Decameter-scale rimmed depressions in Utopia Planitia: Insight into the glacial and periglacial history of Mars

摘要

Currently, Mars appears to be in a frozen state, with the clear majority of the planet's surface maintaining yearround sub-zero temperatures. The discovery of features consistent with landforms found in periglacial environments on Earth, suggests a recent climate history for Mars dominated by the presence of permafrost and/or freeze thaw cycling. Landforms indicative of such periglacial activity include hummocky, polygonized, scalloped, and pitted terrains in the mid-to high-latitudes of both hemispheres. The detection of near-surface and surface ice by the Phoenix lander, excavation of ice by recent impacts, complemented by interpreted results from the SHAllow RADar (SHARAD) instrument, further unveil a landscape in the mid-latitudes enriched in subsurface ice. We report here on the landscape analysis of a region within Utopia Planitia, where we aim to assess the influence of periglacial processes in the formation of a new landform that we have dubbed 'Decameter-scale Rimmed Depressions' or DRDs for short. These features are small-scale depressions surrounded by a narrow rim, ranging from 1 m to no higher than a few metres in height. We categorized 3 different DRD morphologies: ellipse morphologies, labyrinth, and tear-drops. DRDs share many morphological similarities with so-called "brain terrain", although unlike "brain terrain", most of the DRDs we mapped occur on intra-crater plains and not in association with glacial flow features. The presences of isolated fully enclosed ellipse morphologies is also a notable difference. We discuss the possibility that "brain terrain" and DRDs are genetically different features, but given the similarity in morphology and geographic distribution, we find this unlikely. Thus, while we cannot rule out an origin of DRDs through periglacial sorting, we favour a process for DRD formation similar to that proposed "brain terrain", whereby ground ice, likely in the form of buried glacial ice, underwent thermal contraction and differential degradation, resulting in the formation of the landform that we see today. DRDs thus represent a further morphological marker for the presence of ground ice in the northern plains of Mars.