Surface Processes Acting On Airless Bodies

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Surface Processes Acting On Airless Bodies
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In This Lecture

• Regolith Generation
• Turnover timescales
• Megaregolith
• Space Weathering
• Impact gardening
• Sputtering
• Ion-implantation
• Volatiles in a Vacuum
• Surface-bounded exospheres
• Volatile migration
• Permanent shadow

Gaspra Galileo mission
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• All rocky airless bodies covered with regolith
  (rock blanket)

Moon - Helfenstein and Shepard 1999
Itokawa Miyamoto et al. 2007
Miyamoto et al. 2007
Eros NEAR spacecraft (12m across)
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Growth of Regolith

• Crust of airless bodies suffers many impacts
• Repeated impacts create a layer of pulverized
  rock
• Old craters get filled in by ejecta blankets of
  new ones

• Maximum thickness related to depth of largest
• crater to reach equilibrium
• Estimate Deq from a size-frequency plot or solve
• (note surface must saturate so b must be gt2)
• If equilibrium 4 of geometric saturation then
  ceq0.046

Shoemaker et al., 1969
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• Minimum regolith thickness more complicated
• Figure out the fractional area (fc) covered by
  craters D?Deq where (D lt Deq)
• Choose some Dmin where youre sure that every
  point on the surface has been hit at least once
• Typical to pick Dmin so that f(Dmin,Deq) 2
• hmin of regolith Dmin/4
• General case
• Probability that the regolith has a depth h is
  P(h) f(4h?Deq) / fmin
• Median regolith depth lthgt when P(lthgt) 0.5
• Time dependence in heq or rather Deq a time1/(b-2)

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• Mega-regolith
• Fractured bedrock extend down many kilometers
• Acts as an insulating layer and restricts heat
  flow
• 2-3km thick under lunar highlands and 1km under
  maria

• Regolith turnover
• Shoemaker defines as disturbance depth (d)
• time until f(4d, Deq) 1
• Things eventually get buried on these bodies
• Mixing time of regolith depends on depth
  specified
• Cosmic ray exposure ages on Moon
• 10cm in 500 Myr

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• Ponding of regolith seen on Eros
• Regolith grains lt1cm move downslope
• Ponded in depressions
• Possibly due to seismic shaking from impacts

Miyamoto et al. 2007
Robinson et al. 2001
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• Cratered landscapes
• Relief-generation
• Crater formation
• Relief-Reduction
• Seismic shaking
• Impact gardening
• Volcanic flooding

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Space Weathering

• The vacuum environment heavily affects individual
  grains
• Impact gardening micrometeorites
• Comminution (breaking up) particles
• Agglutination grains get welded together by
  impact glass
• Vaporization of material
• Heavy material recondenses on nearby grains
• Volatile material enters atmosphere
• Solar wind
• Energetic particles cause sputtering
• Ions can get implanted
• Cosmic rays
• Nuclear effects change isotopes dating
• Collectively known as space-weathering
• Spectral band-depth is
• reduced
• Objects get darker
• and redder with time

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• Asteroid surfaces exhibit space weathering
• C-types not very much
• S-types a lot (still not as much as the Moon)
• Weathering works faster on some surface
  compositions

• Smaller asteroids (in general) are the result of
  more recent collisions less weathered
• Material around impact craters is also fresher

Ida (and Dactyl) Galileo mission

• S-type conundrum
• S-Type asteroids are the most common asteroid
• Ordinary chondrites are the most numerous
  meteorites
• Parent bodies couldnt be identified, but
• Galileo flyby of S-type asteroids showed surface
  color has less red patches
• NEAR mission Eros showed similar elemental
  composition to chondrites

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• Nanophase iron is largely responsible
• Micrometeorites and sputtering vaporize target
  material
• Heavy elements (like Fe) recondense onto nearby
  grains
• Electron microscopes show patina a few 10s of nm
  thick
• Patina contains spherules of nanophase Fe
• Fe-Si minerals also contribute to reddening
• e.g. Fe2Si Hapkeite (after Bruce Hapke)

• Sputtering
• Ejection of particles from impacting ions
• Solar-wind particles
• H and He nuclei
• Traveling at 100s of Km s-1
• Warped Archimedean spiral
• Implantation of ions into surface may explain
  reduced neutron counts

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• Lunar swirls
• High albedo patches
• Associated with crustal magnetism
• Most are antipodal to large basins
• Model 1
• Magnetic field prevents space weathering
• Model 2
• Dust levitation concentrates fine particles in
  these areas
• Levitation concentrated near terminator
• Photoelectric emission of electrons

Wang et al. 2008
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Volatiles in a Vacuum

• Airless bodies do have atmospheres
• Surface bounded exospheres
• Atoms collide more often with the surface than
  with each other
• mean free path gtgt atmospheric scale height
• (really means that mean free path gtgt trajectory
  of a molecule)
• Molecules ejected from hot surface with a
  Maxwellian velocity distribution
• Launched on an orbital track (if they dont
  escape outright)
• with range
• Particles from hotter regions travel furthest
• Particles continue to hop around until they find
  cold spots (e.g. night-side or shadowed area)

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• The Moon and Mercury both possess tenuous
  atmospheres

Calcium now also seen at Mercury

• Sodium emission at the Moon and Mercury shows
  temporal changes
• Stirring of regolith by small impacts

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• Planetary ices sublimate at a (very) temperature
  dependant rate
• Depends on the vapor pressure
• Psat comes from the Clausius-Clapeyron relation

Andreas, 2007

• Sublimation rates are extraordinarily temperature
  sensitive
• 1m of water ice can survive 1 Gyr at 110K
• but temps of 130K can sublimate 1km of water ice
• lt 1m/Gyr is considered stable

Vasavada et al., 1999
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• H, He, O can be delivered by the solar wind
• Na, K, Ca are derived from the surface mostly
  by impacts
• H2O delivered episodically by comets
• Neutral molecule travels on ballistic bounces
  around the surface until
• It is ionized by a photon solar wind can then
  push it back into the surface or sweep it away
• It hits a cold (night-side or shadowed) area
• Lifetimes depend on ease of ionization and solar
  flux
• 1 hour (9 minutes) for Na on the Moon (Mercury)
• Few 100 days (month or two) for H on the Moon
  (Mercury)
• H2O molecules can be destroyed in many ways
• Sputtering
• Photo-ionization (then removed by charged solar
  wind)
• Photo-disassociation
• or just thermal escape
• Molecules stuck in shadows wait until sunrise
  before being launched again
• But what if the sun never rises?
• What if there was a shadow that was always
  there?......

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• Do permanently shadowed regions exist?
• Yes, Moon and Mercury have low obliquity
• Solar elevations in the polar regions are always
  low
• Even modest craters can have permanent shadow on
  their floors

Margot et al., 1999
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• Modeling (Vasavada et al. 1999) shows
  temperatures in permanently shadowed craters are
  very low
• These cold traps are favored condensation sites

Mercury
Moon
Vasavada et al., 1999
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• Evidence for ice in polar craters of the Moon and
  Mercury
• Evidence for ice at lunar poles
• Clementine bi-static radar
• Lunar prospector neutron data fewer neutrons
  indicates surface hydrogen
• Evidence for ice at poles of Mercury
• VLA radar returns

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• How about other airless bodies asteroids?
• Low gravity is a problem, molecules tend to
  escape
• Largest asteroid Ceres (900km) does show some
  promise
• Current polar loss rates are 1m/Gyr without
  shadowing
• Previous observations of OH- surrounding Ceres
• Interpreted to be photo-dissassociation of water

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• Polar ice on the Moon first suggested by Watson,
  Murray Brown (1961)
• As long as there is an ice deposit there
• Atmospheric pressure will be the Psat over the
  ice
• which depends on Tice
• Higher pressure will cause net condensation,
  lower will cause net sublimation
• If ice is to be sustainable over solar system
  history then it must be delivered at the same
  rate its sublimated.

• Water leaves cold traps by sublimation
• 5-15 returns on Mercury
• 20-50 returns on the Moon
• The rest is lost
• Water can be delivered by meteors and comets
• For Mercury these rates have been estimated
• Balance exists if Tice is 113K

Killen et al., 1997

• Moon/Mercury differences
• Mercurys ice deposits were easily detected
• Lunar ice is probably not abundant barely
  detected
• Mercury may have experienced a recent impact that
  delivered a lot of water

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Regolith Generation

• Regolith generated by continuous impacts
• Depth of regolith can be related to how saturated
  a surface is with craters
• Regolith depth increases with time
• Down-slope movement of regolith on asteroids

Space Weathering

• Objects in space get darker and redder with time
• S-type conundrum solved by realizing that their
  spectra once looked chondritic
• Weathering caused by condensation of nanophase
  iron vaporized by micrometeroites
• Albedo swirls caused by interaction of solar wind
  and crustal magnetism

Volatiles on Airless Bodies

• Surface bounded exospheres
• molecule-surface collisions gtgt molecule-molecule
  collisions
• Vapor pressure (temperature) of volatiles
  determines their stability
• Volatiles can hide in permanently shadowed
  regions polar craters
• Strong evidence for polar ice on Mercury
• but weak evidence on the Moon
• very weak evidence on large asteroids