Space Weathering
on Phobos and Deimos
Carle M. Pieters
Brown Univ/SSERVI
Topics
• Why we suspect space weathering (SPWE) on
Phobos & Deimos.
• Review what we know of SPWE forms.
– Lunar
– Asteroids: S-type; Vesta
• What do we know of Phobos/Deimos’
environment?
• Summary and conjecture…..
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Phobos and Deimos variations are
without obvious spectral features.
4.0
Rivkin et al (2002)
Phobos 2
Phobos Red Unit (IRTF)
Phobos Red unit (Murchie and Erard, 1996)
3.5
Phobos Blue unit (Murchie and Erard, 1996)
Normalized Reflectance
Phobos Blue unit (IRTF)
3.0
Deimos (Murchie et al. 1999)
Deimos (IRTF)
2.5
2.0
1.5
Murchie et al., 1999
1.0
0.5
0.3 0.6 0.9 1.2 1.5 1.8 2.1 2.4 2.7 3.0 3.3 3.6
Wavelength (mm)
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Phobos and Deimos appear to have
affinities
with
asteroid
and
lunar
soils.
Reflectance Spectra of Exploration Targets:
Remote
measurement
of scaled
spectral
properties.
Spectral
range
450 – 2450 nm,
to unity
NEA
Main
Belt
Asteroids
All data are VIS-NIR scaled spectral reflectance.
Just what IS Space Weathering?
SPWE
Space weathering refers to an array of
processes that measurably alter the
character of surfaces with time as a
result of exposure to the space
environment.
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Just what IS Space Weathering?
MANY Processes!
Many Products!
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Just what IS Space Weathering?
S. Noble V1
V4 SPWE
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Just what IS Space Weathering?
Lunar-like SPWE:
accumulation of npFe0 on regolith grains.
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Just what IS Space Weathering?
Vesta-like SPWE: contamination and mixing of exogenic (CC) materials.
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Just what IS Space Weathering?
Which processes are most active on
Phobos and Deimos?
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Observations & Ground Truth: The Moon
L. Keller, S. Noble et al.
1996-present
RELAB
With time, lunar soils…
• Become darker across the visible
• Have weaker absorption bands
• Exhibit a ‘red’ sloped scaled nearIR continuum
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Observations & Ground Truth: The Moon
L. Keller, S. Noble et al.
1996-present
RELAB
With time, lunar soils…
• Become darker across the visible
• Have weaker absorption bands
• Exhibit a ‘red’ sloped scaled nearIR continuum
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Evolution of thought…
Old View
Space weathering occurs
on the Moon, and….
– agglutinates and glass
are the cause of lunar
optical alteration.
– Since agglutinates are
unlikely on asteroids,
space weathering has no
effect.
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Evolution of thought…
Old View (incorrect!)
Current Understanding
Space weathering occurs
on the Moon, and….
– agglutinates and glass
are the cause of lunar
optical alteration.
– Since agglutinates are
unlikely on asteroids,
space weathering has no
effect.
– Nano-phase opaques form on
lunar grains by XX process.
– During soil evolution, npFe0
accumulates on grains while
Fe0 particles become larger
within glass.
• Absorptions weaken
• Albedo reduced in the visible
• Continuum lowers toward short
wavelengths
Formation of npFe0 requires mobilization of Fe atoms…
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TEM Images of Lunar Soil Grains
• npFe0 is observed as (sometimes
multiple) deposits on rims
• Fe0 particles on the interior of
agglutinates are larger.
Plagioclase grain
L. Keller et al., 1996-present
Agglutinate grain
Optical properties and amount of
npFe0 are directly related
Contaminant
npFe0
0.005
0.02
0.07
Low FeO
0.13
0.19
0.30
High FeO
2.1
Lunar Samples: Noble et al., 2001
after Morris 1977
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Optical properties and amount of
npFe0 are directly related
Contaminant
npFe0
0.005
0.02
0.07
Low FeO
0.13
0.19
0.30
High FeO
2.1
Lunar Samples: Noble et al., 2001
Experimental: Noble et al., 2004; 2006
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Lunar-like SPWE
Current Understanding
– Nano-phase opaques form on
lunar grains by XXprocess.
– During soil evolution, npFe0
accumulates on grains while
Fe0 particles become larger
within glass.
• Absorptions weaken
• Albedo reduced in the visible
• Continuum lowers toward short
wavelengths
Formation of npFe0 on grains requires mobilization of Fe atoms…
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Is the same process
likely or unlikely on asteroids?
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S-Asteroids are ABUNDANT!
Ordinary Chondrites are the most abundant meteorites.
S-type V-type C-type
40,000 asteroids
Sloan Digital Sky Survey
Asteroid Types
S-type
Distance from the Sun
Wavelength
NEO Observations & in-situ: Eros
S-Asteroid <–> Ordinary Chondrite?
[Abundant]
NEAR
S-Asteroids have weaker absorption bands
….and a ‘red’ sloped continuum.
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NEO Observations & in-situ: Eros
S-Asteroid <–> Ordinary Chondrite?
[Abundant]
NEAR Results: Eros is a slightly (lunarlike) space weathered ordinary
chondrite body….
[Trombka et al., 2000; Clark et al., 2001; McCoy et al.
2001]
Observations & Ground Truth: Itokawa
Hayabusa orbital and earth-based spectroscopy, suggested
small amounts of lunar-like weathering for Itokawa.
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Binzel et al. 2001, MaPS
predict OC parent body.
Observations & Ground Truth: Itokawa
Hayabusa
returned
regolith
grains to Earth
laboratories
From Hayabusa
orbital
and earth-based
spectroscopy,
0 weathering.
for
analyses
in 2010.
Itokawa
shows
small amounts
of
npFe
Furthermore, half of the grains studied
contain np-opaque rims (both Fe and S).
[Noguchi et al., 2011]
Demonstrated Itokawa is a
LL Chondrite [Nakamura et al.,
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2011]
Moon & NEA summary….
Prior to Dawn’s arrival at the main asteroid belt, a
consistent story of space weathering had evolved:
– Optical alteration is largely dominated by the accumulation
of nano-phase opaques (npFe0 or npOpq) on the surface of
regolith grains.
– Both solar wind and micrometeorite processes are
important (for the Moon, micro-impact vaporization & deposition
may be common).
– NEAs exhibit significantly less space weathering products
than the Moon.
– npFe0 is not the only phase to accumulate on regolith
grains [composition of the host appears to be important].
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Moon & NEA summary….
Prior to Dawn’s arrival at the main asteroid belt, a
consistent story of space weathering had evolved:
– Optical alteration is largely dominated by the accumulation
of nano-phase opaques (npFe0 or npOpq) on the surface of
regolith grains.
– Both solar wind and micrometeorite processes are
important (for the Moon, micro-impact vaporization & deposition
may be common).
– NEAs exhibit significantly less space weathering products
than the Moon.
– npFe0 is not the only phase to accumulate on regolith
grains [composition of the host appears to be important].
BUT….. What of Vesta, a main-belt asteroid??
Observations & Ground Truth: Vesta
• The HED meteorites
(Howardites, Eucrites, Diogenites)
have been associated with
Vesta for decades
(McCord et al., 1970).
• Telescopic data are well
modeled by very finegrained howardites, <25 µm.
[i.e., Space weathering is not needed?]
[Dawn instruments has confirmed the
relation between Vesta and HEDs.]
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Observations & Ground Truth: Vesta
• The HEDs (Howardites,
Eucrites, Diogenites) have
been associated with
Vesta for decades (McCord
et al., 1970).
• Telescopic data are well
modeled by fine-grained
howardites. [i.e., Space
weathering is not needed.]
[Dawn instruments has confirmed
this relation between Vesta and
HEDs.]
However, Vesta does exhibit space weathering !
Vesta
Space Weathering
Pieters et al.
Nature 2012
Prominent variations in band strength and
albedo are observed, but no related variation
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in NIR continuum,  no npOpq.
FC Image
Two Key Questions:
What IS causing the space
weathering at Vesta?
Why has npFe0 NOT
developed at Vesta?
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Darkening by small opaque particles:
Scale is critical…
• Impact darkening: 1-5 µm
[Remobilization of opaques….]
• npFe0 darkening: 1-5 nm
[Vapor deposit or sputtering products]
Neutral color
“Red’ nearIR
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Two Key Questions:
What IS causing the space
weathering at Vesta?
• Vesta exhibits a spectrally
neutral absorbing
species…
• that is well mixed in the
regolith*
• and originates from
contamination by low
albedo asteroid debris
[e.g. water-bearing CC**]
*See Pieters et al. and McCord et al., 2012 Nature
**See de Sanctis et al., 2012 ApJ and Prettyman et al.
2012, Science for OH/H2O discussion
e.g. Zolensky et al 1996 MaPS
CC clasts in HED meteorites
Two Key Questions:
What IS causing the space
weathering at Vesta?
• Vesta exhibits a spectrally
neutral absorbing
species…
• that is well mixed in the
regolith*
• and originates from
contamination by low
albedo asteroid debris
[e.g. water-bearing CC**]
*See Pieters et al. and McCord et al., 2013 Nature
**See de Sanctis et al., 2012 ApJ and Prettyman et al.
2012, Science for OH/H2O discussion
Why has npOpq NOT
developed at Vesta?
• Native Fe0 and FeS are
almost absent on Vesta.
• Pyroxene is more
resistant to formation of
npFe0.
• Impact velocities are
lower at Vesta.
• Vesta is further from the
Sun than Earth/Moon.
• Rheasilvia is too recent.
• Other…..
Two Key Questions:
What IS causing the space
weathering at Vesta?
• Vesta exhibits a spectrally
neutral absorbing
species…
• that is well mixed in the
regolith*
• and originates from
contamination by low
albedo asteroid debris
[e.g. water-bearing CC**]
*See Pieters et al. and McCord et al., 2013 Nature
**See de Sanctis et al., 2012 ApJ and Prettyman et al.
2012, Science for OH/H2O discussion
Why has npOpq NOT
developed at Vesta?
• Native Fe0 and FeS are
almost absent on Vesta.
• Pyroxene is more
resistant to formation of
npFe0.
• Impact velocities are
lower at Vesta.
• Vesta is further from the
Sun than Earth/Moon.
• Rheasilvia is too recent.
• Other…..
Key Lesson:
All processes are active, but the relative effect
varies with each body (location).
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Summary….
The preponderance of nanophase opaques* (npOpq) on
rims of regolith grains is a primary product of space
weathering at 1 AU.
However, cross-contamination and regolith mixing are
now recognized as significant space weathering
processes active on airless bodies.
Estimations of the time scale for observed space
weathering are non-unique and must directly address
the processes and products involved.
Simple parameters such as band strength or continuum
slope cannot be used alone to measure the magnitude
of space weathering.
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*can be Fe0, FeS, C, etc. …..
….so what of Phobos?
Phobos is OLD [Gys]
Stickney is OLD [Gys]
Thus, the surface is old.
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Best view of Phobos
(Mars-side from HIRISE)
Phobos and Deimos appear to have
affinities
with
asteroid
and
lunar
soils.
Reflectance Spectra of Exploration Targets:
Remote
measurement
of scaled
spectral
properties.
Spectral
range
450 – 2450 nm,
to unity
Asteroids
All data are VIS-NIR scaled spectral reflectance.
Phobos and Deimos spectra are….
without obvious features!
Rivkin et al (2002)
4.0
Phobos Red Unit (IRTF)
Phobos Red unit (Murchie and Erard, 1996)
3.5
Phobos Blue unit (Murchie and Erard, 1996)
Normalized Reflectance
Phobos Blue unit (IRTF)
3.0
Deimos (Murchie et al. 1999)
Deimos (IRTF)
2.5
2.0
1.5
1.0
0.5
0.3 0.6 0.9 1.2 1.5 1.8 2.1 2.4 2.7 3.0 3.3 3.6
Fraeman et al.,
2012; 2014
Wavelength (mm)
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….but there is variability at Phobos
Phobos is OLD [Gys]
Stickney is OLD [Gys]
Thus, the surface is old.
But it is not homogeneous.
Old craters exist.
Fresh craters exist.
Boulders exist.
Fresh ‘red’ craters
Fresh ‘blue’ craters
Boulders
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….so what of Phobos?
• Our understanding of Mars two moons is very
data limited [spectra, coverage, resolution].
• However, Phobos and Deimos appear to share
the same regolith optical properties.
– If they both are captured asteroids, they must be
the same type [D?] to produce the same regolith.
– If they accreted from a major Mars impact …
• Both likely started from highly processed material.
• Their space weathering history is similar.
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A SPWE ‘Story’* for Phobos
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*To be tested, revised…
A SPWE ‘Story’* for Phobos
After the Borealis impact event….
• Phobos and Deimos accreted from anhydrous highly
processed material (from Mars’ mantle?).
• Minor heterogeneity was maintained, but neither
evolved beyond a coherent rubble pile.
• A regolith developed and was mixed with dust in the
Mars environment.
• With constant recycling, the regolith developed
abundant lunar-like grain rims [abundant npOpq].
• The local dynamic height configuration redistributes
regolith across the moon in the patterns observed.
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*To be tested, revised…
Questions and Comments?
Dominant SPWE Process?
[Comminution assumed for all]
• The Moon [significant accumulation of npFe0]
• Near-Earth Asteroids [accumulation of npOpq]
• Vesta [contamination and regolith mixing]
• Mercury [Uber-accumulation of npOpq?]
• Phobos [? accumulation of npOpq + contamination]
• Ceres [volatile mobility, + contamination, +
dehydration, + regolith mixing?]
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Dominant Space Weathering Processes?
Differences are important!
– Location
•
•
•
•
Temperature
Solar wind
Velocity of impactors
Dust environment
– Composition
• Presence/absence of iron, FeS….
• Pyroxene/olivine
• Water and volatiles
– Other
•
•
•
•
Mass
Magnetic field
Close encounters
…..
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