PTYS 554 Evolution of Planetary Surfaces Vacuum Processes

PYTS 554 – 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 Gaspra – Galileo mission

PYTS 554 – 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)

PYTS 554 – Vacuum Processes 4 l Impacts create regoliths

PYTS 554 – 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 554 – 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 554 – 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 554 – 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 554 – 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 n Cosmic ray exposure ages on Moon 10cm in 500 Myr About 10 5 yrs to remove

PYTS 554 – Vacuum Processes 10 l Regolith modeled as overlapping ejecta blankets n Number of craters at distance r (smaller than D=2r) n Contributes ejecta of thickness n Where ejecta thickness is: n Results (moon, b=3.4)

PYTS 554 – Vacuum Processes 11 l Sharp boundaries between mare and highlands are maintained over Gyr n Little lateral mixing n E.g. Tsiolkovsky Crater

PYTS 554 – Vacuum Processes 12 What make the lunar landscape look so smooth?

PYTS 554 – Vacuum Processes 13 Phobos

PYTS 554 – Vacuum Processes 14 l..and other airless bodies Vesta Deimos

PYTS 554 – Vacuum Processes 15 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 554 – Vacuum Processes 16 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 554 – Vacuum Processes 17 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 554 – Vacuum Processes 18 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 554 – Vacuum Processes 19 Lunar agglutinate

PYTS 554 – Vacuum Processes 20 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 554 – Vacuum Processes 21 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 554 – Vacuum Processes 22 l New impacts and crater rays darkened over time by space weathering Kuiper Crater, Mercury Kramer et al, JGR, 2011

PYTS 554 – Vacuum Processes 23 Kramer et al, JGR, 2011 l Lunar swirls n High albedo patches n Associated with crustal magnetism n Most are antipodal to large basins

PYTS 554 – Vacuum Processes 24 Kramer et al, JGR, 2011

PYTS 554 – Vacuum Processes 25 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 554 – Vacuum Processes 26 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