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Vacuum Processes

Vacuum Processes. 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.

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Vacuum Processes

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  1. Vacuum Processes

  2. 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

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

  4. Impacts create regoliths

  5. Geometric saturation • Hexagonal packing allows craters to fill 90.5% of available area (Pf) • In reality, surfaces reach only ~4% of this value Log (N) Log (D)

  6. 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

  7. 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 • 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

  8. 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 Deqα time1/(b-2)

  9. 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

  10. 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)

  11. Transport is slope dependent • For ejecta at 45° on a 30° slope • Downrange ~ 4x uprange • Net effect is diffusive transport Downhill

  12. 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

  13. 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

  14. 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

  15. Lunar agglutinate

  16. 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 Clark et al., Asteroids III • Smaller asteroids (in general) are the result of more recent collisions – less weathered • Material around impact craters is also fresher Clark et al., Asteroids III 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 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

  17. Clark et al., Asteroids III • 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

  18. 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

  19. 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 >> 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

  20. Atmosphere, partial pressure P and temperature T • 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

  21. 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

  22. 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

  23. Night Day • Permanent shadow produces low temperatures • Some areas of permanent illumination as well Paige et al., 2010

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

  25. 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

  26. 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 • 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 Killen et al., 1997

  27. 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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