Vacuum Processes

PTYS 411

Geology and Geophysics of the Solar System

Vacuum ProcessesVacuum Processes

PYTS 411 – 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


All rocky airless bodies covered with regolith (‘rock blanket’)

Moon - Helfenstein and Shepard 1999 Itokawa – Miyamoto et al. 2007

Eros – NEAR spacecraft (12m across)

Miyamoto et al. 2007


Impacts create regoliths


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)


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


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


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)


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


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)




Transport is slope dependent

For ejecta at 45° on a 30° slope Downrange ~ 4x uprange

Net effect is diffusive transportD

ow

nh

ill


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


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


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


Lunar agglutinate


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



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



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


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

Volatiles in a Vacuum


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

Atmosphere, partial pressure Pand temperature T


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


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


Permanent shadow produces low temperatures Some areas of permanent illumination as well

Paige et al., 2010

Day Night


Modeling (Vasavada et al. 1999) shows temperatures in permanently shadowed craters are very low for Mercury too

These cold traps are favored condensation sites

Vasavada et al., 1999

Moon

Mercury


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


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

Killen et al., 1997 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


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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Vacuum Processes