PTYS_411_511_4_vacuum_processes - Surface Processes Acting...

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PTYS 411/511 Geology and Geophysics of the Solar System Shane Byrne – shane@lpl.arizona.edu Background is from NASA Planetary Photojournal PIA05578 Surface Processes Acting On Airless Bodies
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PYTS 411/511 – Vacuum Processes 2 Regolith Generation Turnover timescales Megaregolith Space Weathering Impact gardening Sputtering Ion-implantation Volatiles in a Vacuum Surface-bounded exospheres Volatile migration Permanent shadow In This Lecture Gaspra – Galileo mission
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PYTS 411/511 – Vacuum Processes 3 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
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PYTS 411/511 – Vacuum Processes 4 Maximum thickness related to depth of largest crater to reach equilibrium: Estimate D eq from a size-frequency plot or solve (note: surface must saturate so b must be >2) If equilibrium ~4% of geometric saturation then c eq =0.046 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 Growth of Regolith 4 eq eq D h = ( 29 ( 29 2 2 1 2 , , - - - - = = = = = = b eq eq b eq eq sat eq eq sat b cum D c c or c c D so N N D D at D c N and cD N Shoemaker et al., 1969
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PYTS 411/511 – Vacuum Processes 5 Minimum regolith thickness more complicated: Figure out the fractional area (f c ) covered by craters D→D eq where (D < D eq ) Choose some D min where you’re sure that every point on the surface has been hit at least once Typical to pick D min so that f(D min ,D eq ) = 2 h min of regolith ~ D min /4 General case Probability that the regolith has a depth h is: P(h) = f(4h→D eq ) / f min Median regolith depth <h> when: P(<h>) = 0.5 Time dependence in h eq or rather D eq α time 1/(b-2) ( 29 ( 29 2 1 min min 1 2 4 - - + - = b eq eq c b f b h h π ( 29 2 1 2 min 1 2 - - - + = b b eq eq h h h h
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PYTS 411/511 – Vacuum Processes 6 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 Regolith turnover Shoemaker defines as disturbance depth (d) time until f(4d, D eq ) =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
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PYTS 411/511 – Vacuum Processes 7 Ponding of regolith – seen on Eros
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PTYS_411_511_4_vacuum_processes - Surface Processes Acting...

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