Obliquity-coupled Atmospheric Variations and the Small Cratering Record on Mars

Mathematics – Logic

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5407 Atmospheres: Evolution, 5420 Impact Phenomena (Includes Cratering), 5450 Orbital And Rotational Dynamics, 5470 Surface Materials And Properties

Scientific paper

Mars, at its current obliquity of 25.2 degrees, has an atmosphere of ~6.1 mbar (density of 0.021 kg/m3) at reference elevation. Several workers have shown that Mars experiences severe obliquity oscillations on time scales of less than 100 kyr resulting in cyclical growth and retreat of the polar CO2 ice caps. Our calculations and those of others indicate that this has a profound effect on the density of the predominantly CO2 atmosphere. For instance, a conservative treatment of orbital properties indicates obliquity oscillations ranging between 15 and 35 degrees generate atmospheric pressures of 0.3 and 30 mbar, respectively. Other models, taking into consideration resonance effects, predict minimum obliquities approaching zero. At such times, any CO2 would be cold-trapped at the poles and Mars' atmosphere would essentially vanish. At the other extreme, the maximum predicted obliquity of 51.4 degrees would eradicate the permanent CO2 polar caps and create an atmosphere of ~35 mbar (0.093 kg/m3). Higher atmospheric densities enhance atmospheric filtering so that fewer meter-scale impactors reach the surface ballistically and somewhat larger projectiles would produce craters with anomalous morphologies indicative of atmospheric breakup and dispersion. So, times of high obliquity will show a depletion of small craters, indicated by a down-turn in conventional size-frequency curves. During times of low obliquity, however, crater production would be more lunar-like. Consequently, production rates for the smallest resolvable martian craters show temporal variations unlike those expected for any other terrestrial planet. This effect should be considered when using such craters to understand recent geological history of Mars.

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