Conceptual Model for the Removal of Cold‐Trapped H2O Ice on the Mars Northern Seasonal Springtime Polar Cap.

The transport of H2O ice along the retreating north polar seasonal CO2 ice cap has previously been modeled and observed. Spectral observations show that H2O ice forms on the interior of the seasonal cap, while thermal observations show these regions to be consistent with CO2 ice. Prior to the sublimation of the seasonal CO2, the observed H2O ice deposits are diminished—and because H2O ice sublimation rates are extremely slow while in direct thermal contact with CO2 ice, an alternate removal process must be operating. We propose a model where the process of removing these H2O deposits starts with insolation‐induced basal sublimation of the underlying CO2 ice. This sublimed gas would "seep" upward and into the interface between the two ices, increasing pressure until the gas pressure fractures the cold‐trapped H2O ice. Small fragments would be suspended while larger fragments would be pushed aside, exposing the underlying CO2 ice. Plain Language Summary: The atmosphere on Mars is 95% CO2. Over the winter polar region, which is in complete darkness, temperatures drop to the point where the atmosphere itself condenses onto the surface as either CO2 ice or snow. A small amount of H2O is mixed in as part of the seasonal ice. In spring, sunlight returns, and the CO2 ice starts to sublime. H2O, while only a small component in the atmosphere, interacts with the CO2 ice and becomes cold‐trapped. Once cold‐trapped, the H2O ice sublimation rate is constrained while remaining in direct thermal contact with the much colder CO2 ice. Despite this, observations in the late spring show that the H2O ice is being removed while the CO2 ice remains. We present a model where the H2O ice is removed by the activity of subliming CO2 ice. Key Points: We propose a model where the process of removing H2O deposits starts with insolation‐induced basal sublimation of the underlying CO2 iceWe used images of dawn and dusk polar clouds as a tracer for the H2O cycle over the Mars north polar seasonal capH2O vapor, cycled in from the midlatitudes, forms the observed clouds and also becomes cold‐trapped on the seasonal CO2 ice [ABSTRACT FROM AUTHOR]