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Distilling the Effects of Ballistic Fractionation on the Moon and Ceres

Presentation #411.01 in the session Asteroids: Main Belt (Poster)

Published onOct 23, 2023
Distilling the Effects of Ballistic Fractionation on the Moon and Ceres

Volatile compounds like water are stable near the poles of low obliquity airless bodies like the Moon and Ceres for billions of years, providing a unique record of solar system history. One proposed mechanism for delivering volatiles to the polar regions is migration through a series of thermal hops [1]. Molecules are unstable on illuminated terrain at low latitudes and eject from the surface on ballistic trajectories until they are lost to space or land in a cold, permanently shadowed region where sublimation rates are negligible on Gyr-timescales.

The fingerprint of volatile species present within permanently shadowed regions today holds clues to the timing and source of water delivery to the inner solar system, but those origins must be disentangled from the processes governing emplacement and erosion of polar ice deposits [2]. Here we discuss the potential for “ballistic fractionation”, the preferential removal of lighter compounds through gravitational escape and subsequent concentration of heavier compounds in cold traps.

We present results from a Monte Carlo volatile transport model, e.g., [3], applied to the Moon and Ceres, in which particles are tracked through gravitational escape, photodissociation, and cold trapping. Cold trapping fractions are determined for an array of molecules typically found in comets. We show that ballistic fractionation can significantly alter the relative abundance of volatiles that accumulate in the permanently shadowed regions, concentrating trace species like CO2 and masking the original source composition. This process is likely to be more efficient on small, cold bodies like Ceres, where the surface gravity is lower.

References

[1] Watson K., Murray B., and Brown H., JGR, 66, 1598 (1961)

[2] Mandt, K. E. et al., Nat Comm., 13, 642 (2022)

[3] Schörghofer, N. et al., ApJ, 850, 85 (2017)

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