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Overview. Motivation: Why study crater distributions?BackgroundCratering RatesMethods of Crater Degradation (Mars specific)ModelsDescriptionResults. Motivation. Craters investigated for?Dating/ Deducing geologic historyDeducing climate historyNumerical models allow predictions of observed c
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1. Overview of Martian Crater Degradation, including Simple Numerical Models Ge 151a
2004
Colette Salyk
2. Overview Motivation: Why study crater distributions?
Background
Cratering Rates
Methods of Crater Degradation (Mars specific)
Models
Description
Results
3. Motivation Craters investigated for
Dating/ Deducing geologic history
Deducing climate history
Numerical models allow predictions of observed crater fields given age and climate
4. Cratering Rates Determined by lunar crater counts
N(D,t) =C (1+2*10-6) Exp[4.5 (4.5-t)] D-3
3.6 Ga surface on moon has 188 craters
Martian production ~ 2 x lunar production
5. Degradation Mechanisms: Dust Deposition/Erosion Dust trapped in depressions
Sharp surface features eroded by wind
Rates depend on
Wind speed, direction, frequency
Gravity
Particle size
Geometry of topography
Linear deposition/erosion in time-averaged sense
Estimates give ~ 18 ?m/yr deposition
6. Degradation Mechanisms: Sublimation/Condensation Rates strongly dependent on surface temperature
Sublimation estimates: 0.2 pr. mm/yr from polar regions in summer
Similar winter condensation rates, if steady state
Degradation of craters caused by preferential condensation/sublimation due to shadowing
7. Degradation Mechanisms: Viscous Creep Viscous relaxation of material due to small stresses applied for a long time
Craters relax under influence of gravity
Relaxation time, ? = 4? ? /(? g D)
8. Model Description Specify area, time of start, end, ?t, rates of erosion/infill
Create craters using production equation
Calculate depths, rim heights
Allow craters to degrade, via creep, infill/erosion or both
Erase shallow craters
Increment time and repeat
9. Result 1: Viscous Relaxation Relaxation rate depends on diameter Few in upper right Many in lower left