Moats in the residual south polar cap of Mars: Ages, formation, and evolution

Abstract

The residual south polar cap of Mars (RSPC) is a region comprising tabular deposits of solid CO2 averaging a few m thick resting on layered, water-ice rich deposits a few km thick. Estimates of the ages of morphologic features in the RSPC have relied on average rates of change obtained from geographically limited samples. In this work we apply image coverage of the whole RSPC at 6 m/pixel spanning up to seven Mars years to determine how well ages of RSPC forms can be modelled from recent observations, and to explore implications of the calculated chronology. Topographic troughs separating CO2 layers of different ages and thickness, termed moats, are widespread, changing, and dateable features of the RSPC. Although they all develop by the same phenomena, we divide moats into two groups based on their linear dimensions and the span of relevant spacecraft imaging: 1) moats separating large areas of CO2 ice of different age and thickness, termed sinuous boundary moats, that imaging data show are related to post-1972 CO2 deposition. 2) moats separating the walls of pits from thinner, younger, interior mounds, termed pit moats; these are not resolved in early spacecraft images. Our survey of the entire summer RSPC spanning seven Mars years yields modelled formation times for both kinds of moats that cluster largely about Mars Years (MY) 11–13. A population of smaller moats is calculated to have formed near MY 26–28. These times closely follow planet-encircling dust events in MY 9, MY 12, and MY 28 (Earth years 1971, 1977, 2007). High-resolution images show that both pit and sinuous moats are initiated by slow downward erosion of debris left by retreating scarps of CO2 ice that are flanked by brighter, accumulating CO2 ice. We find that the most probable role of the dust events in moat formation is to clear thin CO2 layers from underlying water-ice-rich materials through thermal effects of the atmospheric dust content. The age results severely limit the magnitude of variations in moat scarp retreat rates during the last 25 MY. Pit moats have limited lifetimes because scarp retreat eventually removes the interior mound that defines the inner moat wall. Pits can reach much greater ages than moats; some pits formed prior to MY -100 (Earth year 1765).