Presentation #507.04 in the session Icy and Polar Mars.
The polar regions of Mars experience deposition of seasonal carbon dioxide ice in each respective winter that sublimates during spring and summer. The ice cap, known as the north and south polar seasonal caps, respectively, undergoes transformations in density and albedo as it metamorphoses due to thermal gradients caused by heat released from the substrate and from insolation. Outside of the polar night, sunlight may pass through the ice to warm the substrate, which warms the ice from below, potentially creating gas pockets that can pressurize enough to fracture the ice and entrain dust or sand to be scattered on the surface of the ice. This process of forming jets and fans is called the “Kieffer Model”, and our experiments that fracture the ice are able to constrain the thermal gradient required to create it.
Additionally, in our experiments, we recreate the conditions of the south pole of Mars, ~6 mbar and ~140 K, in the MARs Volatile and Ice evolutioN (MARVIN) chamber at York University in Toronto. Our experiments identify several phases of CO2 ice, including bright frost and transparent ice. We also observe crystalline structure in microscopes. We use reflectance spectroscopy, at 0° phase and with a home built goniometer to have ± 60° incident and reflected light to develop bidirectional reflectance distribution functions (BRDFs) for each phase.
Finally, we test various hypotheses related to the seasonal increase in albedo of the south polar seasonal cap that is not observed at the north polar seasonal cap. In the first scenario, internal fracturing scatters the sunlight, brightening the signal that returns to orbital cameras. In the second scenario, the surface of the ice has large facets that give rise to specular reflection of incoming solar radiation. This second scenario requires increasing surface pressure, a condition met at the south pole during southern spring but not met during northern spring. We observe these facets and surface brightening in our experiments that increase in surface pressure, supporting the second hypothesis, and possibly explaining the polar dichotomy.