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Planetary Decadal Study Community White Paper
Solar System Exploration Survey, 2013-2023
SUBJECT AREA: Outer Planet Satellites
DRAFT DATE: 06/##/2009
Io: Future Exploration for 2013-2023
and Beyond
David A. Williams (School of Earth and Space Exploration, Arizona State University)
David.Williams@asu.edu
Jani Radebaugh (Brigham Young University)
Rosaly M.C. Lopes (NASA Jet Propulsion Laboratory, California Institute of Technology)
Imke de Pater (University of California, Berkeley)
Nicholas M. Schneider (University of Colorado, Boulder)
Frank Marchis (University of California, Berkeley, and the SETI Institute)
Julianne Moses (Lunar and Planetary Institute)
Ashley G. Davies (NASA Jet Propulsion Laboratory, California Institute of Technology)
Jason Perry (Lunar and Planetary Laboratory, University of Arizona)
Jeffrey S. Kargel (Department of Hydrology and Water Resources, University of Arizona)
Laszlo P. Keszthelyi (Astrogeology Science Center, U.S. Geological Survey, Flagstaff)
Coauthor (Affiliation)
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Executive Summary
TBD
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REPORT
I. IO SHOULD BE A PRIORITY FOR FUTURE EXPLORATION
Io is one of the most intriguing bodies in the Solar System, for intrinsic and "extrinsic" reasons.
Intrinsically, there is much to be learned from the only known body to exhibit ongoing volcanic,
tectonic, and gradational processes operating at rates comparable to (or exceeding) those on the
Earth. The extrinsic interest comes from the fact that that Io provides insight into wider issues in
planetary science.
Io's Intrinsic Interest
Jupiter’s satellite Io is the most geologically dynamic solid body in the Solar System. Io
undergoes severe tidal heating, induced by the orbital eccentricity forced by Jupiter and the 4:2:1
Laplace resonance between Io, Europa, and Ganymede. Global heat flow is estimated at >3 W m2
, compared to the 0.06 W m-2 average for Earth. This one remarkable number presages a huge
variety of interconnected phenomena operating at a scale not seen active anywhere elsewhere in
our Solar System. Io offers an extremely rich array of geophysical, geological, geochemical,
atmospheric, and magnetospheric plasma phenomena. Diurnal tidal flexing amplitudes may
reach ~100 meters, dissipating huge amounts of heat in the interior, although the sites and
mechanisms of the dissipation are still unidentified. Lavas of poorly known composition
inundate the surface, while volcanic plumes feed a tenuous, inhomogeneous atmosphere
controlled by a combination of volcanic sources and surface frost sinks. Complementing Io’s
pervasive volcanic landforms, the surface is studded with some of the Solar System's highest and
most dramatic mountains, and by scarps of both tectonic and erosional origin.
Most remarkably, geology can be observed as it happens on Io, and thus we are not restricted, as
on other planetary bodies, to reconstructing geological processes from their long-dead remains.
These processes occur at a variety of time-scales that make changes observable by a near-by
spacecraft. We can watch erupting plumes cover the surface with pyroclastic debris, as observed
at the Tvashtar volcano during the February 2007 flyby of NASA’s New Horizons spacecraft.
We can observe lava flows advance across the surface, as was done (in a limited fashion) by
NASA’s Galileo spacecraft between 1999-2001. We can track chemical changes as surface
materials anneal over time, and with better temporal and spatial coverage, we may also be able to
watch tectonic and erosional processes at work.
Io also exerts the dominant influence on the Jovian magnetosphere, filling it with a dense plasma
of heavy ions, neutral atoms, and dust grains. The processes by which these materials leave Io
and subsequently evolve are complex and still poorly understood. Io is so energetic that many Iorelated processes are observable from Earth, including thermal emission from its volcanoes,
large-scale albedo changes from volcanic plume activity, temporal changes in its atmosphere,
and the ionized and neutral clouds that fill the magnetosphere. Thus, much can be learned
relatively inexpensively from Earth’s surface or from Earth orbit.
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Finally, as one of the most spectacular places in the Solar System, Io has unique public appeal,
and Io exploration offers many opportunities to attract and engage public interest in planetary
science.
Extrinsic Interest: History and Mechanics of Tidal Heating
Io is the body for studying tidal heating, a process that is fundamental to the evolution of giant
planet satellite systems, and one that may greatly expand the habitability zone for extraterrestrial
life in the study of extrasolar planets. By understanding Io, where manifestations of tidal heating
are displayed to extremes and are easily observed, we can better understand the importance of
tidal heating in a wide range of circumstances, including the history of satellites in our own Solar
System, to the possibility of life-sustaining tidal heating elsewhere in the universe. Io is directly
linked to the evolution of the one of most promising sites for extraterrestrial life, Europa (the
primary target of the next NASA Outer Planets Flagship-class mission), via the coupled orbital
evolution of the two bodies. Jovian tidal forces act more strongly on Io than on Europa, but the
Laplace resonance between the bodies helps transfer energy to Europa. Io's current heat flow
appears to be at least a factor of two higher than sustainable by steady-state orbital evolution,
suggesting that time-variable orbital and thermal evolution of Io is likely, if not required. If true,
this would imply time-variable tidal heating of Europa as well, with obvious implications for the
sustainability of its putative subsurface ocean. Indeed, an Io-dedicated mission that operates in
the 2013-2023 timeframe could identify key parameters involved in tidal heating that can be
further investigated by the NASA-ESA Europa-Jupiter System Mission when it enters the Jovian
system in 2025.
Extrinsic Interest: What can we learn from Io about Earth History and Solar System
Volcanism?
Volcanism is a fundamental geologic process in planetary evolution, providing communication
between the planet's interior and exterior. Volcanoes are thought to have a major role in bringing
much of the Earth's volatile inventory to the surface, supplying the modern ocean and
atmosphere. Much of the Earth's crust is made of volcanic and plutonic rocks. Studies of Io's
volcanism can thus help us to understand terrestrial volcanism and crustal evolution.
Io's extreme heat flow is similar to that experienced by the Earth shortly after its formation, at
the time life began, and thus Io provides a window into the Earth's formative years. This analogy
became clear when the Galileo spacecraft detected that at least some of Io's lavas appear to have
an unusually high eruption temperature, and may be similar to the high-temperature komatiite
lavas that were common in the Earth’s Archean Eon (3.85-2.5 Ga), but have been extremely rare
in the past billion years. The present-day Earth contains several types of large igneous provinces
such as flood basalts, which are manifestations of large-scale volcanism that (fortunately) have
not occurred in historical times. Such eruptions can have devastating regional and even global
effects, and have been implicated in mass extinctions. Eruptions on this scale occur much more
frequently on Io than on Earth, and are thus available for direct study. For instance, Io appears to
show examples of large, compound lava flow fields that grow slowly by inflation (at e.g.,
Prometheus Patera), as well as rapidly-emplaced lava flow fields of comparable size resulting
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from short-lived “outbursts” (at e.g., Pillan Patera), as well as periodically overturning lava lakes
(at e.g., Loki Patera).
The other terrestrial planets, particularly the Moon, Mars, and Venus, have undergone similar
massive volcanic eruptions in the past, so Io’s modern-day eruptions will teach us much about
the history of these bodies. In particular, analogies to the Moon, and possibly Mercury, because
they are also airless, may be particularly instructive.
Extrinsic Interest: Atmosphere
Io’s atmosphere is different from any other in the Solar System. It has a central role in both the
Jovian magnetosphere (as the buffer of escaping material) and in shaping Io’s surface (through
frost condensation). It provides a fundamentally contrasting case study for several reasons. First,
because Io’s atmosphere is so closely coupled to its surface temperature via vapor pressure
equilibrium (VPE) of SO2 gas, and because the VPE of SO2 varies by at least 6 orders of
magnitude for the various temperatures appropriate for Io’s surface (i.e., day, night, volcanic hot
spots), Io’s atmosphere provides access to a temperature-pressure regime unlike any other
atmosphere known. Strong horizontal winds that reach hundreds of meters per second resulting
from volcanic plume collapse carry SO2 towards regions of lower pressure. Vertical transport is
also rapid, so that volcanic material reaches the exobase almost immediately. Sulfur dioxide
vapor condensation and sublimation cycles are important and contribute to the heterogeneous
distribution of surface materials and surface spectral reflectance of Io. From Io neutral cloud and
torus observations, and from thermodynamic models of high-temperature magma-vapor
equilibrium, we know that other volatile constituents are also involved in plume emissions and
probably also are included in various volatile cycles on Io, but these are poorly understood.
Many volcanic plumes exceed the scale height of the atmosphere. Second, unlike other
atmospheres, Io’s position deep within the Jovian magnetosphere, surrounded by a dense corotating plasma torus of heavy ions, leads to a wide variety of non-thermal processes involving
charged particles that affect the surface and atmosphere, and ultimately nearly every other aspect
of the Jovian magnetosphere, including the other satellites. These conditions challenge, and
therefore ultimately extend, our ability to understand basic plasma and atmospheric physics.
Extrinsic Interest: Magnetospheric interaction
The interactions of particles within Io’s magnetosphere provide an excellent opportunity to study
basic physical problems of general interest in space plasma physics. Io's plasma torus is a
prominent example of a mass loaded plasma field. Mass loading occurs when ionized material is
introduced into a fast-flowing magnetized plasma. Pickup ions created from Io's atmosphere and
extended neutral clouds generate field-aligned currents that transfer momentum from the ambient
medium to the pickup ions via Alfven waves. Other examples of mass loaded plasmas include
solar wind flow by comets, planets, moons and other solar system objects. Io also represents a
dramatic example of electrodynamic interaction. The plasma flow around Io and its conducting
atmosphere generates a Mega-ampere current across Io that closes along the Jovian magnetic
field lines. These field-aligned currents are propagated toward Jupiter via Alfven waves and
couple Io to Jupiter's ionosphere, resulting in an Ionian magnetospheric footprint in the Jovian
polar regions.
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The coupling between distinct plasma populations (i.e., the Io plasma torus and Jupiter's
ionosphere) is a fundamental and unresolved problem in space physics. In many cases, coupling
is imperfect due to an electric field component parallel to the ambient magnetic field. Parallel
electric fields inhibit the propagation of the momentum-transferring Alfven wave and a magnetic
decoupling (or violation of the frozen-in condition) results. Perhaps the most familiar example of
a decoupled system is the Earth's magnetosphere-ionosphere system where the decoupling is
manifested by electron acceleration and the formation of discrete aurorae. Likewise, the IoJupiter system is at least partly decoupled by parallel electric fields that are manifested by
auroral emissions in Jupiter's atmosphere at the base of Io's flux tube. The persistence of auroral
emissions wakeward of Io's flux tube suggests that decoupling persists for a significant period of
time following the initial interaction of a given flux tube with Io. Io's magnetospheric interaction
is therefore highly appealing for studying the general problem of coupling and electron
acceleration processes.
More parochially, Iogenic plasma completely dominates the Jovian magnetosphere, and via
sputtering and implantation, has a major influence on the surfaces of the other Galilean satellites.
Io creates the plasma environment of Europa, which both creates Europa's atmosphere and
causes it to radiate. Io thus influences the biohazard of hard radiation at Europa. It will not be
possible to fully interpret results from the EJSM without a good understanding of Io and its
various influences on Europa. It is even possible that Iogenic plasma provides a source of
chemical energy for possible Europan life.
Finally, the unique spectral signatures and large spatial scale of emission from a plasma-rich
planetary magnetosphere provides a possible means of detecting and characterizing extrasolar
planetary systems, increasing the importance of understanding the physics of such systems.
II. SCIENCE GOALS FOR IO
Despite visitation of Io by the Voyager (1979; 1980), Galileo (1995 – 2003), Cassini (2000), and
New Horizons (2007) spacecraft, and ongoing ground-based monitoring, there are many
important outstanding questions about Io. Various concept studies for Europa orbiter and Jovian
system missions during the past decade have resulted in a refined set of science goals for future
observations of Io by spacecraft. These include understanding the interior, surface, atmosphere,
and magnetosphere. Below we list the fundamental science goals in bullet form, then discuss in
detail the observations and measurements required to advance our understanding of Io:
1. Understand Io’s heat balance and spatial and temporal distribution of tidal dissipation,
including implications for Europa
2. Understand Io's deep interior structure, especially melt fraction of the mantle, by
characterizing Io’s shape, gravity field, chemistry, and heat transport
3. Understand the eruption mechanisms for Io’s lavas and plumes, and their implications for
volcanic processes on Earth and the other terrestrial planets
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4. Investigate the nature of the lithosphere, especially the processes that form Io’s mountains,
and implications for tectonics under high heat flow conditions that may have existed early in the
history of other planets
5. Understand Io’s surface processes, including the relationships among volcanism, tectonism,
erosion, and depositional processes
6. Understand Io’s surface chemistry, including volatiles and silicates, and implications for
crustal differentiation and mass loss
7. Understand Io’s atmosphere and ionosphere, the dominant mechanisms of mass loss, and the
connection to Io’s volcanism
8. Determine whether Io has a magnetic field and implications for the state of Io’s large core
9. Understand the linkage between Io, the Jovian magnetic field, and the plasma torus and neutral
clouds
Interior Composition and structure
It is clear, based on measurements of the spherical harmonic degree 2 gravity coefficients, that Io
has a metallic core. The size of this core is between about 550 km and 900 km assuming an FeFeS composition, or between 350 and 650 km assuming an Fe composition. The size of the core
is uncertain mainly because we do not know its chemical composition. Io's core is thought to
contain an unknown amount of sulfur; the larger the sulfur content is, the larger Io's core is. An
estimation of the size of Io's core would fix its density and by inference lead to a determination
of core composition, i.e., core sulfur content. Another basic unknown about Io's core is its
physical state, i.e., liquid or solid or partially molten. Evidence from limited Galileo spacecraft
flybys suggests that Io does not have an intrinsic magnetic field, and additional knowledge of the
physical state of the core would enable understanding of why Io has no magnetic field.
Furthermore, an understanding of Io's apparent lack of a magnetic field will contribute
importantly to our general understanding of dynamo action in the cores of Earth and other
terrestrial planets.
The state of Io's mantle is also unclear. Eruption temperatures estimated from Galileo data would
indicate that the mantle is largely molten. But such high degrees of melting would not allow
sufficient tidal dissipation to explain the observed heat flow. One possibility is that Io's mantle is
currently much hotter than an equilibrium model would allow. Alternatively, the magma may
become superheated as it rises through the lithosphere, or the temperature estimates are incorrect.
All three of these possibilities are possible and additional data is needed to improve our current
understanding.
For example, additional gravity data, together with topography data, would tell us about density
anomalies in Io's mantle and the thickness and variations in thickness of Io's crust, and would
constrain crustal density and the degree of isostatic compensation, and thus provide information
on crustal chemistry and the degree of magmatic differentiation, if any. Such data would also
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inform about any anomalous mass concentrations at or near Io's surface associated with lava
flows and magma chambers, for example.
Heat Flow
Io’s heat flow is so prodigious that it can be measured remotely, by quantifying Io’s emitted
infrared radiation. Recent estimates, using a combination of spacecraft and ground-based
observations, have been in the range ≥3 W m-2, whereas steady-state tidal heating rates, as
constrained by historical observations of Io’s orbital evolution, suggest heat flow should be ≤1
W m-2. Possible solutions to this discrepancy include coupled variations in Io’s heat flow,
internal rheology variations, and orbital elements (that would necessarily also involve Europa),
or episodic release of heat despite constant tidal input, due to convective instabilities. Despite
estimates derived from existing Galileo and telescopic data, it is still possible that these heat flow
estimates are incorrect, and more precise estimates are sorely needed in order to address these
questions. In addition, spatial variations in Io’s heat flow provide important clues to its internal
state and the site of tidal dissipation: dissipation in the deep mantle will produce excess heat flow
at the poles, while shallow asthenospheric dissipation will concentrate heat flow at the equator.
Thus, to improve our understanding of Io’s heat flow, we require accurate measurements of the
spatial distribution of Io’s total thermal radiation at all wavelengths and in all directions, and
accurate measurements of its bolometric albedo (that controls the input of solar heat).
Surface chemistry
Io's surface composition remains mysterious, despite multiple spacecraft flybys and more than 70
years of photometric, calorimetric, and spectral observations. Sulfur dioxide frost, with its many
diagnostic infrared absorption features, is the only constituent that has been definitively
identified on Io's surface. Less conclusive evidence as to the identity of the other surface
constituents are provided by (1) the satellite's overall high (and bland) reflectance in the visible
and near-infrared, (2) its strong absorption in the ultraviolet, (3) the identification of atomic and
ionized S, O, Na, Cl, and K in the Io torus and neutral clouds, (4) active volcanism across the
satellite, (5) the variety of colors (yellow, red, orange, black, green) observed in different regions
across the surface, (6) the high temperatures (indicative of mafic to ultramafic silicates) in many
active volcanic centers on Io, and (7) the high bulk density of the satellite (suggesting a silicate
composition for the whole satellite). Elemental cyclo-octal sulfur (S8) has long been a contender
as a dominant surface constituent based on points (1), (2), (3), (4), and (5) above (as have S 2O
and polysulfur oxides). Surprisingly, Io's surface exhibits few spectral features indicative of
silicates, other than a 0.9-μm absorption feature tentatively identified as a Mg-rich silicate (e.g.,
pyroxene) observed in some of the dark spots across Io. The only other infrared spectral features
not linked to SO2 include an absorption band at 3.915 mm that has been tentatively identified as
pure solid H2S or Cl2SO2 diluted in solid SO2, and an enigmatic broad 1-1.5 mm band.
Candidates for the red plume deposits and other red materials include S3-S4, S2O, Cl2S, and
elemental sulfur contaminated by As, Se, or Te. The dominant Na-, Cl-, and K-bearing surface
constituents are still uncertain, although NaCl, KCl, and perhaps alkali sulfides are favored by
models of volcanic outgassing. Many outstanding questions about Io's surface composition
remain. What causes Io's latitudinal albedo variations (e.g., darker and redder poles)? What is
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responsible for Io's complex surface color patterns (e.g., multicolored flows and plume
deposits)? Answers to these questions must await future Io missions with orbital spectrometers
covering the visible and infrared at higher spectral and spatial resolution than earlier data sets.
Tectonic and Surface Processes
Io provides a laboratory for understanding surface processes on bodies without atmospheres, and
in particular for understanding active tectonic processes on a body without Earth-like plate
tectonics. Thus Io is a living laboratory to assess the processes that were active on the Moon,
Mars, Venus, and perhaps the early Earth.
The towering mountains of Io are not volcanoes. Instead, they are tectonic massifs that dwarf Mt.
Everest. However, unlike the Himalayas on Earth, they are largely found in isolated structures
rather than mountain belts. Only a few mountains have been imaged at moderate (<500 m/pixel)
resolution and even fewer have topographic data (from stereo imaging). The few very-highresolution images of Io show a myriad of baffling features, including dune-like “ripples” in
plains material, gullies and fretted terrain suggestive of sapping, and giant lobes indicative of
mass movement off of mountains.
Specific questions important to understanding tectonic processes: 1) What is the thickness and
composition of the crust? 2) What creates and destroys Io’s mountains? (A plausible recent
theory for mountain formation, that they result from crustal compression caused by resurfacing,
needs to be tested by looking for thrust faults, and alternatives explored), and 3) How do
tectonism and volcanism interact? (e.g., are any of the ~500 large paterae on the surface strictly
tectonic in origin?) To answer these questions, we need global panchromatic imaging at ~100
m/pixel, along with topography and ≥4-color data at a similar resolution. These various types of
observations need to be obtained in conjunction, over an extended period of time, so the
processes and the timescales over which they occur can be observed.
Volcanism
Io is the only place beyond the Earth where active silicate volcanism can be studied. A wide
range of Earth-based instruments and spacecraft (e.g., Voyager, Galileo, Hubble Space
Telescope, Cassini, New Horizons) have been used to study the nature of Io's volcanic processes.
Much has been learned, but after three decades of study, new questions with broad implications
have arisen, including:
1. How does volcanism operate in extreme environments (i.e., no plate tectonics, no
atmosphere, low gravity)? Io is the only place in the solar system where we can
observationally test physical models of silicate volcanism that can be applied to Venus,
the Moon, Mars, and asteroids. The key to answering this question is to separate the
influences of magma composition and volatiles, tectono-physical processes (e.g., tidallyinduced superheating, mantle plume ascent), and local environmental conditions. Such
studies will improve not only our understanding of Ionian volcanism but also of planetary
volcanism in general.
2. What is the compositional range of Io’s magmas? High magma temperatures suggest
widespread mafic to ultramafic eruptions, but sulfurous and perhaps sulfur dioxide
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effusive eruptions may also occur. We need to determine magma and gas compositions
via remote sensing and in situ studies, in order to create more realistic physical and
chemical models of eruption processes. Magma chemistry also provides a vital window
on Io’s interior composition and structure.
3. How does volcanism change as the scale of the eruption increases beyond that seen in
historical eruptions on Earth? The key to answering this question is to frequently monitor
active Ionian volcanoes at all timescales (seconds, minutes, hours, days, months, years),
which enables identification of eruption size and style from visible imaging and thermal
emission analyses, and to compare resulting volcanic eruptions, their products, and
morphologies with those on Earth and inferred for the other planets.
4. What is the full range of Io’s volcanic processes? Large-scale eruptions are easily
identified and quantified, but the smallest thermal sources identified in Galileo NIMS and
SSI data are still larger than most current volcanic thermal sources on Earth. Is there a
cutoff, and if so, where is it? What, therefore, is the contribution to Io’s heat flow from
hot spots yet to be discovered? A new mission to Io will be the first with instruments
designed expressly to investigate the full range of volcanic activity found there.
Surface Age and Cratering Timescales
While it is obvious that Io is being resurfaced rapidly (e.g., there are no impact craters visible on
Io’s surface in any image at any resolution), the actual resurfacing rate is not well known. Many
of the large plume deposits are ephemeral, and large areas of the surface remain unchanged since
Voyager. Heat flow considerations suggest a mean resurfacing rate of ~1 cm/year, but much of
this may be repeated resurfacing of patera floors. It is possible that much of Io is resurfaced at a
rate of less than 1 mm/year, allowing the possible survival of a few impact craters that could be
detected with sufficient high-resolution imaging coverage. If these craters were found,
comparison of crater densities with independent estimates of resurfacing rates derived from
techniques such as plume particle size analyses and lava flow thickness measurements would
provide a valuable independent estimate of cratering timescales on the Galilean satellites,
improving age estimates for the surfaces of the other outer planet satellites. Better constraining
the cratering rate on Io would also help in providing an estimate for the transfer rate of Ionian
material to Europa’s icy surface.
Atmosphere
Existing observations only weakly constrain Io’s atmospheric structure, but suggest that it is
controlled by a combination of volcanic and sublimation processes. Models suggest that it is the
only solar system atmosphere with orders of magnitude spatial variation in surface pressure,
producing trans-sonic winds and bulk atmospheric composition that may be completely different
between high and low pressure regions. SO2 in the sublimation component of the atmosphere
condenses at night, whereas the volcanic component is diurnally constant. Lyman-alpha images
of the atmosphere also indicate that it condenses out at the poles. Atmospheric vertical structure
is unusual, being probably dominated by electrical and plasma impact heating.
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In many ways Io’s atmosphere is the “missing link” between its surface and magnetosphere.
Because the observational constraints are so poor, many major unsolved questions persist, whose
answers require new observations. Such questions include:
1. What is the main immediate atmospheric source, volcanism or recycled surface frost, and
what is the dominant volcanic effluent gas?
2. What gases other than SO2, SO, and S2 are present in the high- and low-pressure regions,
and how do they interact?
3. By what mechanisms are the various visible and UV emissions excited?
4. How is the ionosphere maintained and is it global or local?
5. Do particulate aerosols, visible in the plumes, interact with the atmosphere and provide
less volatile constituents such as Na and Cl to the atmosphere and magnetosphere? Are
the plumes the main source of Io’s dust streams?
6. Is the upper atmosphere greatly expanded by the magnetosphere-atmosphere
electrodynamic interaction, as predicted by models?
7. Are there local high-current electrodynamical interactions at the plumes, and if so, do
they affect the plumes or atmospheric mass loss?
8. How does SO2 that condenses out at the poles get recycled back into the atmospheric
circulation? Why are there no extensive polar caps?
Mass Loss and the Torus
While the Io plasma torus has been observed from the Earth’s surface, Earth orbit and by
spacecraft flybys, there remain several critical issues relating to the production, transport and
loss of plasma mass and energy. Different measurements are consistent with a production rate on
the order of 1 ton per second of iogenic plasma, but we do not understand the detailed
mechanism of the near-Io source (e.g., ionization by the impacting plasma, stripping of an
ionosphere, ionization of a sputtered corona, etc.) nor the relative importance of localized
ionization vs. ionization of an extended neutral cloud. Moreover, it is expected that highly
variable volcanism on Io would lead to dramatic changes in its atmosphere, neutral corona and,
hence, ion production rate.
The torus is emitting several terrawatts of energy in the form of EUV emissions. This energy
ultimately comes from Jupiter's rotation, coupled by the magnetic field to torus plasma but the
mechanisms that transfer energy within and between species are poorly understood. Possible
heating mechanisms are ionization, charge exchange, wave-particle interactions and acceleration
by global electric fields.
To first order the plasma is confined and coupled to the planet's 10-hour rotation by Jupiter's
strong magnetic field. But we also know that over time scales of days the plasma is transported
radially outwards (either by global or microscale processes) and the plasma lags a few percent
behind corotation. Whereas these dynamical processes are telling us that the magnetospheric
plasma gets decoupled from the Jupiter fly-wheel, we cannot describe quantitatively the causes
or mechanism(s).
To address issues of plasma production, energization and transport we need to monitor variations
in Io's atmosphere while simultaneously monitoring changes in torus properties. The
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mechanisms enabling energy to flow within and between species requires in situ measurements
of the ion and electron 3-d velocity distributions (particularly separation of the dominant ion
species O+ and S++ which have the same M/Q=16). To understand the coupling between the
torus plasma and Jupiter's rotation we need to measure the heretofore unexplored high latitude
regions between the torus and the planet with polar orbiting spacecraft, such as NASA’s
upcoming Juno mission.
III. HOW WE CAN MAKE PROGRESS
It is important to avoid becoming so focused on missions narrowly dedicated to the search for
life that we lose the chance for the broad new perspectives and serendipitous discoveries that are
likely to come from a more balanced exploration program. Many of the major breakthroughs in
our understanding of life’s place in the universe have come from missions that were not
primarily looking for life or organic chemistry; e.g., our understanding of the history and
importance of planetary impacts derived from studies of our dead moon, the discovery of
outflow channels on Mars by Mariner 9, the discovery of the importance of tidal heating by
Voyager 1 at Io, the discovery of circumstellar dusk disks by IRAS, and so on. Thus, we urge
that NASA pursue a balanced solar system exploration program between life-focused and
general exploration missions.
Missions for 2013-2023
Many of Io’s primary science questions can only be answered by Jupiter- or Io-orbiting craft
making high-resolution observations. From Earth’s surface or even Earth orbit we are unlikely to
achieve spatial resolution better than many tens of km, and we cannot make observations of Io’s
poles or night hemisphere or obtain the detailed compositional analysis possible with in situ
observations. Previous missions that have encountered Io were not equipped with instruments
designed to observe the unexpected, almost unlikely, volcanic processes taking place.
Understanding the subtleties of the evolution of Io, and therefore the Jovian system, requires that
appropriate observations with a carefully crafted instrument payload designed around the
extremities of the broad range of Io’s volcanic activity.
Although Io orbiters have been proposed in the past, e.g., in the 1996 and 1999 Roadmap studies,
the radiation hardness and delta-V issues that have emerged from studies of various Europa
orbiter concepts (including the recent joint NASA-ESA Europa-Jupiter System Mission study)
make an Io orbiter seem very difficult to accomplish with technology available in the decade of
2013-2023. Both the delta-V and radiation issues are even more severe at Io than at Europa. At
the same time, Io’s dynamism is not well suited to study from a one-shot flyby. The next mission
to study Io in detail is thus likely to do so from Jovian orbit. Of course, a Jovi-centric orbit also
allows studies of the other Galilean satellites, and would superficially resemble NASA’s Galileo
orbiter. However, a Jupiter-orbiting ‘Io Observer’ could accomplish much more for less cost due
to advances in instrument capabilities and miniaturization over the 30 years since Galileo was
designed, and the likely 100-fold improvement in data return possible with a functioning highgain antenna. For the Io White Paper prepared for the previous Planetary Science Decadal
Survey, the following Strawman mission concept for an Io-dedicated mission was suggested:
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Orbit: Jovi-centric, eccentric, period roughly 1 month, perijove near Io.
Duration: Galileo survived seven Io flybys, with a radiation dose of roughly 40 krad
each. A four-year mission with 50 monthly Io flybys would accumulate only half the 4
Mrad radiation dose expected for the Jupiter Europa Orbiter (JEO).
Payload: 1000-3000 Ǻ UV spectrometer for atmospheric studies, with solar occultation
capability; high-resolution multicolor visible imager (1-10 m/pixel samples, 100 m/pixel
global); 1-5 μm near-IR spectrometer with 1 km spatial resolution; 10, 20 μm thermal-IR
imager with 10 km spatial resolution; laser altimeter; mass spectrometer; plasma package.
Possibly, a pair of penetrators with 20-hour lifetimes (comparable to DS2), each
including a seismometer, atmospheric mass spectrometer, surface composition package,
and possible ranging capability.
Orbiter Operations: Repeated flybys of the same hemisphere of Io, with similar lighting
geometry, emphasizing studies of time variability. Active regions found in early in the
mission would become the focus of intensive repeated study in later flybys. The
remainder of the orbit would be used for both data playback and distant studies of Io,
using either a scan platform, simple mirror system, or frequent attitude maneuvers.
Penetrator Operations: Penetrators would be released after impact sites had been
chosen on early orbits, and will require retro-rockets to reduce impact speed. Mass specs.
would determine atmospheric composition during entry, and surface composition could
also be determined. Io is likely to be so seismically active that 20 hours of simultaneous
coverage from three stations may be sufficient to map internal structure, though studies
are needed to confirm this. Low-frequency seismometers would allow tidal flexing to be
measured directly.
This Strawman mission, an Io-dedicated, Jovian orbiter with penetrators, clearly falls within the
Flagship-class (large) mission category. However, it is not feasible because the next Outer
Planets Flagship, the joint NASA-ESA Europa-Jupiter System Mission (EJSM), was selected in
February 2009 for development in the 2013-2023 decade (ideal launch scheduled in 2020).
Furthermore, a Titan-Saturn System Mission is a more likely candidate for the following Outer
Planets Flagship in the 2023-2033 decade. Although considerable Io observations are planned by
EJSM between 2025-2028, we believe that a better goal for the next Decadal Survey is to
support a more modest ‘Io Observer’ mission of the Discovery-class (small-sized) or New
Frontiers-class (medium-sized) in the next decade.
In 2008, NASA requested mission proposals for a Discovery-class mission enhanced by two
Advanced Stirling Radioisotope Thermoelectric Generators (ASRGs) provided as governmentfunded equipment. A candidate ‘Io Observer’ mission, called “Io Volcano Observer (IVO)”, was
proposed and is currently under study for the next Discovery opportunity (AO release in late
2009). The IVO mission concept is very similar to that of the Io Strawman mission given above.
It would include a single Jovian orbiter (JOI in 2021) with 7 Io flybys over 16 months, with an
extended mission option with additional Io flybys during a 1-year period. Flybys would range
from closest approach distances of 1000 to 200 km, with ~20 Gb of science data near Io returned
per flyby. The science payload would include a Radiation-hard Narrow-Angle Camera (10
mrad/pixel, 15 bandpasses from 200-1100 nm), Thermal Mapper (125 mrad/pixel, 10 bandpasses
for thermal mapping and silicate compositions), Ion and Neutral Mass Spectrometer (mass range
1-1000 amu/q, M/DM ranges from 300-1000), two Fluxgate Magnetometers (sensitivity 0.01
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nT), with payload enhancement options for a second INMS, a wide-angle camera, a near-IR
spectrometer, or a dust detector. We support the IVO mission as a candidate for a Discoveryclass ‘Io Observer’ mission, consistent with previous Decadal Survey and current Io science
goals.
A 2008 NRC report (“Opening New Frontiers in Space: Choices for the Next New Frontiers
Announcement of Opportunity”) now supports an ‘Io Observer’ as a first-tier option for the
NASA New Frontiers-class (medium-sized) missions during the next decade. Unfortunately, the
2009 New Frontiers-3 Announcement of Opportunity (Release date: April 2009) does not
include mission concepts including Radioisotope Power Systems (RPS). It is commonly agreed
that an ‘Io Observer’ mission requires RPS to justify mission costs and accomplish significant,
useful Io science. Nevertheless, future New Frontiers Announcements of Opportunity for later in
the 2013-2023 decade are anticipated to include mission concepts with RPS. Thus, we advocate
for New Frontiers mission concepts for a ‘Io Observer’ mission later this decade, consistent with
previous Decadal Survey and current Io science goals.
Missions for Beyond 2023
Whereas a Jovi-centric ‘Io Observer’ mission of either Discovery-class or New Frontiers-class
could go a long way towards answering some of the questions posed above, an Io orbiter could
accomplish even more. Laser altimetry from an Io orbiter would provide regional topographic
information and lava flow thicknesses, but even from Jovicentric orbit it conceivably could
measure tidal flexing. Such measurements should be attempted during at least one Io encounter
by the Jupiter Europa Orbiter (JEO) in the late 2020s. Spectroscopy of fresh lavas, and direct
sampling and UV spectroscopy of atmospheric gases would constrain interior composition and
differentiation. Repeated wide-area imaging at uniform illumination conditions and at consistent
spatial resolution, multicolor photometry of plume dust, and laser altimetry would constrain
resurfacing rates, and repeated detailed observations of active volcanic centers would reveal the
true nature of Io’s surface features, as well as eruption mechanisms. Mid-IR mapping would
determine global heat flow and its spatial distribution, thus constraining dissipation mechanisms.
UV and mass spectroscopic and solar occultation mapping would reveal the spatial distribution
and dynamics of the atmosphere, and its sources and sinks. Thus, we suggest that an Io orbiter be
considered as a mission concept in the future, pending results from any future jovi-centric ‘Io
Observers’.
Eventually, in situ measurements may be required to understand the full range of compositions
and processes active on Io’s surface and interior. For example, penetrator seismometers would
reveal Io’s interior structure, and by measuring the core size could distinguish between Fe and
FeS core composition. Deploying penetrators with a surface composition package in Io’s three
types of plains material (yellow, white, and red-brown), and/or in the dark diffuse material or on
Io’s dark and bright flow materials, would unambiguously reveal the compositions of Io’s
eruptive products, provide in situ temperature measurements, and consequently lead to
constraints on internal processes. Alternatively, it may be more cost effective to deploy a rover
rather than 3-4 penetrators, at a location where all of Io’s various surface materials are in close
proximity. Continued technology developments in RPS, radiation-hardening, and optical
communications over the next 15 years suggest that active Io in situ equipment could become a
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reality. Thus, we suggest that in situ Io missions, perhaps penetrators, landers, or rovers, be
considered as mission concepts in the future after ‘Io Observer’ missions.
Space-Based Telescopes
Space-based telescopes provide a critical capability that is largely complimentary to both
ground-based telescopes and in situ spacecraft observations. Fundamental contributions to our
understanding of both the Io atmosphere, and its interaction with the plasma torus, have been
made by several space-based facilities in the past 20 years. For example, discovery of the atomic
sulfur and oxygen emissions at Io, which are a basic diagnostic of the plasma interaction with the
satellite, was made using the IUE satellite in 1986. Observations with the Hubble Space
Telescope (HST) have provided the only quantitative spatially resolved information about Io’s
SO2 atmosphere, and determined to first order its basic spatial distribution. HST and groundbased telescopes have also provided the spectroscopic observations of volcanic plumes; in
particular, HST discovered molecular sulfur (S2) in the Pele plume. The UV imaging capability
of HST has provided detailed information about the plasma interaction with the satellite via its
images of the atomic emission morphology, which provides basic ground truth for the detailed
MHD models of the interaction. EUVE observations of the Io torus discovered Na+ emission.
None of the spacecraft sent to the Jovian system (Pioneer, Voyager, Galileo, Cassini, New
Horizons) has had the ability to make these measurements.
Generally speaking, the space-based capability that has proven most useful for studying Io and
its plasma torus is access to the ultraviolet portion of the electromagnetic spectrum. There are
two reasons for this: 1) SO2 and other atmospheric molecules have strong absorption bands
throughout the UV spectral region; and 2) the electronic ground state transitions of both sulfur
and oxygen occur in the ultraviolet. Additionally, the Io plasma torus radiates primarily at UV
wavelengths. A UV capability combined with the superior spatial resolution and small
spectroscopic slits on HST instruments have provided the most comprehensive results compared
with other more specialized facilities such as EUVE or FUSE. It is therefore of very high priority
for continued success in unraveling Io’s secrets to strongly endorse a widely accessible successor
to HST with UV capability. An obvious next step would be to have diffraction-limited capability
in the UV, which HST lacks, and UV detectors with higher quantum efficiency than HST’s. The
James Webb Space Telescope’s lack of UV capability, its tailoring to astrophysical, and
particularly cosmological, problems, and its lack of planetary capability (specifically, the ability
to track moving targets) will give it only limited usefulness for Io observations. Thus, we
advocate for a space-based UV telescope with diffraction-limited capability to study Io and other
planetary targets.
A second important capability that is dearly needed for progress in Io studies is a long-term
synoptic monitoring capability in space-based facilities. Observations such as the ones described
above have given us only the shortest glimpse (e.g., the science time available in one HST orbit
is only about 40 minutes) at the true nature of Io’s atmosphere and torus interaction. While the
first order spatial structure of Io’s SO2 atmosphere is now known, these observations also make it
obvious that, as expected, the atmosphere is highly variable. What they are unable to untangle,
due to the general scarcity of the observations, is how much of the variability is due to actual
physical causes (e.g., the volcanoes) and how much is due to factors due only to the viewing
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geometry (such as correlation with specific features on Io’s surface). Space-based facilities such
as HST that provide only a brief, in-depth snapshot will never be capable of telling the full story.
A facility that provides long term synoptic monitoring capability, such as the proposed JMEX
(Jupiter Magnetospheric Explorer) or JIST (Jovian Imaging and Spectroscopic Telescope)
missions, or a more general-purpose UV telescope, provides the best hope of allowing us to
explain the complex, and intricately interconnected, Io-torus-magnetosphere system. A Jupiterorbiting space telescope, orbiting at (for example) Ganymede’s orbit, equipped with a 0.5-1.5
meter, MIDAS-like (Multiple Instrument Distributed Aperture Sensor) optical system, would
also provide excellent ability to monitor Io, as well as Jupiter’s atmosphere, rings, and other
satellites. Alternatively, if funding limitations inhibit development of these space-based
missions, then distant Io observations by planetary spacecraft cruising through the Jovian system
for extended periods (e.g., JEO will perform a 30-month Jovian system tour with distant Io
monitoring) could provide similar data. Thus, we advocate for space-based missions that enable
long-term (years) monitoring of Io over a range of time scales (seconds, minutes, hours, days,
months, years) and spatial and spectral resolutions.
Ground-Based Telescopes
Many Io phenomena, such as thermal emission from its volcanoes, emissions from its
atmosphere and torus, its reflectance spectrum, and even its large-scale albedo patterns, can be
studied from the Earth’s surface, providing the cheapest way to study Io’s time variability. Io’s
dynamism requires frequent observations to capture and understand the full range of phenomena
that it exhibits; for instance, the largest infrared volcanic “outbursts” are seen only a few percent
of the time. Such monitoring is most easily done on smaller telescopes like the IRTF that have
limited spatial resolution, but queue scheduling can allow frequent snapshots even on heavily
subscribed large 8-10-meter-class telescopes. Multi-wavelength infrared observations of the
volcanic thermal emission can constrain magma temperatures, eruption mechanisms, spatial
distributions, and the time evolution of these quantities. Over the last decade, advances in
adaptive optics (AO) with large telescopes providing 20 or more pixels across Io’s disk have
opened a new and exciting field of ground-based disk-resolved studies. New instrumentation also
enables new observational possibilities: e.g., high-spectral-resolution 7-8 micron spectroscopy of
Io, coupled with AO on a 30-meter-class telescope, which could directly map the SO2
atmospheric distribution and characterize its temporal variability.
Thus, we recommend that NASA expand the time available for general planetary science on 810-meter class telescopes, by purchasing more time on existing facilities, or by constructing a
dedicated large planetary telescope with nighttime AO capabilities. Creative scheduling,
including queue-scheduled, remote, service, and daytime (non-AO) IR observing, can maximize
the efficiency of these expensive facilities, particularly for bright objects like Io where
integration times are often short. Continued support for smaller facilities that can do crucial
temporal studies is also important. Telescope time on “smaller” facilities may become available
as these are replaced by the next generation of giant telescopes: for instance Caltech time on the
Palomar 5-meter might be available for purchase by NASA if the Thirty Meter Telescope (TMT)
is built. Furthermore, we recommend that future NASA Io-dedicated space missions should
include in their budgets support for ground-based monitoring programs that can enhance the
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spacecraft science return, e.g., by providing better temporal coverage of volcanic eruptions, for
a small fraction of the mission cost.