PowerPoint PresentationSlide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Lunar agglutinateSlide 20Slide 21Slide 22Slide 23Slide 24Slide 25Slide 26PTYS 554Evolution of Planetary SurfacesVacuum ProcessesVacuum ProcessesPYTS 554 – Vacuum Processes2Regolith GenerationRegolith growthTurnover timescalesMass movement on airless surfacesMegaregolithSpace WeatheringImpact gardeningSputteringIon-implantationGaspra – Galileo missionPYTS 554 – Vacuum Processes3All rocky airless bodies covered with regolith (‘rock blanket’)Moon - Helfenstein and Shepard 1999Itokawa – Miyamoto et al. 2007Eros – NEAR spacecraft (12m across)PYTS 554 – Vacuum Processes4Impacts create regolithsPYTS 554 – Vacuum Processes5Geometric saturationHexagonal packing allows craters to fill 90.5% of available area (Pf)In reality, surfaces reach only ~4% of this value  NSATArea( )=Pf4pD2=1.15D- 2or log NSAT/ Area( )=- 2log D( )+logPf4pæ è ç ö ø ÷ Log (D)Log (N) NSATArea( )=EfPf4pD2=0.046 D- 2NSATArea( )=ceqD- 2PYTS 554 – Vacuum Processes6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 Deqµ t1b- 2 Ncum=cD- band Nsat=ceqD- 2at D =Deq, N =Nsat, so Deq=cceqæ è ç ö ø ÷ 1 b- 2( )or c =ceqDeqb- 2( )impliesPYTS 554 – Vacuum Processes7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/4Shoemaker et al., 1969Growth of RegolithPYTS 554 – Vacuum Processes8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) hmin=heq4 b- 2( )fminpbceq+1é ë ê ù û ú - 1b- 2( )h =heq12heqhminæèçöø÷b- 2+1æèççöø÷÷éëêêùûúú- 1b- 2( )PYTS 554 – Vacuum Processes9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 Moon10cm in 500 MyrAbout 105 yrs to removePYTS 554 – Vacuum Processes10Regolith modeled as overlapping ejecta blanketsNumber of craters at distance r (smaller than D=2r)Contributes ejecta of thicknessWhere ejecta thickness is:Results (moon, b=3.4)¶T¶t(>r) =6.8r- 0.66m/ Gyrd r, D( )=aD0.742rDæèçöø÷- 3dT >r( )=- 2pr dr d r, D( )-dNcumdDæèçöø÷02ròdDPYTS 554 – Vacuum Processes11Sharp boundaries between mare and highlands are maintained over GyrLittle lateral mixingE.g. Tsiolkovsky CraterPYTS 554 – Vacuum Processes12What make the lunar landscape look so smooth?PYTS 554 – Vacuum Processes13PhobosPYTS 554 – Vacuum Processes14..and other airless bodiesVestaDeimosPYTS 554 – Vacuum Processes15Transport is slope dependentFor ejecta at 45° on a 30° slopeDownrange ~ 4x uprangeNet effect is diffusive transportDownhillPYTS 554 – Vacuum Processes16Ponding of regolith – seen on ErosRegolith grains <1cm move downslopePonded in depressionsPossibly due to seismic shaking from impactsMiyamoto et al. 2007Robinson et al. 2001PYTS 554 – Vacuum Processes17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 mariaPYTS 554 – Vacuum Processes18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 isreducedObjects get darker and redder with timeSpace WeatheringPYTS 554 – Vacuum Processes19Lunar agglutinatePYTS 554 – Vacuum Processes20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 missionClark et al., Asteroids IIIClark et al., Asteroids IIIPYTS 554 – Vacuum Processes21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 reddeninge.g. Fe2Si Hapkeite (after Bruce Hapke)SputteringEjection of particles from impacting ionsSolar-wind particlesH and He nuclei Traveling at 100’s of Km s-1Warped Archimedean spiralImplantation of ions into surface may explain reduced neutron countsClark et al., Asteroids IIIPYTS 554 – Vacuum Processes22New impacts and crater rays darkened over