Science options and
priorities for the exploration
of Phobos and Deimos
Compiled from:
Scientific Objectives for the MPD
Mission, Beaty et al., 2012
Phobos and Deimos: Achieving scientific
goals and objectives with robotic and
human exploration, Murchie et al., 2014
Scott Murchie
The Johns Hopkins University/Applied Physics Laboratory
Nancy Chabot and Andy Rivkin (JHU/APL)
Abigail Fraeman and Ray Arvidson (Washington University)
David Beaty, Deborah Bass, Julie Castillo-Rogez (NASA/JPL)
Paul Abell (NASA/JSC)
Ruthan Lewis (NASA/GSFC)
Tony Colaprete (NASA//ARC)
Phobos and Deimos …
Red Unit
Blue Unit
Phobos
Deimos
Size
27 x 21 x 19 km
15 x 12 x 10 km
Orbital Period
7.66 hrs
30.3 hrs
Density
1.9 g/cm3
1.5 g/cm3
Normal albedo,
0.55 µm
7%
7%
• Are the only terrestrial
planet satellites besides
the Moon and provide
insights into terrestrial
planet formation.
• Reconnaissance by
several missions gives us
working knowledge of
the moons’ outstanding
science issues
2
• The moons resemble
outer main belt and
Trojan bodies, and could
provide insights into
primitive bodies and
sources of volatiles and
organics
3
Phobos – the better known moon
• Properties well known globally
-
Mass
Global morphology at moderate (~10s m) scale
Volume from imaging
Density (1870±20 kg/m3)
Murray et al.
2007
4
Phobos – the better known
moon
• Known only regionally or at low
resolution
-
-
Morphology at meter scale
VISIR spectral reflectance
 Weak absorptions, D-type
 OH is present at optical surface
 Best analog desiccated clay +
carbon or opaques
Relationship of color to morphology
 Hints of subsurface structure
Thermal emission / thermal inertia
 Nearly black-body spectrum
 Fine grained regolith
500 m
5
Deimos – the lesser known moon
• Properties well known globally
-
Mass
6
Deimos – the lesser known moon
• Poorly known globally
-
Global morphology at ~10-m scale
Volume
Density (1490±190 kg/m3)
• Known only locally or at low
resolution (lower than Phobos)
-
Morphology at meter scale
VISIR spectral reflectance
Relationship of color to morphology
7
Morphologically Distinct, Spectrally Similar
•
First-order differences: shape, surface morphology, density
-
Deimos’s lower density and smooth surface may result from
fragmentation/ejecta blanketing by south polar impact
Basic Deimos measurements are needed: volume to improve density
determination, global/spectral imaging at lighting geometries to assess
morphology and subsurface structure
Stickney
Grooves
Bright
regolith
streamers
South polar concavity may be impact scar
8
Morphologically Distinct, Spectrally Similar
•
Deimos is spectrally similar to Phobos’ red unit – but we can’t
tell if composition/origin is the same or different
-
Different compositions may hide in bland spectrum – issue for D bodies
An origin outside the Mars system does not require any relation
Moons’ relation requires at least in situ measurement of both moons,
though returned samples would provide greater insight
OH
Fraeman et al. (2014)
Fe-clay?
9
Relevance to Decadal Survey Goals
Building new worlds: Understanding solar system beginnings.
• The composition of the moons indicates their origin and elucidates accretion and
moon formation
Planetary habitats: Searching for the requirements for life:
• If the moons are captured primitive bodies they sample the carbon and volatiles
brought by incoming impactors
Workings of solar systems: Revealing planetary processes through time.
• The moons’ distinct regolith evolutions, and possibly different internal
structures suggested by density, provide a laboratory for divergent small
body regolith characteristics.
• Groove formation is a widespread but controversial process and Phobos is a
type example.
• Understanding microphysical effects of space weathering calibrates the
interpretability of bland D-type spectra
10
Top-Level Phobos and Deimos Exploration Objectives
1) Study Phobos and Deimos as planetary bodies that provide
insight into small bodies and terrestrial planet evolution
-
Some investigations can be accomplished robotically; others are assisted
by a human presence
Initial robotic results would help plan human exploration
2) Close strategic knowledge gaps for future human exploration
-
Very high overlap with highest-priority robotic objectives
Effectively same as #1
3) Use as a support area for human exploration of Mars
-
Teleoperation, weather monitoring, etc.
ISRU
11
Objective 1: Origin and evolution of Phobos and
Deimos, providing insight into Mars & small bodies
1) Determine origin of moons evidenced by composition
-
-
Proposed origins have radically differing implications for composition
 If the moons originate outside the Mars system, they may provide
insights into how volatiles and/or organics were delivered
 If they originate in the Mars system, they may sample Mars’ earliest
crust
Can be determined by basic mineral and elemental abundance
Solvable with a precursor mission – frames human science objectives
Origin
Plausible Composition
Capture of organic- and water-rich outer
solar system body
Primitive composition; like CI or CM
Capture of organic and water-poor outer
solar system body
Anhydrous silicates plus elemental C
Capture of inner solar system body, or coSpace weathered ordinary chondrite
accretion with Mars
Giant impact on Mars
Space-weathered Mars crust or mantle
12
-
Major elements easily separate a chondritic origin from a differentiated
composition
13
-
Minor elements separate ordinary chondrite-like compositions from
different carbonaceous compositions
Mn, Zn, C, and S are particularly diagnostic
14
-
-
Major and mineral
minerals are
complementary
information and
distinguish subtypes
Phyllosilicates and
carbonates are critical to
distinguish primitive
compositions
15
Objective 1: Origin and evolution of Phobos and
Deimos, with insight into Mars and small bodies
2) Conduct detailed studies of surface
chemistry



Determine absolute ages of materials
Constrain conditions (P, T, redox) of formation
Quantify the amount, compositions of material
from Deimos, possible extinct moons, Mars
• Mars contribution est. as ~150 ppm
Inventory & characterize organic & volatile phases

3) Determine the microphysical effects of
space weathering
-
These require analysis of returned samples in
Earth’s laboratories
Stickney ejecta, non-Sitckney ejecta, fresh and
mature regolith
Human participation invaluable
16
Objective 1: Origin and evolution of Phobos and
Deimos, with insight into Mars and small bodies
4) Determine regolith & internal structure, processes affecting
them – if either moon is a rubble pile, there is a fuzzy line
 Test models for formation of grooves
Murray (2007)
Stickney Ejecta (Thomas)
Tidal Stress (Dobrovolskis)
Stickney Rolling Boulders (Head & Wilson)
Secondary Impacts from Mars (Murray)
Stickney Fracturing (Fujiwara & Asada, Thomas)
Map of Phobos’ Grooves (Murray)
Objective 1: Origin and evolution of Phobos and
Deimos, with insight into Mars and small bodies
Fracture with Drainage
• No raised rims
• Loose fracture fill
matching composition of
surrounding regolith
-
Fracture with Degassing
• Raised rims
• Loose fracture fill with
composition from depth
Secondary Cratering
• Raised rims
• Compacted regolith inside
grooves
Key surface properties can be determined robotically
Cross-sectional variations in composition, competence assisted by human
participation: sampling transects, seismic studies and/or GPR
18
Objective 1: Origin and evolution of Phobos and
Deimos, with insight into Mars and small bodies
 Determine the moons’ internal structure
-
-
Phobos’ and Deimos’ differing densities suggest differing internal structure
• Orbital radio science, spectral mapping
• Radar and/or seismic sounding
• Sampling transects, chemical/isotopic analysis of returned samples
Human participation in sampling /
sounding could elevate data quality
Notional internal structural model
from initial analyses of Phobos 2 data
(Murchie et al. 1991)
19
Objective 1: Origin and evolution of Phobos and
Deimos, with insight into Mars and small bodies
 Quantify regolith processes
-
Some features resemble more S-asteroids like Eros, e.g. streamers
Albedo variegation is distinct - suggests processes in “primitive” composition
20
Objective 2: Determine the moon’s properties that
are important to planning human exploration
“A human mission to the Phobos/Deimos surface would require a precursor mission
that would land on one or both moons.” (Finding #2 of P-SAG Report, 2012)
Objectives of a precursor
- Hazards
- In situ resources
- Tactical data to plan surface
operations
- Science to frame human
scientific exploration
Phobos and Deimos dust belts from Hamilton (1996)
21
Objective 2: Determine the moon’s properties that
are important to planning human exploration
Strategic Knowledge Gap
(P-SAG, 2012)
Relevant Measurements
A3-1. Orbital particulate
environment
• Particle size-frequency distribution of dust belts
C1-1. Surface composition and
potential for ISRU (C, H)
• Elemental composition, including C and H
• Mineral composition, including hydrous phases
• Global spectral imaging or elemental abundance
mapping, for context
C2-1. Charged particle
environment
• Near-moon total dose and energy measurements
C2-2. Gravitational fields
• Mass, mass distribution from radio science
• Global shape through stereo imaging or lidar
C2-3. Regolith geotechnical
properties
• Thickness, rock abundance from imaging, radar
• µm to cm scale structure, particle size
• Regolith mechanical properties experiment
22
Objective 3: Use the moons as a staging area to
support human exploration of Mars
•
•
A platform to support Mars science - somewhat redundant to
lower-cost robotic exploration
-
Teleoperation of rovers
-
Mars weather / atmosphere monitoring
-
Retrieve Mars samples from Mars orbit
In situ resource utilization – powerful but yet to be proven
(by robotic precursor?)
-
Amount of C, OH still speculative
23
Take-away Messages - Science
•
Phobos and Deimos are relevant to important science questions
-
•
“Exploring the outer solar system from Mars orbit”
Some questions can be addressed robotically
-
Would close SKGs
•
More challenging questions benefit from human participation
•
The science is distinct & complementary to Mars surface science
24
Take-away Messages - Tactical
•
Science value in visiting both moons
•
Phobos appears to be of greater value
•
Depending on timeline, visit both but with more time at Phobos
•
Precursor data are needed to formulate detailed objectives
•
Robotic mission to Phobos and/or Deimos is required prior to a human
mission.
•
The precursor should interact with the surface of Phobos and/or
Deimos to understand geotechnical properties
•
Robotic Phobos or Deimos sample return from prior to a human
mission is judged NOT to be necessary
25
Precursor Concepts
MERLIN: Deimos flybys Phobos
rendezvous and land
PANDORA: Deimos and Phobos
rendezvous
26
BACKUP
27
Relevance to SBAG Themes and Objectives
Solar System Origins: May be remnant Mars building blocks & contain key info on
Mars’ accretional environment. May be captured asteroidal materials linked to early
planetesimal formation.
Solar System Dynamics: May be related to population of late-accreting planetesimals
involved in early bombardment and offer insights into exchange of material from Mars to
moons, to each other, and also from outside Mars system.
Current State of the Solar System: Present potential relationship with D-type
asteroids, Tagish Lake chondrite, other primitive carbonaceous meteorites, and as well as
Mars’ surface dust. Also understanding of regolith processing.
In Situ Resource Utilization: OH and/or water suggested by spectroscopy
Hazards: Do not present an impact hazard; physical studies can help better understand
relationships to near-Earth asteroids (i.e., contribution to their near-surface geotechnical
properties and internal structure).
Astrobiology: May represent pre-Noachian Mars ejecta or water-rich asteroids; may be
repository for Mars’ meteorites ejected through time; may offer insights/comparisons
into delivery of organics/volatiles to early Earth.
28