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.



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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