Carbon Dioxide Ice within the Subsurface of Mars

We present initial results from the UK version of the LMD Mars Global Circulation Model using a newly developed subsurface model showing the present day distribution of subsurface carbon dioxide (CO₂) and water ice. A comparison of the modelled subsurface CO₂ distribution with either observations or previous models is not possible due to the lack of studies on its subsurface distribution. As a result, the present day subsurface water ice distribution is compared to subsurface water observations (which reach up to one meter in depth) and other models to test the accuracy of the results.

Studies of subsurface ice on Mars have mostly focused only on the distribution of water ice, because the amount of subsurface CO₂ ice present has been considered insignificant. This is because present day surface and subsurface temperatures are only temporarily low enough for the presence of CO₂ ice outside of the polar regions. Studies [e.g. 1, 2] have shown that water ice becomes more widespread across the surface at high obliquities. This is because of the increase in sunlight at the poles sublimating the CO₂ polar cap and allowing for the sublimation and equatorward transport of water [e.g. 2, 3]. Conversely, at low obliquities, studies show that the CO₂ polar cap is perennial and increases in size [e.g. 2, 3]. In all obliquity cases, it has been assumed that CO₂ can only occur as surface or subsurface ice at the poles, vapour within the atmosphere or adsorbed in the regolith. We have included CO₂ ice physics to account for the potential for subsurface CO₂ ice outside of the polar regions which has been neglected to date [e.g. 1, 4]. This could impact the modelled distribution of water ice at these obliquities and the expected locations for water ice deposits in the present day. Before a study of the distribution of subsurface CO₂ ice over the obliquity cycle can be undertaken, an understanding of the present day distribution of CO₂ ice in the subsurface is needed. We demonstrate that our model can be used to understand the present day subsurface CO₂ cycle and map regions of accumulation and loss of subsurface CO₂ ice throughout a martian year.

[1] Mellon, Jakosky, JGR:Planets 1995

[2] Head et al., Nature 2003

[3] Pollack, Toon, Icarus 1982

[4] Steele et al., Icarus 2017