venera 6
1969
Hard-lander; stopped transmitting at ~20 km (320 °C / 27 bar); 51 min descent
(Russia)
Venera 7
1970
First to soft land on surface; parachute failure, rough landing, landed on the side;
55 min descent / 23 min on surface (Russia)
Venera 8
1972
Performed as designed; soft-lander; 55 min descent / 50 min on surface (Russia)
Mariner 10
1973
Flyby en route to Mercury (US)
Venera 9
1975
Orbiter (moves out of radio range); soft-lander; first to return photos of surface;
20+55 min descent / 53 min on surface (Russia)
Venera 10
1975
Orbiter (moves out of radio range); soft-lander; 20+55 min descent / 65 min on
surface (Russia)
Pioneer-Venus 1
1978
Orbiter with radar altimeter; first detailed radar mapping of surface (US)
Pioneer-Venus 2
1978
Four hard-landers (US)
Venera 11
1978
Flyby, soft-lander; 60 min descent / 95 min on surface (Russia)
Venera 12
1978
Flyby, soft-lander; 60 min descent / 110 min on surface (Russia)
Venera 13
1981
Orbiter, soft-lander; first color images of surface; 55 min descent / 127 min on
surface (Russia)
Venera 14
1981
Orbiter, soft-lander; 55 min descent / 57 min on surface (Russia)
Venera 15
1983
Orbiter with radar mapper (Russia)
Venera 16
1983
Orbiter with radar mapper (Russia)
Vega 1
1984
Flyby, atmospheric balloon probe (Russia / International)
Vega 2
1984
Flyby, atmospheric balloon probe (Russia / International)
Magellan
1989
Orbiter with radar mapper (mapped 98% of the surface); first high-resolution global
map of Venus (US)
Venus Express
2005
Orbiter – ongoing mission (ESA)
Planet-C (VCO)
2010
Venus Climate Orbiter “Planet-C” – in development (JAXA)
Venera-D
2016
Orbiter with lander and balloons (Russia)
19
Pathways for Venus Exploration: 2009
Artist’s concept of Venus Express spacecraft operating at Venus since 2006. Courtesy of ESA. Vignette 3: Recent Venus Express VIRTIS Results
Surface Temperatures.
(left) Black-body temperatures
measured for the surface
correlate well with topography
(right), due to decreases of
surface temperature with
height. Slight variations in this
correlation may indicate differences in the surface rock
emissivities. Courtesy of ESA.
20
Pathways for Venus Exploration: 2009
Vignette 3: Recent Venus Express VIRTIS Results (continued)
Day and night images of the south pole of Venus.
Daytime images (left side of each image) show
high-altitude clouds of small particles near the
70-km level. Night images (right side of each
image) show thick clouds of relatively large
particles near the 50-km level. Clouds at night are
seen in silhouette against the glow of Venus’ hot
lower atmosphere, using near-infrared thermal
radiation near 1.7-µm wavelength. Following the
dark (cloudy) and bright (less cloudy) regions, as
they
move
around
the
planet,
yields
measurements of Venus’ winds near the 55-km
level. Comparison with 70-km altitude winds as
measured by the movements of dayside clouds
yields wind shears, providing clues to the
processes powering Venus’ enigmatic system of
super-rotating winds.
Polar Vortex Phenomena. Venus Express confirmed that the Venusian south pole has a complex and
variable vortex-like feature, sometimes taking the shape of a dipole, but at other times morphing into
tripolar, quadrupolar, and amorphous, indistinct shapes. Temperatures near the 60-km level are shown in
the nighttime portions of 5-µm images, revealing the dipole to be notably hotter than its surroundings,
likely due to compression of descending air. (Bottom left image, taken in daytime conditions, is
overexposed by the Sun). Right-hand, close-up image shows filamentary nature of the dipole, which
changes shape constantly in the dynamically active atmosphere. The dipole is offset from the pole by
several degrees of latitude and rotates with a period of about 2.4 days.
21
Pathways for Venus Exploration: 2009
Vignette 4: Lessons Learned from Pioneer Venus Orbiter and Huygens
Pioneer Venus Orbiter 1978–1992. Venus orbiter with comprehensive payload for remote sensing
and in situ aeronomy.
1. Showed that the greenhouse effect operates much more efficiently on Venus. Data from the four
atmospheric probes led to a greenhouse model that closely matches the observed vertical
temperature profile.
2. Measured long-term changes in atmospheric minor constituents above the clouds. These indicate
forcings on decades-long timescales. Possible causes are volcanic activity and variable dynamics
of the middle atmosphere.
3. Measured upper atmosphere’s response to solar cycle.
Pioneer Venus demonstrated the need to examine the long-term stability of the current climate and to
probe all altitudes during an entire solar cycle. In addition, the nature of the middle and deep atmosphere
remains to be examined via remotely sensed spectral signatures or long-duration in situ probes.
Huygens 2005. Titan lander with cameras, spectrometers, and in situ atmospheric and surface
science instruments.
1. Huygens provided vertical resolution and sensitivity impossible from remote sensing by the
Cassini orbiter, thus providing direct measurements of wind and chemical profiles from >200 km
altitude down to the surface and measurement of volatiles entrained within surface materials.
2. Huygens descent images, when combined with other remote observations, allowed identification
of dune fields by their distinctive color. This, in turn, yielded the exact lander location and ground
truth for remote sensing as well as provided regional context for the landing-site measurements.
Also, radar identification of fields of linear dunes on Titan allowed comparisons to similar features on
Earth, Venus, and Mars. Comparisons to Earth analogs in turn have increased understanding of surface
processes on both bodies.
Pioneer Venus Orbiter and Probes. Artist’s Conception of Huygens Probe. Courtesy of ESA. 22
Pathways for Venus Exploration: 2009
2. VENUS EXPLORATION MISSIONS
In parallel with providing the context for Venus exploration, it is useful to examine past,
current, and future Venus missions and to examine the Venus missions identified in the 2003
NRC Decadal Survey and Crisp paper [1, 2], the 2006 NASA Solar System Roadmap [3], the
2007 NASA Science Plan [4], and the 2008 Venus STDT study [7]. These reports also identify
technologies that must be developed in order to accomplish the endorsed science measurements.
These technologies, in turn, are closely linked to the proposed new missions and their
implementations through various mission architectures. The relevant missions and their science
traceability—the mapping of the science goals and objectives against various proposed
missions—is discussed in this section. Vignette 5 provides additional information on balloonborne in situ exploration.
2.1
Discovery, New Frontiers, and Flagship Missions
Venus exploration is discussed in the NRC Decadal Study [1] and in the NASA Solar System
Exploration Roadmap [3], which endorsed missions to solar system bodies under three mission
classes:



The Discovery Program consists of PI-led smaller missions that provide opportunities for
targeted investigations with relatively rapid flight missions.
The New Frontiers Program consists of PI-led medium-class missions addressing specific
strategic scientific investigations endorsed by the NRC Decadal Survey.
Flagship missions address high-priority investigations so challenging that they must be
implemented with resources significantly larger than those allocated to New Frontiers
missions.
2.1.1 Discovery-Class Missions
The Discovery Program, which began in the early 1990s, consists of PI-led missions that
provide opportunities for targeted investigations with relatively rapid flight missions. Ten full
missions and four missions of opportunity (instruments and investigations flown on a non-NASA
spacecraft as well as extended missions for NASA spacecraft) have been selected. The Discovery
program is open to proposals for scientific investigations that address any area embraced by
NASA’s Solar System Exploration program, including the search for planetary systems around
other stars. This provides an excellent means for tapping the creativity of the planetary science
community. Details on these past and current missions can be found on the Discovery Program
web site at http://discovery.nasa.gov/index.html.
2.1.2 New Frontiers Missions
The New Frontiers program comprises medium-class missions that address objectives
identified by the NRC Decadal Survey [1]. In particular, the Venus In Situ Explorer (VISE) was
endorsed by the NRC in its 2003 Decadal Survey and the NRC Committee for New
Opportunities for Solar System Exploration (NOSSE) in 2007 [8]. Although exploration of the
surface and lower atmosphere of Venus will be a significant technical challenge, it also will yield
high scientific rewards. Venus is considered Earth’s sister planet, and there is much to learn
about Earth by studying Venus’ tectonics, volcanism, surface-atmospheric processes,
atmospheric dynamics, and chemistry. Furthermore, technology demonstrations on VISE could
23
Pathways for Venus Exploration: 2009
Vignette 5: Experiencing Venus by Air: The Advantages of Balloon-Borne
In Situ Exploration
Balloons provide unique, long-term platforms from which to address such fundamental issues as the
origin, formation, evolution, chemistry, and dynamics of Venus and its dense atmosphere. As successfully
and dramatically demonstrated by the USSR’s twin Vega balloons in 1985, such aerial vehicles can
uniquely measure Venus’ dynamic environment in three dimensions, as they ride the powerful, convective
waves in Venus’ clouds near the 55-km level. Also, by sampling over an extended period, balloons can
measure the abundances of a plethora of tell-tale chemical and noble gases, key to understanding Venus’
origin, evolution meteorology, and chemistry.
While the Vega balloons successfully pioneered the use of aerial platforms to explore planets, weight
restrictions prevented their measuring abundances of diagnostic chemicals or noble gases. The new,
highly miniaturized instrument technologies of the 21st century allow such measurements to be made.
Our knowledge of the origin, formation, and evolution of all the planets—including Venus—relies
primarily on knowledge of the bulk abundances and isotopic ratios of the noble gases—helium, neon,
argon, krypton, and xenon—as well as on the isotopic distributions of light gases such as nitrogen. For
example, xenon, with its nine tell-tale isotopes, along with krypton (Kr) and argon (Ar) and their isotopes,
can together reveal a range of ancient cataclysms on Venus and other planets. These include the nature
of (1) any global atmospheric blowoff by intense solar EUV radiation, and (2) any major impacts by large
(>200-km diameter) comet-like planetesimals from the outer solar system. On the other terrestrial planets
where xenon has been adequately measured—Earth and Mars—one or more such major cataclysmic
events occurred early in their histories. Similar measurements for Venus would reveal whether
cataclysmic events occurred on our sister planet as well. As these key tell-tale noble elements have no
appreciable spectral signature, in situ sampling is the only means by which to measure them. Thus, to
reach into the planet’s past, one must sample Venus directly, with typical precisions of better than 5% for
both isotopic ratios and bulk abundances.
Such detailed and precise isotopic measurements can be more than adequately achieved by today’s
lightweight balloon-borne instrumentation suspended for several days in the middle atmosphere near an
altitude of 55 km. Riding the strong winds of Venus near the Earth-like 297-K, 0.5-bar pressure level,
hundreds of high-precision, mass-spectroscopy measurements can be acquired and transmitted during
the balloon’s two-day transit across the face of Venus as viewed from Earth, thus achieving the requisite
tight constraints on isotopic abundances of all the noble gases and many light elements. In addition,
vertical profiles of chemically active species can be obtained as the balloon rides the planet’s dynamic
array of gravity waves, planetary waves, and convective motions, thus providing unique insights into
photochemical and thermochemical processes. Additionally, the planet’s sulfur-based meteorology can be
explored, for example, by measuring over time and altitude both cloud particles and their parent cloudforming gases, as well as lightning frequency and strength.
As was done by the Vega balloons, both local dynamics and planet-scale atmospheric circulation can
be investigated via radio-tracking of the balloon from Earth. Today’s improved interferometric and Doppler
tracking together with well-calibrated onboard pressure sensors can yield knowledge of all three
components of balloon velocity an order of magnitude more accurately than achieved by Vega, that is,
better than 10 cm/s on time scales of a minute in the vertical and an hour in the horizontal. Such
accuracies can provide fundamental measurements of the amplitude and power of gravity waves and the
latitude/longitude characteristics of zonal and meridional winds at known pressure levels. All of these are
key to understanding the processes powering Venus’ super-rotating circulation.
Beyond providing unique insights into the origin/evolution, dynamics, and chemistry of Venus,
exploring Venus by balloon provides valuable experience for flying the skies of other worlds. Experiencing
Venus for days and perhaps weeks by the first airborne rovers could well lead to a new era of “aeroving”
the distant skies of Titan and the many gas giants of the outer solar system.
24
Pathways for Venus Exploration: 2009
pave the way for a future flagship class mission to the surface and lower atmosphere of Venus, as
well as for a subsequent Venus Surface Sample Return (VSSR) mission.
In the 2009 New Frontiers AO, NASA identified a number of key science objectives and
solicited a mission that would address all or a subset of these. Specifically, a New Frontiers
Venus mission should be designed to:






Understand the physics and chemistry of the Venus atmosphere, especially the
abundances of its trace gases, sulfur, light stable isotopes, and noble gas isotopes.
Constrain the coupling of thermochemical, photochemical, and dynamic processes in
Venus’ atmosphere and between the surface and atmosphere to understand radiative
balance, climate, dynamics, and chemical cycles.
Understand the physics and chemistry of the Venus crust.
Understand the properties of the Venus atmosphere down to the surface and improve our
understanding of Venus’ zonal cloud-level winds.
Understand the weathering environment of the Venus crust in the context of the dynamics
of the atmosphere and the composition and texture of its surface materials.
Search for planetary scale evidence of past hydrological cycles, oceans, and life and for
constraints on the evolution of the Venus atmosphere.
The 2009 New Frontiers AO identified eight potential medium-class missions, including the
Venus In Situ Explorer (VISE). A New Frontiers mission will be selected and launched not later
than the end of 2018.
2.2
Venus Flagship-Class Missions
Certain high-priority investigations are so challenging that they cannot be achieved within
the constraints of the Discovery and New Frontiers programs. With costs significantly larger than
those of New Frontiers missions, flagship missions represent major national investments that
must be strategically selected and implemented. Examples include comprehensive studies of
planetary bodies, such as those undertaken by Voyager, Galileo, Cassini, and the Mars rovers.
Thus, flagship missions conduct in-depth studies of solar system bodies as well as sample return
from planetary surfaces. These missions generally require large propulsion systems and launch
vehicles. In addition, flagship missions often require significant focused technology development
prior to mission start, extended engineering developments, and extensive pre-decisional trade
studies to determine the proper balance of cost, risk, and science return.
2.2.1 Venus Flagship Design Reference Mission
NASA Headquarters conducted a Venus flagship mission study in 2008–2009 based on
recommendations identified by the 2003 NRC Decadal Survey [1] and the NASA Solar System
Exploration Roadmap [3], in parallel with a finding from VEXAG. This study was conducted by
a NASA-appointed Venus STDT, an international group of scientists and engineers from France,
Germany, Japan, the Netherlands, the Russian Federation, and the United States. JPL supported
this study with a dedicated engineering team and the Advanced Project Design Team (Team X).
The STDT assessed science goals and investigations, leading to the Venus Design Reference
Mission (VDRM)—which includes a notional instrument payload, subsystems, and
technologies—implemented using an orbiter, balloons, and landers (Figure 2-1).
25
Pathways for Venus Exploration: 2009
Figure 2‐1. Artist’s concept of Venus flagship orbiter, balloons, and landers—elements of the Venus Design Reference Mission, developed by the Venus STDT in 2008–2009. Artwork by Tibor Balint. 26
Pathways for Venus Exploration: 2009
NASA guidelines for this study specified a launch between 2020 and 2025 with a cost of $3B
to $4B. Although the study assumed no international contributions, it is expected that a future
NASA Venus flagship mission would, in fact, be conducted with international collaboration.
This mission would revolutionize our understanding of the climate of terrestrial planets
(including the coupling between volcanism, tectonism, the interior, and the atmosphere); the
habitability of planets; and the geologic history of Venus (including the existence of a past
ocean). The mission would address top-level science questions:















Is Venus geologically active today?
How does the Venus atmospheric greenhouse work?
What does the surface say about Venus’ geological history?
How does the Venus atmospheric super-rotation work?
How do the surface and atmosphere interact to affect their compositions?
How are the clouds formed and maintained?
How is sunlight absorbed in the Venus atmosphere?
What atmospheric loss mechanisms are currently at work?
What kind of basalts make up Venus’ lava flows?
Are there evolved, continental-like rocks on Venus?
How is heat transported in the mantle, and how thick is the thermal lithosphere?
What happened on Venus to erase 80% of its geologic history?
Did Venus ever have oceans and, if so, for how long?
Did the early atmosphere of Venus experience catastrophic loss, either due to
hydrodynamic escape or a large impact?
Did Venus have a magnetic field, and does it have a remnant one now?
These questions translate to 3 major themes:
What Does the Venus Greenhouse Tell Us About Climate Change? Addressed by
characterizing the dynamics, chemical cycles, and radiative balance of the Venus atmosphere and
by placing constraints on the evolution of the Venus atmosphere.
How Active is Venus? Addressed by identifying evidence for active tectonism and volcanism in
order to place constraints on evolution of tectonic and volcanic styles, characterizing the
structure and dynamics of the interior in order to place constraints on resurfacing, and by placing
constraints on stratigraphy, resurfacing, and other geologic processes.
When and Where Did the Water Go? Addressed by identifying evidence of past environmental
conditions, including oceans, and characterizing geologic units in terms of chemical and
mineralogical composition of the surface rocks in context of past and present environmental
conditions.
27
Pathways for Venus Exploration: 2009
Figure 2‐2. Overviews of Venus Missions Endorsed by the Solar System Exploration Roadmap 28
Pathways for Venus Exploration: 2009
The flagship mission to address these questions, the Venus Design Reference Mission
(VDRM), consists of two launched spacecraft, one being an orbiter and the other delivering two
entry vehicles, where each entry vehicle carries dual landers and balloons (Figure 2-1). In this
dual-launch configuration, two Atlas V launches are required to send these spacecraft to Venus.
The first launch vehicle delivers the two landers and the two balloons to Venus on a Type-IV
trajectory. The second launch vehicle delivers the orbiter on a Type-II trajectory to Venus. The
orbiter arrives at Venus first, with sufficient time for checkout and orbit phasing before the
landers and balloons arrive 3.5 months later. The orbiter supports two functions. First, it acts as a
telecommunication relay to transmit data to/from the landers and balloons to Earth during the in
situ observations. Once the landers and balloons complete their observations, the orbiter
transitions from its telecom relay phase to an orbital science phase with a 2-year remote sensing
mission. The landers are designed for a 1-hour atmospheric descent followed by 5 hours of
operation on the surface. The balloons and their payloads are designed to operate for 1 month at
an altitude of 55 km, circumnavigating the planet several times, while gradually drifting from
mid latitudes towards the polar vortex.
VDRM can be implemented with modest technology developments, such as those for sample
acquisition and handling; aerial mobility; and high temperature–tolerant components (e.g.,
sensors, electronics, mechanisms, instruments, and power storage). This mission lends itself to
spinoffs, as various elements could be implemented as precursor Discovery or New Frontiers
missions. Continuation of the flagship study would further refine science objectives, and
technology development planning based on technology needs for this and other mission
architectures requiring long-lived mission elements. The fact sheet for NASA’s Flagship Mission
to Venus follows the appendices.
2.2.2 Other Future Venus Flagship-Class Missions
Flagship missions beyond the 2015–2020 timeframe (such as those shown in Figure 2-2) will
be defined and selected based upon the results of earlier missions. Other high-priority flagshipclass missions for Venus endorsed by the NRC Decadal Survey [1] and NASA Solar System
Roadmap [3] include the following:

Venus Mobile Explorer (VME)—a long-range, long-duration air mobility platform—
would perform up to several months of extensive measurements at the Venus surface,
including a search for granitic and sedimentary rocks. This would be accomplished by in
situ analysis of the crust for meta-stable hydrated silicates and measurements of the
oxidation and mineralogical state of iron. VME and VISE could determine how long ago
an ocean may have existed on Venus and, therefore, how long Venus may have had to
nurture life. Equipped with visual imaging and a targeted set of geochemical sensors,
VME would use mobile scientific exploration to sample different surface sites. VME
would have the advantages of mobility demonstrated by the Mars Exploration Rovers, as
these rovers have accomplished extraordinary advances in the understanding of
geochemistry and hence past climate conditions on Mars. A similar understanding for
Venus would be enabled by VME.
29
Pathways for Venus Exploration: 2009


A Venus Surface Sample Return (VSSR) mission would return a Venus surface sample
to Earth for further analysis. This enables high-precision measurements of the isotopic
composition of oxygen in surface rocks, as well as trace elements, in order to characterize
the age of rocks and core-mantle formation. This mission requires significant technology
development of sample acquisition and handling instrumentation, including a multi-stage
ascent air-mobility system to lift the sample to launch altitude. In orbit, rendezvous
expertise could be inherited from a Mars Sample Return mission.
A Venus Geophysical Network mission would investigate the internal structure and
seismic activity of the planet as well as monitor the circulation of the atmosphere. It
would provide insight into the causes and effects of the apparent global climate change
that Venus experienced in the distant past, as discussed at the Chapman Conference on
Venus Terrestrial Planet [5]. Key technologies for this long-lived in situ mission include
high-temperature telecom systems, actively cooled radioisotope power systems, and
highly efficient thermal-management and pressure-mitigation systems. Another challenge
is transmitting large data volumes back to Earth.
For further details on the VME, VSSR, and Venus Geophysical Network missions, see their
respective overviews (Figure 2-2).
2.3
Traceability of VEXAG Science Objectives to Future Flagship Missions
Potential science returns from the future Venus missions described above are shown in a
traceability matrix (Table 2-1), which maps the missions to the VEXAG science, goals, and
objectives (Appendix A). Each objective is grouped into one of three main VEXAG goals, while
missions are divided into the flagship mission classes. Green dots  indicate that a mission can
produce major contributions to meeting VEXAG science objectives, while light-blue triangles 
identify the missions that can produce contributory science to these objectives.
Artist’s concept of balloon explorers flying in the Venusian skies. Such mobile vehicles, riding the strong winds of Venus under Earth‐like temperature and pressure conditions, can explore the dynamics and active chemistry of Venus while also uncovering tell‐tale clues to Venus’ past locked in isotopic distributions of noble and light gases. 30
Pathways for Venus Exploration: 2009
Table 2‐1. Traceability Matrix of Objectives Met with Venus Flagship Missions Seek evidence for past changes in interior dynamics

Determine if Venus was ever habitable

Goal II.













VSSR

VNET
Short Lived
Lander(s)

VME
Mid-Altitude
Balloon(s)

Goal I.
Other Missions
Enhanced
Lander(s)
Venus Orbiter
Understand atmospheric evolution
Objectives
Enhanced
VDRM
Enhanced
Balloon(s)
VDRM Mission
Elements
Flagship Class Missions
Origin and Evolution





Venus as a Terrestrial Planet
Understand what the chemistry and mineralogy of the
crust tell us about processes that shaped the surface of
Venus over time

Assess the current structure and dynamics of the
interior

Characterize the current rates and styles of volcanism
and tectonism, and how have they varied over time

Characterize current processes in the atmosphere

Characterize the Venus Greenhouse
























Determine if there was ever liquid water on the surface
of Venus






Characterize how the interior, surface, and atmosphere
interact






Goal III.
Climate Change and the Future of Earth
Convention:


 Supporting Contributions
 Major Contribution
VDRM – Venus Design Reference Mission (Orbiter + 2 short lived landers + 2 balloons):
Venus Orbiter – Science & relay telecon orbiter
Mid-Altitude Balloon(s) – Superpressure balloons in the 52 km-to-70 km altitude range
Short lived landers – Descent science + 5 hours on the surface
Enhanced VDRM:
Enhanced Balloon(s) – ASRG powered superpressure balloon, continuous operation
Enhanced Lander(s) – Long-lived lander, 90 days on the surface, humans-in-the-loop, seismic measurements, active
cooling and radioactive power source.
Enhanced VDRM Architectures:
VME – Venus Mobile Explorer near surface, using bellows; possibly periodic surface access; 90-day aerial traverse
VNET – Venus Network Explorer; seismic network with 4 or more landed elements
VSSR – Venus Surface Sample Return; multiple balloons, short surface stay
31

Pathways for Venus Exploration: 2009
Artist’s concept of lightening on Venus. Courtesy of ESA. The Venus surface observed by the Russian Venera lander showing a platey basaltic surface. 32
Pathways for Venus Exploration: 2009
2.4
ESA Cosmic Vision—European Venus Explorer
The European Venus Explorer (EVE) mission was proposed to the European Space Agency
(ESA) for launch in 2016–2018 in response to 2007 ESA’s Cosmic Vision AO. Although ESA’s
Venus Express mission will answer many questions about Venus, including those relating to the
isotopic-ratio and cloud-chemistry objectives, other questions relating to Venus need to be
addressed. The answers to these questions are key to understanding climate evolution on Venus.
Consequently, the EVE mission focuses on understanding the evolution of Venus and its climate,
with relevance to terrestrial planets.
The proposed EVE mission consists of a balloon platform floating at an altitude of
50–60 km; a short-lived lander provided by Russia; and an orbiter with a polar orbit, which
would perform science observations as well as relay data from the balloon and short-lived lander.
The balloon lifetime of 7 days enables one full transit around the planet. This is significantly
longer than the 48 hours of data returned from Russia’s Vega balloons. Earth-based VLBI and
Doppler measurements would provide tracking information for the orbiter, allowing
measurement of the variations in the planet’s gravity field, while the balloon and short-lived
lander will yield wind measurements in the lower atmosphere. The probe’s descent through the
atmosphere would last 60 minutes, followed by 30 minutes operation on the surface. The
Japanese space agency (JAXA) also proposed to augment this mission with a small water-vaporinflated balloon, which would be deployed at 35-km altitude and then communicate directly to
Earth.
Thus, the EVE mission was proposed as an international project, with mission elements from
Europe, Russia, and Japan. While it was not selected as the next medium-class Cosmic Vision
mission, the EVE proposal was recognized for its high science value and resulted in a European
investment for balloon technology. This development will greatly benefit EVE when proposed
again as a potential subsequent Cosmic Vision mission.
2.5
Russian Space Agency Venera-D
The Russians are contemplating a new Venus mission, Venera-D, with a target launch from
Baikonur in 2016 on Soyuz-Fregat. This mission would have an orbiter, two balloons,
microprobes, and a lander for precise measurements of





Structure and chemical composition of the atmosphere
Clouds, structure, composition, chemistry
Surface composition, mineralogy, geochemistry
Dynamics and nature of super-rotation
Upper atmosphere, ionosphere, electrical activity, magnetosphere, escape rate
The orbiter is 540 kg, with 70–80 kg for science instruments. The balloons are Vega type,
3.4-m diameter, deployed at 48–50 km and 55–60 km, with lifetimes of 8 days and payloads of
5–15 kg. Microprobes are 2 kg with payloads of 0.5 kg, deployed from the balloon with a
lifetime of 30 minutes. The lander, with a payload of 20–25 kg, has a lifetime on the surface of
one hour. The Venera-D mission invites international collaboration.
33
Pathways for Venus Exploration: 2009
3. VENUS LABORATORY MEASUREMENTS
In addition to the missions for future Venus exploration described in the previous section,
new laboratory measurements are needed to maximize the science return from current and future
Venus missions. These measurements, shown in Table 3-1, can be divided into two categories:
Category 1 are laboratory data necessary for retrieving Venus system variables from calibrated
instrument data, and Category 2 are laboratory data necessary for characterizing fundamental
Venus processes based on newly revealed Venus system variables.
There are four basic physical regimes for the new laboratory measurements: (1) the
atmosphere above the clouds, in which the temperature and pressure conditions are similar to
those in the terrestrial atmosphere; (2) the sulfuric-acid-laced cloud layer; (3) the atmosphere
below the clouds, in which the temperature and pressure range is unique for solar system
exploration; and (4) the super-heated surface. Many of these laboratory measurements could be
conducted in a Venus Environmental Test Facility, which would simulate pressure, temperature,
and atmospheric composition as a function of altitude. This would provide insights into how
elements behave in the Venus environment and would also enable development and testing of
new instruments and subsystems to operate under relevant conditions.
Table 3‐1. New Laboratory Studies to Support Future Venus Exploration Context
Category 1 Measurements of Venus System
Variables
Category 2 Measurements of Venus
System Processes
Atmosphere above
the clouds
Trace constituent atmospheric sounding:
mm/sub-mm spectral line pressure-broadening
coefficients
Molecular spectral parameters: frequency,
transition strengths (cross sections) in IR,
submm, etc.
Excited atom/molecule-molecule reaction
rates, for example, O* + CO2
Cloud layer
Cloud composition: optical properties of sulfuric
acid aerosols under the conditions experienced
in the clouds of Venus, especially at the lower
temperatures of the upper clouds
Cloud composition: effects of various likely
impurities (i.e., sulfur allotropes and other
photochemical byproducts) on the scattering
and absorbing properties of these aerosols
Reaction rate parameters for sulfur- and
chlorine-containing species in a CO2 –
dominated atmosphere
Aerosol formation and properties
Cloud microphysics: critical saturation for
nucleation under Venus cloud conditions
Cloud microphysics: charging properties of
the cloud aerosols could be investigated in
a manner similar to terrestrial aerosol
charging
34
Pathways for Venus Exploration: 2009
Context
Atmosphere below
the clouds
Category 1 Measurements of Venus System
Variables
Atmospheric IR opacity: Very high-pressure,
high-temperature CO2 and H2O spectroscopy,
isotopologues, O3, O2, H2, etc.
Category 2 Measurements of Venus
System Processes
Molecular spectral parameters: frequency,
transition strength (cross sections), line
shape, pressure-induced absorption,
particularly CO2 and its isotopologues
Near-surface atmospheric sounding: cm
wavelength properties of CO2 and OCS >30
bars
Supercritical CO2 in new temperature range at
high pressures
Surface
Chemical weathering of surface materials
(basalts): reaction rates, decomposition rates
Spectroscopic (visible, near-IR) characteristics
of various ferric/ferrous, silicate, sulfate, and
hydroxide under Venus conditions
Surface conductivity sounding: dielectric loss
properties at 750 K for various basalts and
other major rock types
Atmospheric IR opacity: Very high-pressure,
high-temperature CO2 and H2O spectroscopy,
isotopologues, O3, O2, H2, etc.
Fundamental thermophysical data: specific
heat, speed of sound, equation of state,
thermal expansion coefficients
Technical issues
Stability of spacecraft materials, and rates of
reaction/corrosion with hot supercritical CO2SO2 gas
Chemical transfer of elements from surface into
atmosphere (and onto spacecraft windows?)
Scattering properties
A Venus Environmental Test Facility would enable:


Understanding the chemistry in the atmosphere above the cloud tops: There is a shortage
of laboratory measurements under Venus atmospheric conditions that would enable
accurate determinations of the atmospheric properties. In addition, for understanding
what acquired measurements reveal about atmospheric processes, there is a shortage of
laboratory measurements for key parameters of relevant reaction processes, particularly
those unique to a sulfur-rich atmosphere.
Understanding the physical and chemical properties of the sulfurous cloud layers: There
is a shortage of laboratory measurements at Venus cloud conditions related to the optical
properties of different candidate cloud aerosols. Thus, new laboratory measurements
concerning aerosol formation and properties are required to understand the formation of
these clouds.
35
Pathways for Venus Exploration: 2009

Understanding the significance of the composition in the atmosphere below the clouds: A
region of high temperature and pressure, new laboratory measurements on the optical
properties of different molecular constituents, including sulfur compounds, are required.
In addition, new laboratory studies under Venus surface conditions are required to obtain
rates of chemical weathering of potential surface materials, spectroscopic parameters for possible
Venus surface materials, measurements of conductivity of surface materials, and fundamental
thermophysical data. Laboratory investigations and studies of analog environments on Earth will
provide the necessary information to support future Venus measurements and their interpretation.
Facilities for laboratory investigations at extreme Venus temperature and pressure conditions can
be small and devoted to particular investigations. If they were made available, larger chambers
for spacecraft and instrument testing under Venus conditions would enable the general scientific
community to perform laboratory investigations. In summary, new laboratory measurements are
needed to maximize the science return from current and future Venus missions.
Diagram showing the possible atmospheric interactions taking place in the Venus lower atmosphere
36
Pathways for Venus Exploration: 2009
4. REFERENCES AND WHITE PAPERS FOR NEXT DECADAL SURVEY
[1] National Research Council New Frontiers in the Solar System: An Integrated Exploration
Strategy (also known as the NRC Decadal Survey for Solar System Exploration) (2003):
http://www.nap.edu/catalog.php?record_id=10432#toc
[2] Crisp, D., et al., “Divergent Evolution Among Earth-like Planets: The Case for Venus
Exploration,” The Future of Solar System Exploration, 2003–2013, Community
Contributions to the National Research Council (NRC) Solar System Exploration Survey
(Mark Sykes, Ed.), ASP Conference Series, vol. 272, pp. 5–34, 2002.
[3] NASA Solar System Exploration Roadmap (2006):
http://solarsystem.nasa.gov/multimedia/downloads.cfm
[4] NASA Science Plan for NASA’s Science Mission Directorate 2007–2016:
http://science.hq.nasa.gov/strategy/Science_Plan_07.pdf
[5] “Exploring Venus as a Terrestrial Planet,” Guest Editors: L. Esposito, E. R. Stofan, and T. E
Cravens, Venus Chapman Conference Special Issue, Journal of Geophysical Research,
Vol. 112, No. E4, 2007.
[6] Treiman, A., et al., Report on the LPI Workshop: “Venus Geochemistry: Progress,
Prospects, and Future Missions,” April 2009,” http://www.lpi.usra.edu/vexag/
[7] NASA’s Flagship Mission to Venus: Final Report of the Venus Science and Technology
Definition Team, April 2009, http://vfm.jpl.nasa.gov
[8] Opening New Frontiers in Space: Choices for the Next New Frontiers Announcement of
Opportunity, National Research Council Committee on New Opportunities in Solar System
Exploration (NOSSE), Space Studies Board, The National Academies Press, Washington,
D.C., 2008.
Most of the these reports can be accessed via the Reports section of VEXAG website.
Other references of interest are:
Special Issue on Pioneer Venus Orbiter, Journal of Geophysical Research, Vol. 85, No. A13,
December 30, 1980.
VENUS, D. M. Hunten, L. Colin, T. M. Donahue, and V. I. Moroz (eds.), Space Science Series,
The University of Arizona Press, Tucson, Arizona, 1143 pages, 1983.
“Magellan at Venus,” Journal of Geophysical Research, Vol. 97, No. E8, August 25, 1992, Vol.
97, No. E10, October 25, 1992, The American Geophysical Union.
J. G. Luhmann, J. B. Pollack, and L. Colin, “The Pioneer Mission to Venus,” Scientific
American, pp. 90–97, April 1994.
VENUS II, Geology, Geophysics, Atmosphere, and Solar Wind Environment, S. W. Bougher, D.
M. Hunten, and R. J. Phillips (eds.), The University of Arizona Press, Tucson, Arizona, 1376
pages, 1997.
"Venus and Venus Express,” Planetary and Space Science, F. W. Taylor (ed.), Volumes 54 and
55, 2006 and 2007.
37
Pathways for Venus Exploration: 2009
Venus White Papers for the Planetary Sciences Decadal Survey
1. “Venus Exploration Goals, Objectives, Investigations, and Priorities”
Sanjay Limaye, Suzanne Smrekar, and VEXAG Executive Committee
2. “Venus Atmosphere: Major Questions and Required Observations”
Sanjay Limaye, Mark Allen, Sushil Atreya, Kevin H. Baines, Jean-Loup Bertaux, Gordon
Bjoraker, Jacques Blamont, Mark Bullock, Eric Chassefiere, Gordon Chin, Curt Covey,
David Grinspoon, Samuel Gulkis, Viktor Kerzhanovich, Stephen Lewis, Kevin McGouldrick,
W. J. Markiewicz, Rosalyn A. Pertzborn, Christopher Rozoff, Giuseppe Piccioni, Gerald
Schubert, Lawrence A. Sromovsky, Colin F. Wilson, Yuk Yung
3. “Venus: Constraining Crustal Evolution from Orbit Via High-Resolution Geophysical and
Geological Reconnaissance,”
James Garvin, Lori Glaze, Sushil Atreya, Bruce Campbell, Don Campbell, Peter Ford,
Walter Kiefer, Frank Lemoine, Greg Neumann, Roger Phillips, Keith Raney
4. “Comparative Planetary Climate Studies”
David Grinspoon, Mark Bullock, James Kasting, Janet Luhmann, Peter Read, Scot Rafkin,
Sanjay Limaye, Kevin McGouldrick, Gordon Chin, Samuel Gulkis, Feng Tian, Eric
Chassefiere, Hakan Svedhem, Kevin Baines
5. “Venus Geochemistry: Progress, Prospects, and Future Missions”
Allan Treiman, David Draper, M. Darby Dyar
6. “Previously Overlooked/Ignored Electronic Charge Carriers in Rocks”
Friedemann Freund
7. “Mission Concept: Venus in situ Explorer (VISE)”
Larry W. Esposito and the SAGE Proposal Team
8. “Venus Atmospheric Explorer New Frontiers Mission Concept”
Kevin Baines, Sushil Atreya, Tibor Balint, Mark Bullock, David Crisp, David Grinspoon,
Jeffery Hall, Gary Hunter, Sanjay Limaye, Viktor Kerzhanovich, Paul Mahaffy, Christopher
Russell, David Senske, Stuart Stephens, Chris Webster
9. “The Venus Science and Technology Definition Team Flagship Mission Study”
Mark Bullock, David Senske, Tibor Balint, Alexis Benz, Bruce Campbell, Eric Chassefiere,
Anthony Colaprete, Jim Cutts, Lori Glaze, Stephen Gorevan, David Grinspoon, Jeff Hall,
George Hashimoto, Jim Head, Gary Hunter, Natasha Johnson, Viktor Kerzhanovich, Walter
Kiefer, Elizabeth Kolawa, Tibor Kremic, Johnny Kwok, Sanjay Limaye, Steve Mackwell,
Mikhail Marov, Adriana Ocampo, Gerald Schubert, Ellen Stofan, Hakan Svedhem, Dimitri
Titov, Allen Treiman
38
Pathways for Venus Exploration: 2009
10. “Technologies for Future Venus Exploration”
Tibor Balint, James Cutts, Mark Bullock, James Garvin, Stephen Gorevan, Jeffery Hall,
Peter Hughes, Gary Hunter, Satish Khanna, Elizabeth Kolawa, Viktor Kerzhanovich, Ethiraj
Venkatapathy
11. “Thermal Protection System Technologies for Enabling Future Venus Exploration”
Ethiraj Venkatapathy, Helen H. Hwang, Bernard Laub, Joseph L. Conley, James Arnold,
Christine E. Szalai, Jim Tibaudo, Robert Knudsen, Andrew Chambers, David Atkinson,
Sushil K. Atreva, Joseph M. Vellinga, William H. Willcockson, Janine M. Thornton, Nicholas
G. Smith, Richard A. Hund, John Dec, Max L. Blosser, Michelle M. Munk, Robert Maddock,
Prasun N. Desai, Walter Engelund, Stephen Sandford, David A. Gilman, Steven W. Gayle,
John Kowal, Christopher B. Madden, Stan Bouslog, Brian J. Remark, Donald Curry, Scott
Coughlin, Adam J. Amar, Kevin H. Baines, Tibor Balint, Bernard Bienstock, George T.
Chen, James A. Cutts, Jeffery L. Hall, Samad A. Hayati, Pamela J. Hoffman, Linda Spilker,
Romasso P. Rivellini, Robert Manning, Eric M. Slimko, Adam D. Steltzner, Thomas Spilker,
Jeffrey Umland, Charles Kiskiras, Duane Baker, Thomas Foster, Dominic Calamito, James
B. Garvin, Timothy A. Sauerwein, Sharon Seipel, Lori S. Glaze, Spencer Stolis, Mark
Lippold, Francis Schwind, James Thompson, Raj Narayan, Thomas Andrews, Conley
Thatcher, Edwin B. Curry, John McKinney, Robert Frampton, Todd Stever, Charley Bown,
William Congdon, Jennifer Congdon, Daniel M. Empey, Joe Hartman, Dinesh Prabhu,
Nancy L. Mangini, Kristina A. Skokova, Margaret M. Stackpoole, Tood White, Howard
Goldstein, Melmoth Covington, Robin A. Beck, Carol W. Carroll, Charles A. Smith, Deepak
Bose, Anthony Colaprete, David M. Driver, Edward Martinez, Donald T. Ellerby, Matthew J.
Gasch, Aga M. Goodsell, James Reuther, Sylvia M. Johnson, Dean Kontinos, Mary
Livingston, Michael J. Wright, Harry Partridge, George A. Raiche, Huy K. Tran, Kerry A.
Trumble
All of the these white Papers can be accessed via the White Papers section of VEXAG website.
A composite of the hemispheric vortex on Venus left, a polar stereographic version of a Venus Express ultraviolet image) and Hurricane Frances, a tropical cyclone on Earth showing the remarkable similarities in the vortex structure. Given this similarity, it may become feasible to better understand the vortex structure on Venus from its terrestrial analog despite the difference in the energy source. 39
Pathways for Venus Exploration: 2009
5. ACRONYMS AND ABBREVIATIONS
AO
Announcement of Opportunity
ASPERA
Venus Express fields and particles experiment
ASRG
Advanced Stirling Radioisotope Generator
CCD
charge-coupled device
DSN
Deep Space Network
ESA
European Space Agency
EUV
Extreme Ultraviolet
EVE
European Venus Explorer (Cosmic Vision proposal, 2007)
JAXA
Japanese Aerospace Exploration Agency
JPL
Jet Propulsion Laboratory
LPI
Lunar and Planetary Institute
MAG
Venus Express magnetometer experiment
NASA
National Aeronautics and Space Administration
NOSSE
NRC Committee for New Opportunities for Solar System Exploration
NRA
NASA Research Announcement
PVO
Pioneer Venus Orbiter
R&A
research and analysis
ROSES
Research Opportunities in Space and Earth Sciences
STDT
Science and Technology Definition Team
USSR
Union of Socialist Soviet Republics
VCO
Venus Climate Orbiter
VDAP
Venus Data Analysis Programs
VDRM
Venus Design Reference Mission
Vega
Russian Halley/Venus Lander and Orbiter Mission
VeRa
Venus Express radio science experiment
VEXAG
Venus Exploration Analysis Group
VIRTIS
Visible and Infrared Thermal Imaging Spectrometer
VISE
Venus In situ Explorer
VMC
Venus Monitoring Camera
VLBI
very long–baseline interferometry
VME
Venus Mobile Explorer
VSSR
Venus Surface Sample Return
40
Pathways for Venus Exploration: 2009
APPENDIX A. VENUS GOALS, OBJECTIVES, AND INVESTIGATIONS
Excerpts from Venus White Paper: VEXAG Goals, Objectives, and Investigations
Why Venus now?
Venus proximity to Earth and its similarity in size and bulk density to Earth’s have earned it
the title of “Earth’s twin”. As well, the lack of seasons and overall regular nature of the
surface—with no land/water contrasts to help generate weather nor large oceans to help transport
heat and momentum—seemingly renders Venus a relatively simple planet to understand. Yet we
understand very little about this very alien world next door. Indeed, the contrast between Venus’
hellish 450°C surface temperature, sulfuric acid clouds, and its divergent geologic evolution has
challenged our fundamental understanding of how terrestrial planets, including Earth, work. The
absence of plate tectonics on Venus helped move models away from an emphasis on buoyancy to
an understanding of the function of lithospheric strength, convective vigor, and the role of
volatile history in controlling these processes. Venus is the planet where the importance of the
greenhouse effect was first realized, and where winds blow with hurricane force nearly
everywhere across the planet, from the first km above the ground to above 100 km altitude and
from the equator to the high polar region. What powers such global gales when the planet itself
rotates at a speed slower than the average person can walk on Earth is unknown?
The study of the links between surface, interior, and climatic processes on Venus has
reinforced the idea that Venus could represent the fate of the Earth. The realization that two such
similar planets could produce this extreme range of processes and conditions makes Venus an
essential target for further exploration as we move out in the universe and discover Earth-like
planets beyond our solar system. Recent results from Mars show that liquid ground water was
limited to the first billion years of its evolution, during its geologically active period. Venus
Express has provided new reasons to explore Venus now. Surface thermal emissivity
observations suggest tantalizing evidence of more evolved crustal plateaus, suggesting past
oceans. Observations of atmospheric cyclones show structure nearly identical to those on Earth.
As climate evolution comes into sharp focus on Earth, we must resume exploration of the planet
that serves as an extreme end member.
Overarching Theme for Venus Exploration
With the context provided by the 2003 NRC Decadal Survey [1, 2], the 2006 Solar System
Roadmap [3], and the 2007 NASA Science Plan [4], VEXAG has adopted an overarching theme
for Venus exploration: Venus and Implications for the Formation of Habitable Worlds. This
theme is supported by three goals and prioritized objectives and investigations (Table 1-1).
1. Origin and Evolution: How did Venus originate and evolve, and what are the
implications for the characteristic lifetimes and conditions of habitable environments on
Venus and similar extrasolar systems?
2. Venus as a Terrestrial Planet: What are the processes that have shaped and still shape
the planet?
3. Climate Change and the Future of Earth: What does Venus tell us about the fate of
Earth’s environment?
41
Pathways for Venus Exploration: 2009
Table 1‐1. Venus and Implications for the Formation of Habitable Worlds Venus as a Terrestrial Planet
Origin and Evolution
Goal
Objective
Investigation
Characterize elemental composition and isotopic ratios of noble gases in the
Venus atmosphere, especially Xe, Kr, 40Ar, 36Ar, Ne, 4He, 3He, to constrain
Understand
origin and sources and sinks driving evolution of the atmosphere.
atmospheric
Determine isotopic ratios of H/D, 15N/14N, 17O/16O, 18O/16O, 34S/32S and
evolution
13
C/12C in the atmosphere to constrain paleochemical disequilibria,
atmospheric loss rates, the history of water, and paleobiosignatures.
Characterize the structure, dynamics, and history of the interior of Venus,
including possible evolution from plate tectonics to stagnant-lid tectonics.
Seek evidence for Characterize the nature of surface deformation over the planet's history,
particularly evidence for significant horizontal surface movement.
past changes in
interior dynamics Characterize radiogenic 4He, 40Ar and Xe isotopic mixing ratios generated
through radioactive decay to determine the mean rate of interior outgassing
over Venus’ history.
At the surface, identify major and minor elemental compositions (including H),
petrology, and minerals in which those elements are sited (for example,
Determine if
hydrous minerals to place constraints on past habitable environments).
Venus was ever
habitable
Characterize gases trapped in rocks for evidence of past atmospheric
conditions.
Characterize geologic units in terms of major, minor, and selected trace
elements (including those that are important for understanding bulk volatile
composition, conditions of core formation, heat production, and surface
emissivity variations), minerals in which those elements are sited, & isotopes.
Characterize the chemical compositions of materials near Venus’ surface as
Understand what a function of depth (beyond weathering rind) to search for evidence of
the chemistry and paleochemical disequilibria and characterize features of surface rocks that
mineralogy of the may indicate past climate or biogenic processes.
crust tell us about
Assess the petrography (shapes, sizes, & mineral grain relationships) &
processes that
petrology (formation characteristics) of surface rocks to aid in interpretation of
shaped the
chemical and mineralogical characterization.
surface of Venus
Determine the physical properties and mineralogy of rocks located in a
over time
variety of geologic settings, including meteoritic and crater ejecta, volcanic
flows, aeolian deposits, and trace metals in the high radar reflectivity
highlands.
Characterize surface exposure ages through measurements of weathering
rinds.
Characterize the current structure and evolutionary history of the core.
Place constraints on the mechanisms and rates of recent resurfacing and
volatile release from the interior.
Determine the structure of the crust, as it varies both spatially and with depth,
through measurements of topography and gravity to high resolution.
Assess the
Measure heat flow and surface temperature to constrain the thermal structure
current structure
of the interior.
and dynamics of
Measure the magnetic field below the ionosphere and characterize magnetic
the interior
signature of rocks in multiple locations.
Characterize subsurface layering and geologic contacts to depths up to
several km.
Determine the moment of inertia and characterize spin-axis variations over
time.
42
Pathways for Venus Exploration: 2009
Table 1‐1. Venus and Implications for the Formation of Habitable Worlds Climate Change and the Future of Earth
Venus as a Terrestrial Planet
Goal
Objective
Investigation
Characterize active-volcanic processes such as ground deformation, flow
emplacement, or thermal signatures to constrain sources and sinks of gases
affecting atmospheric evolution.
Characterize the Characterize active-tectonic processes through seismic, ground motion, or
current rates and detailed image analysis.
styles of
Characterize the materials emitted from volcanoes, including lava and gases,
volcanism and
in terms of chemical compositions, chemical species, and mass flux over
tectonism, and
time.
how have they
Characterize stratigraphy of surface units through detailed topography and
varied over time
images.
Assess geomorphological, geochemical, and geophysical evidence of
evolution in volcanic styles.
Characterize the sulfur cycle through measurements of abundances within
the Venus clouds of relevant gaseous and liquid/solid aerosol components
such as SO2, H2O, OCS, CO, and sulfuric acid aerosols (H2SO4).
Determine the mechanisms behind atmospheric loss to space, the current
rate, and its variability with solar activity.
Characterize local vertical winds and turbulence associated with convection
and cloud-formation processes in the middle cloud region, at multiple
locations.
Characterize
Characterize superrotation through measurements of global-horizontal winds
current processes over several Venus days at multiple-vertical levels (day and night) from
in the atmosphere surface to thermosphere.
Investigate the chemical mechanisms for stability of the atmosphere against
photochemical destruction of CO2.
Characterize local and planetary-scale waves, especially gravity waves
generated by underlying topography.
Measure the frequencies and strengths of lightning and determine role of
lightning in generating chemically-active species (e.g., NOx).
Search for and characterize biogenic elements, especially in the clouds.
Determine radiative balance as a function of altitude, latitude, and longitude.
Measure deposition of solar energy in the atmosphere globally.
Characterize the
Determine the size, distribution, shapes, composition, and UV, visible, and IR
Venus
spectra, of aerosols through vertical profiles at several locations.
Greenhouse
Determine vertical-atmospheric temperature profiles and characterize
variability.
Determine isotopic ratios of H/D, 15N/14N, 17O/16O, 18O/16O, 34S/32S 13C/12C in
Determine if there solid samples to place constraints on past habitable environments (including
was ever liquid
oceans).
water on the
surface of Venus Identify and characterize any areas that reflect formation in a geological or
climatological environment significantly different from present day.
Determine abundances and height profiles of reactive atmospheric species
Characterize how (OCS, H S, SO , SO , H SO , S , HCl, HF, SO , ClO and Cl ), greenhouse
2
2
3
2
4
n
3
2
2
the interior,
gases, H2O, and other condensibles, in order to characterize sources of
surface, and
chemical disequilibrium in the atmosphere.
atmosphere
Determine rates of gas exchange between the interior, surface and
interact
atmosphere.
43
Pathways for Venus Exploration: 2009
Goal 1. Origin and Evolution: How did Venus originate and evolve, and what are the
implications for the characteristic lifetimes and conditions of habitable environments on
Venus and similar extrasolar systems?
Goal 1 involves understanding the origin and evolution of Venus, from its formation to
today. Like Earth and Mars, the atmosphere of Venus today seems to have substantially evolved
from its original composition. Whether the major processes that shaped the atmospheres of Earth
and Mars—such as impacts of large bolides and significant solar wind erosion—also occurred on
Venus is largely unknown. Detailed-chemical measurements of the composition of the
atmosphere (in particular, the noble gases and their isotopes) will provide fundamental insight
into the origin and evolution of Venus.
The surface of Venus appears to have been shaped, for the most part, within the geologically
recent past, likely within the past 500 million to one billion years. Venus’ surface may contain
evidence of the planet’s earlier history and origin (which may be accessible through a more
complete characterization of the surface than previously accomplished), as well as a deeper
understanding of the nature and evolution of the interior dynamics. In addition, detailed-chemical
measurements of the composition of the atmosphere (in particular, the noble gases and their
isotopes) provide additional information about the origin and evolution of Venus. Of particular
interest is the possibility that Venus, early in its history, had long-lived oceans and a climate
amenable to the development and evolution of life—possibilities that are not excluded by current
knowledge.
The objectives of Goal 1 are to:
(1) understand the sources of materials that formed Venus and their relationship to the
materials that formed the other terrestrial planets,
(2) understand the processes that subsequently modified the secondary (or original)
atmosphere, leading to the current inventory of atmospheric gases (which is so unlike those
present on Earth), and
(3) determine whether Venus was ever habitable.
Goal 2. Venus as a Terrestrial Planet: What are the processes that have shaped and still
shape the planet?
Although Earth and Venus are ‘twin’ planets in size and mass, Venus’ surface at this time is
clearly hostile to carbon-water-based organisms. Venus’ atmosphere, which is far denser than
Earth’s, is composed mostly of carbon dioxide with abundant sulfur oxides and a significant
deficit of hydrogen. Venus’ atmosphere moves (everywhere except within a few hundred meters
of the surface) with hurricane-force velocities reaching 60 times planetary rotation speed near the
cloud tops. How a planet that revolves more slowly than a normal walking speed can generate
such winds globally is an enigma. Venus’ surface is composed mostly of Earth-like igneous
rocks (basalt) at an average temperature of ~460 ºC, precluding the presence of liquid water.
Venus’ highlands are mantled by deposits of an electrically-conductive or semiconductive
material.
Venus’ geologic processes are also largely dissimilar from those on Earth, aside from
volcanic eruptions. The surface of Venus appears to have been resurfaced within the past 500
44
Pathways for Venus Exploration: 2009
million to one billion years, obscuring possible signatures of earlier geological episodes. The
nature and duration of this resurfacing remain enigmatic. Subsequent to resurfacing, styles of
tectonism and volcanism evolved as the planet cooled, such that the thermal/dynamic regime of
the planet is now thought to be a convection under a stagnant or sluggish lid. There is no
manifestation of the global-plate tectonic processes like those on Earth. Analyses of gravity and
topography data suggest that Venus has a comparable number of active large mantle plumes as
Earth, as well many hundreds of smaller scale plumes that may also be active. Although there is
little information on current levels of volcanic or tectonic activity, some atmospheric data
suggest that Venus is still volcanically active. Exploring and characterizing processes on and in
Venus will help us understand dynamical, chemical, and geologic processes on other planets
throughout our galaxy.
The objectives of Goal 2 are to:
(1) understand what the chemistry and mineralogy of the crust tell us about processes that
shaped the surface of Venus over time,
(2) assess the current structure and dynamics of the interior, and
(3) characterize the current rates and styles of volcanism and tectonism, and how they have
varied over time, and (4) characterize current processes in the atmosphere.
Goal 3. Climate Change and the Future of Earth: What does Venus tell us about the fate of
Earth’s environment?
Although the terrestrial planets formed at about the same time within the inner solar system,
from similar chemical and isotopic reservoirs, they have followed very different evolutionary
paths. In particular, Venus and Earth, which formed at similar distances from the Sun with nearly
identical masses and densities, currently have vastly different atmospheres, surface
environments, and tectonic styles. It has been suggested that Venus may have been more Earthlike earlier in its history and then evolved to its current state, and that Earth may ultimately
transform to a hot, dry, inhospitable planet like Venus. Thus, understanding the interior
dynamics and atmospheric evolution of Venus provides insight into the ultimate fate of Earth.
Objectives within Goal 3 are to:
(1) characterize the present-day greenhouse of Venus,
(2) determine if liquid water ever existed on the surface of Venus, and
(3) characterize how the Venus interior, atmosphere, and surface are interacting.
It has become clear that, as on Earth, the climate balance of Venus reflects a dynamic balance
between geologic and atmospheric processes.
45
Pathways for Venus Exploration: 2009
Courtesy NASA/JPL‐Caltech Courtesy of National Oceanic and Atmospheric Administration 46
Pathways for Venus Exploration: 2009
APPENDIX B. COMPARATIVE CLIMATOLOGY OVERVIEW
Investigating global warming and climate change on Earth has raised consciousness about the
potential instability of terrestrial climate systems and the value of understanding the Venus
greenhouse. A key finding from the 2006 Chapman Conference [5] is that Venus may have had
an ocean and could have been habitable for much of its history. Thus, Venus provides
climatologists with an opportunity to test state-of-the-art models simulating the mechanisms and
processes that led to Venus’ extreme climatology. A new comparative-climatology initiative
cosponsored by NASA’S Earth and planetary science programs would motivate and encourage
work in this increasingly relevant area.
Venus is Earth’s closest planetary neighbor, and a near twin in terms of bulk properties such
as mass and size. Their densities and inventories of carbon and nitrogen are similar, suggesting
similar primordial origins. Mars, Earth’s next nearest neighbor, also has a wide range of
meteorological and geological phenomena that are recognizable as variations on familiar
terrestrial processes. Current understanding of planetary formation, volatile accretion, and the
well-preserved ancient geological record of Mars all suggest that these three planets started out
with comparable surface environments, geological processes, and atmospheric compositions.
Yet, despite their close proximity and similar origins, they have evolved into very different
states. Today’s rotation state, magnetic field, surface temperature and pressure, atmospheric
inventories of radiatively active gasses, total water inventory, polar deposits, and global patterns
of geological activity are among the properties that differ so dramatically.
An understanding of the evolutionary histories and current states of the Venus and Mars
climates is directly relevant for studies of the past and future climates of Earth. As extreme
examples of very different climate on otherwise similar, nearby planets, Venus and Mars provide
opportunities to improve and validate our knowledge of planetary climate modeling. For
example, Venus can provide a test bed for an extreme case of global warming where nonlinear
effects have evidently played an important and irreversible role in climate evolution. Mars has a
climate history where “Milanković cycles on steroids” have resulted in a history of extreme
climate change.
This synergism between Venus, Earth, and Mars goes both ways: Our understanding of
Venus and Mars would benefit from use of the best Earth models and the expertise of the larger
community of Earth climatologists. A much deeper understanding of these very different global
climate systems should be possible, given the techniques developed to understand climate
change on Earth. At the same time, these extreme cases can help to validate the crucial ability of
terrestrial models to correctly predict variations from the current atmospheric composition and
climate conditions of Earth, increase the ability of Earth modelers to work with unforeseen
climate feedbacks, and expose potential weaknesses or limitations in our current Earth climate
models. Although Venus and Mars represent very different evolutionary paths for terrestrial
planets, they provide our closest analogs for many important processes and planetary
mechanisms operating on Earth. This unique combination of similar initial conditions and bulk
properties, with radically divergent evolutionary outcomes, makes Venus-Earth-Mars
comparative studies a uniquely fruitful area for expanding and testing our knowledge of
planetary science and global climate change.
47
Pathways for Venus Exploration: 2009
APPENDIX C. ENHANCING AND ENABLING TECHNOLOGIES
FOR VENUS EXPLORATION
Excerpts from Venus Technologies White Paper [a],
submitted to the NRC Decadal Survey Inner Planets Sub–panel
This appendix provides an overview of technologies required for future Venus exploration
missions. These technologies will focus on mission-enabling and -enhancing capabilities for in
situ missions, because most orbiter-related subsystems are considered heritage technologies. This
appendix draws heavily on the 2008–2009 Venus Flagship Mission study [b] that identified key
technologies required to implement its Design Reference Mission (DRM). These technologies
include surface sample acquisition and handling; mechanical implementation of a rotating
pressure vessel; and a rugged–terrain landing system. Also, a large-scale environmental test
chamber is needed to validate these technologies under relevant Venus–like conditions. Other
longer–term Venus flagship missions will require additional new capabilities; namely, a Venus–
specific radioisotope power system, active refrigeration, high-temperature electronics, and
advanced thermal insulation.
The chosen mission architectures —whether large flagship, medium New Frontiers, or small
Discovery-class missions—are primary drivers for Venus technologies. The Venus Flagship
Mission study [b] recommends a multi–element mission architecture of an orbiter, two cloud–
level balloons, and two short–lived landers, which have been successfully used for past Venus
exploration missions. In addition, the Venus flagship DRM used heritage technologies and, in
turn, minimized the number of new technologies required for this mission’s implementation.
This multi–element architecture also allows designers to utilize appropriate technologies for
smaller (New Frontiers or Discovery-class) missions, which would use similar mission elements.
However, NASA’s SSE Roadmap identified other missions, including the near-surface Venus
Mobile Explorer, a seismic network, a New Frontiers–class VISE, and Discovery-class balloon
missions ultimately leading to a Venus Surface Sample Return mission. Discovery and New
Frontiers missions are not expected to include a significant amount of new technologies and
could be designed without them; although they could benefit from new technologies if they were
made available as part of a technology development program for a future flagship mission.
Table C-1 summarizes enabling and enhancing technologies for potential future Venus
missions, with emphases on in situ elements. Further information can be found in the
“Technologies for Future Venus Exploration” White Paper [a] and in the Venus Flagship
Mission study final report [b].
Appendix C References
a. Balint, T., Cutts, J., et al., “Technologies for Future Venus Exploration,” VEXAG White
Paper to the NRC Decadal Survey Inner Planets Sub-Panel, September 9, 2009.
b. Hall, J.L., Bullock, M., Senske, D.A., Cutts, J.A., Grammie, R., “Venus Flagship Mission
Study: Report of the Venus Science and Technology Definition Team,” National Aeronautics
and Space Administration, Jet Propulsion Laboratory, California Institute of Technology,
Task Order NM0710851, April 17. 2009.
48
Pathways for Venus Exploration: 2009
Table C‐1. Technologies for Future Venus Exploration Legend: Bold italic Bold Italic Roman Capability
The highest‐priority technology items—those that would enable the mission to survive for 5 hours on the surface—as recommended by the Venus Science and Technology Definition Team Technologies that would enhance the DRM by extending its lifetime up to a day are in italics with light red shading New technologies that would extend the lifetime to up to several months are in regular text with light green shading Technologies that would further enhance future Venus exploration missions Current state of the
art (TRL)
Surface
sample
acquisition
and handling
(VDRM)
TRL 2–3
Heritage Soviet–
derived systems are
not available off the
shelf, but they
demonstrate a
feasible approach.
Rotating
pressure
vessel
(VDRM)
TRL 2
Rotating pressure
vessel concept is
powerful but
technologically
immature.
Rugged
terrain
landing
(VDRM)
TRL 2
Russian landers
provide proof of
concept, however,
these landed at
benign surfaces and
used a drag plate
instead of
parachutes.
TRL 2-6
Two small Venus
environment test
chambers are
operational at JPL;
A small Venus test
chamber setup is
underway at GSFC;
Proof of concept
Testing
facility
(VDRM)
Technology development
needs to enable Venus
missions
Surface sample
acquisition system at high
temperature and pressure
conditions;
Vacuum–driven sample
transfer is demonstrated
on Venera, but requires
development for NASA.
Full scale design and
testing of a rotating
pressure vessel with a
driver motor and mounted
sampling system.
Design and test a landing
system that can account
for a large variety of
unknown landing hazards
using parachutes.
Large test chamber
doesn’t exist;
Develop large Venus test
chamber for full scale in
situ elements
(probe/lander) testing;
Simulate transient
atmospheric conditions;
composition.
49
Benefits to future Venus
missions
Drilling, sample collection
and sample handling are
enabling for the Venus
Flagship Mission.
It minimizes the external
components, such as drill
arms, actuators, motors,
sampling systems; and
the heat leakage from the
outside through the
number of windows
required for panoramic
imaging.
Tessera and other rugged
areas on Venus cannot be
reliably accessed unless a
properly engineered
rugged terrain landing
system is developed and
tested.
The 12.5 km anomaly on
the Pioneer–Venus
mission demonstrates the
critical need for an
environmental chamber
using relevant
atmospheric composition
and conditions;
It can test spacecraft
Pathways for Venus Exploration: 2009
Capability
Current state of the
art (TRL)
Technology development
needs to enable Venus
missions
from Russian test
chamber
(decommissioned).
Advanced
passive
thermal
control
(enhancement
to VDRM)
TRL 3–9
Venera and PV era
insulation and phase
change materials are
mostly available.
Alternate insulation and
phase change material
technologies are needed
to increase lander
lifetimes beyond 2–5 hour
operation.
High–T and
Medium–T
components,
sensors, and
electronics
(new
capabilities)
TRL 2–4
Geophones could
operate up to 260°C;
High-temperature
pressure,
temperature,
anemometers used
on Venera/VEGA and
Pioneer–Venus;
Silicon based high–T
components are
designed for up to
350°C for the
automotive and oil
drilling industry;
Limited number of
components and
integrated circuit
capability
demonstrated for SiC
at 500°C;
Limited electronics
packaging at 500°C;
Data storage, ADC,
power converters,
and other needed
components never
demonstrated.
High–temperature MEMS
technology for
seismometers could
operate at surface
temperatures;
SiC and GaN high
temperature sensors and
electronics require
development to operate at
surface temperatures;
Development of data
acquisition, processing
and storage capability,
and packaging;
Development of high–T
power management;
Demonstration of
reliability and long life.
Power
generation
(new
capabilities)
TRL 4
Demonstrated single
Stirling convertor
operation for 300 hours
Cold side temperature must
be raised from 90°C to
480°C with high conversion
efficiency preserved (e.g.,
50
Benefits to future Venus
missions
components; validate and
calibrate science
instruments; test
operating scenarios under
realistic conditions.
Achievement of 12 to 24
hour lander lifetimes
would enable humans–in–
the–loop operation by
ground controllers;
Improved thermal
insulation will decrease
refrigeration requirements
for truly long–term lander
missions.
Long life on the surface is
desirable (especially, for
meteorology,
seismometry);
Sensors, actuators,
instruments directly
interfacing with the
environment cannot be
sufficiently protected, and
therefore, high
temperature components
can enable operations
and science
measurements (e.g., long
lived meteorology,
seismometry) that
otherwise cannot be
achieved;
High temperature data
processing and storage,
and power electronics
results in a drastic
reduction in refrigeration
requirements, even at
moderately high
temperatures (>250°C);
Low power dissipation at
300°C and long life
reduces environmental
tolerance requirements
for components.
Required for long life
operation;
Venus specific RPS with
active cooling could enable
Pathways for Venus Exploration: 2009
Capability
Current state of the
art (TRL)
with a 850°C hot–side
temperature and 90°C
cold–side, 38%
efficiency and 88 W
power output with heat
input equivalent to 1
GPHS module.
Technology development
needs to enable Venus
missions
maintaining ∆T through
increased hot end
temperature, which would
required materials or design
development);
Material testing, system
development and validation
for reliable operation in
Venus surface environment.
Active
refrigeration
(new
capabilities)
TRL 4
Cryocoolers are space
qualified, but high
temperature operation
is not demonstrated at
the system level.
Adopt Stirling conversion
based coolers for Venus
surface conditions;
High efficiency duplex
Stirling system must be
produced that integrates the
heat engine and refrigerator
functions into a high
efficiency and high reliability
device;
Refrigeration system should
be coupled with the power
source;
Low mass and low vibration
is desirable.
Pressure
control
TRL 4–9
Titanium pressure
vessel is space
qualified;
New lightweight
materials need
development.
TRL 4–9
Aerogels, MLI, PCM
are space qualified, but
not for high g–load
entries and high
temperatures.
Advanced materials (e.g.,
beryllium, honeycomb
structures) could reduce
structural mass.
TRL 4
Demonstrated LiAl–
FeS2, Na–S, and Na–
metal chloride
secondary batteries
with specific energy in
Adapt high temperature cell
and battery designs for
space applications;
Address stability of seals
and terminals;
Minimize the corrosion of
Thermal
control
(passive)
Power storage
High performance thermal
insulation for Venus
environment is required for
mission lifetimes beyond
Venera demonstrated
lifetimes.
51
Benefits to future Venus
missions
long lived missions,
operating for months;
Low mass version could
power near surface aerial
mobility systems;
It could power long lived
seismometers and
meteorology stations on the
surface (117 days
minimum).
Almost every long–duration
(~25 hrs+) in situ platform
will require some amount of
refrigeration to survive;
Focus should be on
radioisotope–based duplex
systems that produce both
refrigeration and electrical
power;
Low mass version would
allow for near surface aerial
mobility (metallic bellows);
Low vibration version would
enable a seismic network
(on multiple landers) (117
days minimum);
Extended mission life allows
humans in the loop.
Mass saving translates to
higher payload mass
fraction for the same entry
mass.
Improvements in passive
thermal control could extend
mission lifetime from ~2
hours to 5 hours or maybe
more.
(Beyond that active
refrigeration and a power
source is required.)
High temperature batteries
operating at Venus surface
temperatures would make it
possible to keep the power
storage outside of the
pressure vessel, thus
Pathways for Venus Exploration: 2009
Capability
Instruments (in
situ) for the
Venus
Flagship
Mission
Upper
atmosphere
Balloons
Near surface
balloons
Descent
probes and
sondes
High–T
Telecom
Current state of the
art (TRL)
the 100–200 Wh/kg
range;
Short lived missions
could use high TRL
primary batteries.
TRL 2–9
Descent probe
instrument heritage
from Pioneer–Venus;
New in situ contact
instrument need
development.
TRL 5–7
Russian VEGA
balloons successfully
operated for 48 hrs
over 20 year ago;
Large super–pressure
balloon have been built
and tested at JPL and
at CNES;
Development for a
mid–altitude balloon is
underway at JAXA.
TRL 2–3
Metallic bellows proof–
of–concept was built at
JPL and tested at high
temperatures.
TRL 2–9
Pioneer–Venus probe
heritage for large
probes
Microprobes have
been designed but not
yet tested.
TRL 2
Demonstrated 2 GHz
operation at 275°C
using SiC;
SiC and vacuum tube
based oscillator
Technology development
needs to enable Venus
missions
current collectors at high
temperatures;
Optimize the electrolyte
composition to improve
performance and reliability.
Several Venus Flagship
Mission instruments, e.g.,
heat flux plate, XRD/XRF
are at medium TRL;
High–T seismometry and
high–T meteorology are at
low TRL;
G–load tolerance during
atmospheric entry should
also be addressed.
Cloud level balloons are
considered mature, but
further development, testing,
verification and validation
are required to address
lifetime and reliability issues
for a 30–day mission;
Materials must tolerate high
temperatures, corrosive
environment (sulfuric acid
droplets in clouds).
Benefits to future Venus
missions
reducing volume and
thermal requirements for the
pressure vessel.
In situ instruments are key
drivers for Venus missions
and are required for mission
success.
The Venus Flagship Mission
balloons are designed for
30–days operation;
An ASRG powered balloon
mission could operate for
months, circumnavigating
the planet and continuously
measure dynamics and
atmospheric composition.
Development is needed to
build and test a metallic
bellows system and test it
under Venus surface
pressure and temperature
conditions;
Near surface operation must
address altitude change and
surface access.
Develop small drop sondes
that could be released from
a balloon platform (also work
as ballast).
A near surface mobile
platform could traverse
hundreds of kilometers over
a 90–day mission, image
the surface at high
resolution and periodically
access the surface for
sampling.
Development efforts should
address SiC based RF
components for transmitters;
Miniaturized vacuum tube
technology for power
amplifiers;
High temperature telecom
on the surface would
drastically reduce cooling
requirements;
It would enable long lifetime
(117 days minimum);
52
Drop sondes can enhance
science by providing vertical
slice measurements to
complement balloon
constant altitude
measurements of the
atmosphere.
Pathways for Venus Exploration: 2009
Capability
Current state of the
art (TRL)
demonstrated at
~500°C.
Technology development
needs to enable Venus
missions
SiC based RF components
for transmitters.
Orbiter
instruments
and telecom
TRL 3–9
Magellan, Venus
Express, Pioneer–
Venus heritage;
Venus Flagship
Mission InSAR needs
development.
Development is required for
InSAR; passive infrared and
millimeter spectroscopic
techniques; and cloud
LIDAR
Atmospheric
entry
TRL 5-9
Carbon-Phenolic (CP)
used on Pioneer–
Venus and Galileo
probe; Provides
heritage for use in
steep entry flight path
angle (EFPA)
missions; Special
rayon needed to make
heritage CP; This
rayon is out of
production; Current
arc jet capabilities are
limited; Mars and Titan
TPS, lower density,
could be useful for
lower EFPA.
TRL 4–6
Autonomous operation
have been tested in
previous missions
(e.g., Pioneer–Venus
probes), but at a lower
complexity than
required for a Venus
flagship mission.
See above
1. Re-establish test
capabilities;
Autonomy
Cross cutting
technologies
2. Periodic verification of
Industry capability to
remanufacture heritage CP;
3. Establish alternate to
heritage CP TPS, since
heritage rayon is not made
anywhere, anymore and
current supply in hand is
limited;
4. Assessment of lower
density TPS be performed
for shallow EFPA missions.
Benefits to future Venus
missions
High data rate (~4.5 kbps)
would support seismic
operations;
However, high temperature
data storage at Venus
surface temperature may
represent a significant
technology challenge.
InSAR is a key instruments
on the Venus Flagship
Mission;
Ultra–fine resolution radar
mapping and cloud LIDAR
could provide high
resolution science data on
the surface and clouds, and
highly desirable by science.
TPS is essential and
enabler;
High entry flight path angle
(EFPA) entries result in high
heat flux, pressure and gloads;
Limited supply of heritage
CP enables unrestricted
access to the planet;
Lower density TPS can
provide significant mass
savings, but constrain the
EFPA and thus the mission
architecture.
Develop and test reliable
autonomous operation for a
Venus surface mission,
including control of the
rotating pressure vessel; drill
site selection; sample
acquisition; instrument
operations; reliable telecom.
Short lived missions (up to 5
hours) does not support
humans in the loop;
TPS; pressure vessel
materials; passive thermal
control (insulation; phase
change materials).
These technologies can
benefit a number of
planetary missions, e.g.,
probes to Venus and deep
probes to the Giant Planets.
53
Autonomous operation is
required for all science
measurements and
subsystem control.