Geomorphological characterisation and interpretation of a mid-latitude glacier-like form: Hellas Planitia, Mars

We describe and interpret the surface terrain types associated with a widely-reported ∼4. km long, mid-latitude martian viscous flow feature (VFF). The feature is located in the southern hemisphere, on the poleward-facing rim of a ∼60. km-diameter crater in eastern Hellas Planitia. High Resolution Imaging Science Experiment (HiRISE) images, analysed in both 2D and 3D, reveal that the upper margin of the feature is bounded by steep (∼30°) headwalls, typically some tens of metres high, that are formed from unconsolidated material and characterised by a series of slope-parallel linear incisions. Below these incised headwalls, the feature flows at a general angle of ∼10° from a broad upper basin to a confined lower tongue that is bounded by a nested sequence of elongate raised ridges. These characteristics are typical of several VFFs in the region and are strikingly similar to moraine-bounded valley glaciers on Earth, and we sub-classify this feature as a 'glacier-like form' (GLF). 1This terminology contrasts slightly with Hartmann's original usage in that we refer to the feature as a 'form' rather than a 'flow'. This preference is based on several considerations, including that: (i) although inferred, flow has not yet been measured at such features, (ii) supplementary (basal) motion components cannot yet be ruled out, and (iii) the term 'flow' can refer to both a form and a process of ice-mass motion, which could lead to confusion.¹. The GLF comprises five distinctive surface terrain types that contrast sharply with surface characteristics outside its bounding moraines. Four of these terrains (scaly terrain, polygonized terrain, linear terrain and mound-and-tail terrain) are located within the GLF's innermost bounding moraine, while the fifth (rectilinear-ridge terrain) is located between its frontal moraines. These terrains are mapped, characterised and associated with possible mechanisms of formation to draw inferences about the GLF's glaciology and glacial history. This analysis suggests that the GLF reached its maximal extent in the geologically-recent past, and that it may have been partially wet-based at that time. Subsequent to this phase, the GLF experienced an extended period of general recession that has been punctuated by several episodes of still-stand or advance. Currently, the GLF's basin appears to be composed of a lower zone that is dominated by an exposed former glacier bed and an upper zone that may still contain a now-degraded and dust-mantled viscous mass, similar to many partially-glacierized basins on Earth.