PTYS 554

Evolution of Planetary Surfaces

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

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)

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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 Cosmic ray exposure ages on Moon

10cm in 500 Myr

About 105 yrs to remove

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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)

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Sharp boundaries between mare and highlands are maintained over Gyr Little lateral mixing E.g. Tsiolkovsky Crater

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What make the lunar landscape look so smooth?

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Phobos

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..and other airless bodies

Vesta Deimos

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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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New impacts and crater rays darkened over time by space weathering

Kuiper Crater, Mercury

Kramer et al, JGR, 2011

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Kramer et al, JGR, 2011

Lunar swirls High albedo patches Associated with crustal magnetism Most are antipodal to large basins

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Kramer et al, JGR, 2011

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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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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