Seasonal Variation of the Cold and Bright Anomalies on the North Polar Layered Deposits

Cold and Bright Anomalies (CABA) are cold and bright regions on the northern polar layered deposits that appear annually during early summer. Interestingly, during late summer, these regions rapidly darken in less than one Sol becoming some of the darkest locations on the residual H₂O cap and slightly warmer than their surroundings. In this study, we monitored one CABA at 87°N and 330°E and compared it to a nearby normal location that does not experience this anomalous behavior. We used the Mars Color Imager (MARCI) to build an albedo timeline of the two locations, thermal images from the Thermal Emission Imaging System (THEMIS) to build a temperature timeline as well as THEMIS visual images to observe small-scale features, and finally, the Mars Orbiter Laser Altimeter (MOLA) to identify local topographical features.

Early in the northern spring, the CABAs behave similarly to non-CABA regions – similar albedo values and warm at similar rates. Starting in late spring, L_s ~ 85°, the CABA location warms up at a slower but relatively linear than the normal location, but the albedo difference don’t appear until the summer solstice, L_s = 90°. The temperature and albedo differences peak around L_s = 110°, where the CABA is ~20°K cooler and ~18% brighter than the normal location. During late summer, there are three darkening events, during which the CABA’s albedo decreases by ~31% and becomes ~5°K warmer than the normal location. We noticed a bright halo centered at the darkened anomaly, and the anomaly is present on a local topographical raise of ~50m. Of note, a local dust storm appears at the same time as the darkening event and moves southwards from the CABAs.

We hypothesize that CABAs are regions that experience weaker surface winds during spring, resulting in more, brighter surface ice that is more reflective than the surrounding regions, creating temperature differences. Then, the temperature difference is reinforced by cold trapping, which condenses small-grained, highly reflective ice making these regions brighter than their surroundings. During late summer, we hypothesize that the surface winds driven by high-altitude eddies will shear the surface ice as they move over the local raise, exposing a warm dust layer underneath. The bright halos are regions where the winds are not fast enough to shear the ice, leaving reflective ice surrounding the darkened CABA. The locations of the CABAs, then, are dependent on local geology, especially surface slop, which drives the springtime winds