SEASONAL VARIATION OF THE COLD AND BRIGHT ANOMALIES ON THE NORTH POLAR RESIDUAL CAP

Abstract:

The north polar region of mars is one of the most active regions on the planet, with seasonal surface activity seen on the scale of 5000 km down to a few meters. These seasonal activities affect the entire planet during the formation and sublimation of the seasonal frost cap that changes the atmospheric pressure and composition. This project focuses on the Cold And Bright Anomalies (CABA), regions that appear of the north polar residual cap (NPRC) that, during spring and summer, ~20 K colder than their surroundings. Equally anomalous, during late summer, the CABA quickly darken in three distinct darkening events and ~3 K warmer than their surroundings.

For this project, we used large-scale visual observations from Mars Color Imager (MARCI) to determine the albedo of the CABA to compare with a non-anomalous region. This was combined with temperature and small-scale visual observations from Thermal Emission Imaging System (THEMIS) and local topographical observations from Mars Orbiter Laser Altimeter (MOLA).

Following work by others, we find that during late spring, the CABA warm more slowly and remain brighter than their surroundings. This is likely due to small frost grains that are not removed, whereas they are removed in non-anomalous regions. Modeling results find that weaker surface winds are present at the CABA regions in spring compared to elsewhere, perhaps explaining the reduced ablation. Towards late summer, the differences between the two regions tends towards zero because of a period of refrosting, where the entire cap becomes covered by small-grained water ice.

During late summer, darkening events dominate the NPRC. These events coincide temporarily and spatially with major dust storms emanating from the NPRC. We find that the darkened CABA are located on local topographical raises that may enhance the lifting of frost and dust by strong late summer winds, that strip surface ice and expose a warm dust layer underneath. The winds during late summer coincide with large transient eddies and differ in origin from the late spring surface winds, resulting in differing spatial patterns of frost and dust removal. Future work will simulate the relationship between the CABA and the winds using the Mars Mesoscale Model.