Martian South Polar Deformation and Sublimation Processes

Introduction: The first glacial deformations of an extraterrestrial ice sheet have been discovered and documented at the Martian south polar cap in Mars Orbital Camera (MOC) images [1-4]. Within the south polar cap various structural deformational features have been found, including boudinage, folds, and faults [1-2] (Fig. 1). These features are also found in terrestrial glaciers [5-7]. Mars presents a unique environment where there is a known existence of dry ice (CO 2) and water ice and, possibly, either a metastable coexistence of these two ices or stable CO 2 clathrate hydrate. The south polar cap is laterally heterogeneous in these ices (at least the first two) and vertically structured with a layered sequence of what may be interbedded ices. This is a form of glacier that is not known on Earth. In addition to boudinage, folds, and faults, distinctive forms of ice and dry-ice sublimation features exist. A comparison of these features with terrestrial glacial features and thermokarst will be particularly interesting, as they were formed under different gravi-tational conditions, different climatic environments, and in a variety of ice comp ositions. Sublimation erosion of the layered sequence of ices (H 2 O, CO 2 , and possibly CO 2 clathrate hydrate) [8] allows us to observe the internal structure of an active polar cap. Kargel and Tanaka [1-2] note that some of these layered ices are deformed. Faults, folds, boudinage, thrust faults, and elastic flexural bulges are common features of deformation in the south polar ice cap. Ice also undergoes brittle failure under certain temperature and stress conditions, as indicated by ice deformation experiments in the lab and by formation of leads in arctic sea ice and crevasses in glaciers [9]. Martian polar caps also exhibit faults, fractures, and rarely crevasses that are indicative of brittle failure [2]. Each ice type has unique properties that cause different stress responses and thus result in differing deformation behavior [10-14]. CO 2 clathrate hydrate is the strongest of the three ices and is suspected to be the unit which forms boudins, water ice is moderate in strength, but will form boudins if it is the strongest ice present during deformation, and CO 2 ice is the weakest of the three and does not form boudins.