PTYS 411
Geology and Geophysics of the Solar System
Vacuum ProcessesVacuum Processes
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Regolith Generation Regolith growth Turnover timescales Mass movement on airless surfaces 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
Eros – NEAR spacecraft (12m across)
Miyamoto et al. 2007
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Impacts create regoliths
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Geometric saturation Hexagonal packing allows craters to fill 90.5% of available area
(Pf)
In reality, surfaces reach only ~4% of this value
Log (D)
Lo
g (
N)
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Equilibrium saturation: No surface ever reaches the geometrically saturated limit. Saturation sets in long beforehand
(typically a few % of the geometric value) Mimas reaches 13% of geometric saturation – an extreme case
Craters below a certain diameter exhibit saturation This diameter is higher for older terrain – 250m for lunar Maria This saturation diameter increases with time
implies
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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
Regolith grows when crater breccia lenses coalesce
Assume breccia (regolith) thickness of D/4
Maximum thickness of regolith is Deq/4 , but not in all locations
Smaller craters are more numerous and have interlocking breccia lenses < Deq/4
Shoemaker et al., 1969
Growth of Regolith
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Minimum regolith thickness: Figure out the fractional area (fc) covered by craters D→Deq where (D < Deq) Choose some Dmin where you’re 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 <h> when: P(<h>) = 0.5 Time dependence in heq or rather Deq α time1/(b-2)
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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
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Regolith modeled as overlapping ejecta blankets Number of craters at distance r (smaller than D=2r) (scales as r2)
Thickness of their ejecta (scales as (r/D)-3) (scales as D0.74)
Results (moon, b=3.4)
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Transport is slope dependent
For ejecta at 45° on a 30° slope Downrange ~ 4x uprange
Net effect is diffusive transportD
ow
nh
ill
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Ponding of regolith – seen on Eros Regolith grains <1cm move downslope Ponded in depressions Possibly due to seismic shaking from impacts
Miyamoto et al. 2007
Robinson et al. 2001
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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
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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
Space Weathering
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Lunar agglutinate
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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
S-type conundrum… S-Type asteroids are the most common asteroid Ordinary chondrites are the most numerous
meteorites Parent bodies couldn’t 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
Ida (and Dactyl) – Galileo mission
Clark et al., Asteroids III
Clark et al., Asteroids III
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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
10’s 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 100’s of Km s-1
Warped Archimedean spiral Implantation of ions into surface
may explain reduced neutron counts
Clark et al., Asteroids III
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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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Airless bodies do have ‘atmospheres’ Surface bounded exospheres Atoms collide more often with the surface than with each other
mean free path >> atmospheric scale height
(really means that mean free path >> trajectory of a molecule)
Molecules ejected from hot surface with a Maxwellian velocity distribution
Launched on an orbital track (if they don’t 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)
Ejection rate is slow & range is small When the sun comes up they start hopping around again
Volatiles in a Vacuum
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Sublimation/condensation of ices Molecules in the atmosphere
impact the surface at a rate that depends on P and T
Molecules leave the surface at a rate that depends on T
Mean molecular speeds are
Solid, temperature T
Atmosphere, partial pressure Pand temperature T
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Do permanently shadowed regions exist? Yes, Moon and Mercury have low obliquity
1.6° for the Moon ~0° for Mercury
Solar elevations in the polar regions are always low
Surrounding topography is high compared with solar elevations
Even modest craters can have permanent shadow on their floors
Mazarico et al. 2011
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Permanently shadowed regions in the lunar polar regions 12,866 and 16,055 km2, in the north and south poles respectively
Mazarico et al. 2011
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Permanent shadow produces low temperatures Some areas of permanent illumination as well
Paige et al., 2010
Day Night
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Modeling (Vasavada et al. 1999) shows temperatures in permanently shadowed craters are very low for Mercury too
These cold traps are favored condensation sites
Vasavada et al., 1999
Moon
Mercury
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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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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 it’s 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 Mercury’s 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 Turnover timescales Megaregolith
Space Weathering Impact gardening Sputtering Ion-implantation
Volatiles in a Vacuum Surface-bounded exospheres Volatile migration Permanent shadow
Gaspra – Galileo mission