Seeing Below The Surface Of Mars: Volatile sublimation in the martian regolith

Patel, Narissa (2022). Seeing Below The Surface Of Mars: Volatile sublimation in the martian regolith. PhD thesis The Open University.

DOI: https://doi.org/10.21954/ou.ro.00013f22

Abstract

The discovery of buried carbon dioxide (CO₂) ice between water (H₂O) ice layers within the martian south polar layered deposits has renewed interest in subsurface CO₂ ice. In this thesis, subsurface CO₂ ice stability is explored using a 1-D thermal and vapour diffusion numerical model that simulates three phases of H₂O, two phases of CO₂, and adsorption of both for the first time.

Numerical experiments were run to examine how these two ices influence one another, under a variety of ice-layer configurations that are expected to be valid for Mars. The results demonstrate that an overlying near-surface H₂O ice-filled regolith layer increases subsurface CO₂ ice stability by an order of magnitude. This stability increases further with the addition of an underlying H₂O ice-filled regolith layer. The initial porosity and geological materials used to represent the subsurface also have a large influence on CO₂ ice stability. The porosity limits the vapour diffusion rate, while the geological materials influence thermal conductivity and, therefore, subsurface temperatures.

Simulations at different orbital obliquities demonstrate that CO₂ ice stability in the polar regions is greatest at low obliquities and smallest at high obliquities. The reverse is true for the equatorial regions. At higher obliquities (>45◦) and atmospheric pressures, the results suggest subsurface CO₂ ice deposition could occur in the equatorial region.

The model results suggest that a 0.7–27 km CO₂ ice layer could sublimate away while 1 m of low-porosity H₂O ice forms (in 14–550 kyr depending on method) in the south polar layered deposits. The results also suggest CO₂ ice sublimation is dependent on obliquity: ∼0.15 km sublimates at low obliquity and ∼1.9 km sublimates at high obliquity over 100 kyr.

The subsurface model is a useful tool for future investigations into the historical behaviour of ices on Mars, particularly during the Noachian period when the CO₂ frost-point temperature was higher.

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