Possible Atmospheric Water Vapour Contribution from Martian Swiss Cheese Terrain

Mars’ South Polar Residual Cap (SPRC) is a several meters thick CO₂ ice cap with a variety of morphological features, including quasi-circular depressions known as ‘Swiss cheese’ features, which could potentially expose water-ice through removal of the overlying CO₂. Swiss cheese features have been suggested as a source for the observation of unusually high water vapour during the southern summer of MY 8 (1969) (Barker et al, 1970).

We map the current extent of these features to estimate their contribution to atmospheric water vapour through sublimation and evaluate if these could be a source of the Barker et al. (1970) observation.

We use data from the Mars Reconnaissance Orbiter’s Context Camera and Mars Climate Sounder to map individual features within areas of Swiss cheese terrain and obtain surface temperatures to estimate areas of exposed water ice, and infer the amount of water vapour lost under typical south-polar summer conditions. We find that there is a negligible impact on total atmospheric water vapour from sublimation with the current coverage (0.2% of the total SPRC area) and temperatures (on average ~161 K) of Swiss cheese terrain. We determine that at typical current conditions, complete removal of the CO₂ layer from 91% of the total area of the SPRC would be required to sublimate enough water vapour to reproduce the MY 8 observation. However, the sensitivity of sublimation rate to temperature means that a relatively modest increase in temperature to ~193 K could lead to a dramatic increase in sublimation rate, requiring defrosting from only 2.4% of the SPRC area to recreate the MY 8 observation. These higher temperatures have been observed in surface water ice elsewhere on Mars, but such temperatures are likely transient enough within a season to not have noticeably increased the total sublimated water vapour from the south pole in the last ~30 Mars years.

Barker, E. S., Schorn, R. A., Woszczyk, A., Tull, R. G., & Little, S. J. (1970). Mars: Detection of Atmospheric Water Vapor during the Southern Hemisphere Spring and Summer Season. Science, 170(3964), 1308–1310. https://doi.org/10.1126/science.170.3964.1308