PTYS 411 Geology and Geophysics of the Solar System Vacuum Processes

PYTS 411 – Vacuum Processes 2 l Regolith Generation n Regolith growth n Turnover timescales n Mass movement on airless surfaces n Megaregolith l Space Weathering n Impact gardening n Sputtering n Ion-implantation l Volatiles in a Vacuum n Surface-bounded exospheres n Volatile migration n Permanent shadow Gaspra – Galileo mission

PYTS 411 – Vacuum Processes 3 l All rocky airless bodies covered with regolith (‘rock blanket’) Moon - Helfenstein and Shepard 1999 Itokawa – Miyamoto et al Eros – NEAR spacecraft (12m across) Miyamoto et al. 2007

PYTS 411 – Vacuum Processes 4 l Impacts create regoliths

PYTS 411 – Vacuum Processes 5 l Geometric saturation n Hexagonal packing allows craters to fill 90.5% of available area (P f ) n In reality, surfaces reach only ~4% of this value Log (D) Log (N)

PYTS 411 – Vacuum Processes 6 l Equilibrium saturation: n No surface ever reaches the geometrically saturated limit. n Saturation sets in long beforehand (typically a few % of the geometric value) n Mimas reaches 13% of geometric saturation – an extreme case l Craters below a certain diameter exhibit saturation n This diameter is higher for older terrain – 250m for lunar Maria n This saturation diameter increases with time implies

PYTS 411 – Vacuum Processes 7 l Crust of airless bodies suffers many impacts n Repeated impacts create a layer of pulverized rock n Old craters get filled in by ejecta blankets of new ones l Regolith grows when crater breccia lenses coalesce l Assume breccia (regolith) thickness of D/4 l Maximum thickness of regolith is D eq /4, but not in all locations l Smaller craters are more numerous and have interlocking breccia lenses < D eq /4 Shoemaker et al., 1969 Growth of Regolith

PYTS 411 – Vacuum Processes 8 l Minimum regolith thickness: n Figure out the fractional area (f c ) covered by craters D→D eq where (D < D eq ) n Choose some D min where you’re sure that every point on the surface has been hit at least once n Typical to pick D min so that f(D min,D eq ) = 2 n h min of regolith ~ D min /4 l General case n Probability that the regolith has a depth h is: P(h) = f(4h→D eq ) / f min n Median regolith depth when: P( ) = 0.5 n Time dependence in h eq or rather D eq α time 1/(b-2)

PYTS 411 – Vacuum Processes 9 l Regolith turnover n Shoemaker defines as disturbance depth (d) time until f(4d, D eq ) =1 n Things eventually get buried on these bodies n Mixing time of regolith depends on depth specified

PYTS 411 – Vacuum Processes 10 l Regolith modeled as overlapping ejecta blankets n Number of craters at distance r (smaller than D=2r) n (scales as r 2 ) n Thickness of their ejecta n (scales as (r/D) -3 ) n (scales as D 0.74 ) n Results (moon, b=3.4)

PYTS 411 – Vacuum Processes 11

PYTS 411 – Vacuum Processes 12

PYTS 411 – Vacuum Processes 13 l Transport is slope dependent l For ejecta at 45° on a 30° slope n Downrange ~ 4x uprange l Net effect is diffusive transport Downhill

PYTS 411 – Vacuum Processes 14 l Ponding of regolith – seen on Eros n Regolith grains <1cm move downslope n Ponded in depressions n Possibly due to seismic shaking from impacts Miyamoto et al Robinson et al. 2001

PYTS 411 – Vacuum Processes 15 l Mega-regolith n Fractured bedrock extend down many kilometers n Acts as an insulating layer and restricts heat flow n 2-3km thick under lunar highlands and 1km under maria

PYTS 411 – Vacuum Processes 16 l The vacuum environment heavily affects individual grains l Impact gardening – micrometeorites n Comminution: (breaking up) particles n Agglutination: grains get welded together by impact glass n Vaporization of material wHeavy material recondenses on nearby grains wVolatile material enters ‘atmosphere’ l Solar wind n Energetic particles cause sputtering n Ions can get implanted l Cosmic rays n Nuclear effects change isotopes – dating l Collectively known as space-weathering n Spectral band-depth is reduced n Objects get darker and redder with time Space Weathering

PYTS 411 – Vacuum Processes 17 Lunar agglutinate

PYTS 411 – Vacuum Processes 18 l Asteroid surfaces exhibit space weathering n C-types not very much n S-types a lot (still not as much as the Moon) n Weathering works faster on some surface compositions n Smaller asteroids (in general) are the result of more recent collisions – less weathered n Material around impact craters is also fresher l S-type conundrum… n S-Type asteroids are the most common asteroid n Ordinary chondrites are the most numerous meteorites n Parent bodies couldn’t be identified, but… n Galileo flyby of S-type asteroids showed surface color has less red patches n NEAR mission Eros showed similar elemental composition to chondrites Ida (and Dactyl) – Galileo mission Clark et al., Asteroids III

PYTS 411 – Vacuum Processes 19 l Nanophase iron is largely responsible n Micrometeorites and sputtering vaporize target material n Heavy elements (like Fe) recondense onto nearby grains n Electron microscopes show patina a few 10’s of nm thick n Patina contains spherules of nanophase Fe n Fe-Si minerals also contribute to reddening e.g. Fe 2 Si Hapkeite (after Bruce Hapke) l Sputtering n Ejection of particles from impacting ions n Solar-wind particles H and He nuclei Traveling at 100’s of Km s -1 Warped Archimedean spiral n Implantation of ions into surface may explain reduced neutron counts Clark et al., Asteroids III

PYTS 411 – Vacuum Processes 20 l Lunar swirls n High albedo patches n Associated with crustal magnetism n Most are antipodal to large basins l Model 1: n Magnetic field prevents space weathering l Model 2: n Dust levitation concentrates fine particles in these areas n Levitation concentrated near terminator wPhotoelectric emission of electrons Wang et al. 2008

PYTS 411 – Vacuum Processes 21 l Airless bodies do have ‘atmospheres’ n Surface bounded exospheres n 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) n Molecules ejected from hot surface with a Maxwellian velocity distribution n Launched on an orbital track (if they don’t escape outright) with range: n Particles from hotter regions travel furthest n Particles continue to hop around until they find cold spots (e.g. night-side or shadowed area) wEjection rate is slow & range is small n When the sun comes up they start hopping around again Volatiles in a Vacuum

PYTS 411 – Vacuum Processes 22 l Sublimation/condensation of ices n Molecules in the atmosphere impact the surface at a rate that depends on P and T n Molecules leave the surface at a rate that depends on T n Mean molecular speeds are Solid, temperature T Atmosphere, partial pressure P and temperature T

PYTS 411 – Vacuum Processes 23 l Do permanently shadowed regions exist? n Yes, Moon and Mercury have low obliquity w1.6° for the Moon w~0° for Mercury n Solar elevations in the polar regions are always low n Surrounding topography is high compared with solar elevations wEven modest craters can have permanent shadow on their floors Mazarico et al. 2011

PYTS 411 – Vacuum Processes 24 l Permanently shadowed regions in the lunar polar regions n 12,866 and 16,055 km 2, in the north and south poles respectively Mazarico et al. 2011

PYTS 411 – Vacuum Processes 25 l Permanent shadow produces low temperatures n Some areas of permanent illumination as well Paige et al., 2010 Day Night

PYTS 411 – Vacuum Processes 26 l Modeling (Vasavada et al. 1999) shows temperatures in permanently shadowed craters are very low for Mercury too n These cold traps are favored condensation sites Vasavada et al., 1999 Moon Mercury

PYTS 411 – Vacuum Processes 27 l Evidence for ice in polar craters of the Moon and Mercury n Evidence for ice at lunar poles wClementine bi-static radar wLunar prospector neutron data – fewer neutrons indicates surface hydrogen n Evidence for ice at poles of Mercury wVLA radar returns

PYTS 411 – Vacuum Processes 28 l Polar ice on the Moon first suggested by Watson, Murray & Brown (1961) l As long as there is an ice deposit there n ‘Atmospheric’ pressure will be the P sat over the ice n …which depends on T ice n Higher pressure will cause net condensation, lower will cause net sublimation l If ice is to be sustainable over solar system history then it must be delivered at the same rate it’s sublimated. l Water leaves cold traps by sublimation n 5-15% returns on Mercury n 20-50% returns on the Moon n The rest is lost l Water can be delivered by meteors and comets n For Mercury these rates have been estimated n Balance exists if T ice is ~113K Killen et al., 1997 l Moon/Mercury differences n Mercury’s ice deposits were easily detected n Lunar ice is probably not abundant – barely detected n Mercury may have experienced a recent impact that delivered a lot of water

PYTS 411 – Vacuum Processes 29 l Regolith Generation n Turnover timescales n Megaregolith l Space Weathering n Impact gardening n Sputtering n Ion-implantation l Volatiles in a Vacuum n Surface-bounded exospheres n Volatile migration n Permanent shadow Gaspra – Galileo mission