Geomorphic Evidence for Permafrost on Mars

Mars is currently dry and cold, with year-average surface temperatures well below the freezing point of water. By definition, it is a permafrost planet, and the geomorphic evidence for permafrost and ice-related surface and near-surface processes is rapidly growing. Images with resolutions as good as ~30 cm/pixel reveal a rich inventory of landforms that are morphologically analogous to terrestrial cryogenic features. Basically all known terrestrial glacial and periglacial phenomena have been hypothesized for the case of Mars, including thermal contraction cracks, solifluction lobes, pingos, thermokarst features, rock glaciers, and glaciers with associated landforms such as crevasses, trim lines and moraines. Other landforms and processes, e.g., debris flows and associated erosional and depositional features, have been linked by some authors to snow and/or ice melt and would also belong to the realm of the cryosphere. Finally, there are certain classes of surface features (e.g., so-called “thumbprint terrain”) that do not have terrestrial morphological analogues, but may be related to subsurface volatiles such as water ice. Most of these phenomena appear to be relatively young, based on the paucity of superposed impact craters and the pristine morphology. Some rock glacier-like features display surface ages of a few hundred mission years, but some debris flows in young craters may be less than a million years old. Nevertheless, the exact dates and in particular the relative ages of some of the features are still poorly known. Some of the features are observed in close spatial vicinity and in an apparent chronological sequence, raising the question whether a change in climate (perhaps episodic or even periodic) was responsible to trigger their formation. An attractive hypothesis is that changes in the planet’s obliquity and orbital parameters (e.g., eccentricity) forced climatic changes that redistributed volatiles on a global scale. If that was indeed the case, then it may be feasible to reconstruct climate cycles and associate them to the sequential formation of specific landforms (analogous to glacial cycles on Earth). This approach requires a good understanding of the relative stratigraphy between cryogenic landforms. In our review, we present examples of Martian permafrost features from the northern lowlands and from craters in the southern highlands, which we compare to terrestrial lowland and mountain permafrost phenomena, respectively. We will present some ideas on how glacial, periglacial, and paraglacial scenarios could be combined to produce the observed assemblage of landforms on Mars and their inferred chronological sequence. We will also outline possible tests of the climate change hypothesis to further constrain the recent volatile history of Mars