Formation and evolution of cold and bright anomalies and warm and dark anomalies on the north polar layered deposits

Abstract

The Cold and Bright Anomalies (CABAs) and Warm and Dark Anomalies (WADAs) on the North Polar Layered Deposit (NPLD) are distinct seasonal phenomena on Mars. CABAs, which are colder and brighter during early summer, and WADAs, which are warmer and darker in late summer, were analyzed across Mars Years (MYs) 29–35, focusing on solar longitudes (L_s) between 30° and 170°. Investigating temperature, albedo, wind patterns, and topography at multiple sites, we found that CABAs emerge around L_s = 78°, coinciding with peak katabatic wind activity, reach maximum difference from non-anomalous sites at L_s = 110°, and gradually dissipate by L_s = 140°. During late summer, after L_s = 140°, we observe three significant darkening events, approximately 10° of L_s dates apart, during which CABAs transition into WADAs, with albedo reductions of up to 38 % and intense regional dust activity. WADAs align spatially with bright halos (circumferential remnants of CABAs) surrounding elevated terrain (10–120 m). With mesoscale modeling, we find that WADAs form when transient eddies are prominent across the region, suggesting that these transient eddies strip away the CABA bright fine-grained surface ice, exposing darker larger-grained subsurface layers. The repeatable seasonal events begin in late spring, when the CABA sites experience weaker katabatic winds than nearby regions. This reduces wind shear and thus ice loss, allowing them to retain their brightness and cooler temperatures and act as cold traps. Late, during early summer, cold trapping further enhances these anomalies by condensing fresh ice, slowing the warming process. This dynamic interplay between atmospheric conditions, ice deposition and loss, and topography signifies the complexity of Martian polar climates and their evolution.