SOUTH POLAR CO2 ICE ON MARS: REVISITING "SEASONAL" AND "RESIDUAL"

The uppermost surface of the southern polar ice dome has long been known to be carbon dioxide ice. In the Viking era, this high albedo unit was considered to be a "residual" or permanent ice cap, distinct from the seasonal CO₂ ice deposit that recurs annually when the pole is in winter. Called the residual south polar cap (RSPC), the broad outline of this high albedo unit has remained largely stable; however, strong erosion of its varied morphology was first observed in MOC images and monitored by HiRISE and CTX cameras over the following two decades. Past work has noted variable rates of erosion, new deposition, migration of linear trenches, debris accumulation, and relief inversion, all of which appear to be in approximate equilibrium consistent with a mass balance near zero. Recent work has found that the margins of the canonical high albedo unit are quite dynamic and in at least one year observed by CRISM (MY28), the upper surface retains a distinct spectral character early in the summer season that is most consistent with seasonal CO₂ ice. This suggests that the annual deposition and removal of CO₂ ice is not perfectly repeatable, even though the rates and timing of seasonal retreat are highly consistent between MY, except in years surrounding large dust storms. Thus, on local scales the RSPC is not a permanent or "residual" ice "cap" reflecting a largely unchanging surface that is left behind after the annual deposit sublimes. Nor does it reflect a wholly "seasonal" process of cycling with the current atmosphere each year. Rather, these upper layers record conditions over many years and climatic events. It may be more appropriate to refer to this as “multi-year” CO₂ ice, a transitional surface between annual deposition and sublimation events and burial and compaction of the CO₂ ice, which may underlie this upper surface and ultimately transition to the underlying water ice. The presentation will review evidence for the dynamic nature of this high albedo deposit and its evolution on seasonal and annual timescales.