Geomorphological analysis of gullies on the central peak of Lyot Crater, Mars

The central peak region of Lyot Crater provides an intriguing case study of Martian gully formation on local topographic highs. To better understand how these gullies formed, we carried out a detailed morphological analysis using a HiRISE stereo image pair and a digital terrain model. Gully lengths range from c. 2.2 to 4.3 km, with maximum depths from 17 to 54 m. Alcove slopes range from c. 20 to 22°, channel slopes range from 12 to 16° and apron slopes range from 10 to 14°. In general, these slopes are much lower than both the angle of repose (c. 33°) required to initiate dry flows and the apex slope required to keep these flows from depositing (>21°) under Mars gravity. We find that the observed gully morphology and spatial associations are consistent with an origin by liquid flow. Apron volumes are c. 10-40% of the gully volume, which we suggest indicates significant volatile loss (water and/or CO 2) in the slope materials in which they formed because apron volumes emplaced by dry gravitational flows would equal or exceed the gully volumes due to their lower packing density. These observations, coupled with the gullies' unique micro-environmental setting, lead us to favour a fluvial origin. In addition, we find that the gullies formed almost exclusively on the western slope region, which suggests a possible orographic component. Potential water sources may have been supplied locally in the recent geological past by dry winds blowing across an ice-covered lake situated in a low elevation area just west of the central peak or more globally during periods of higher obliquity. In either case, the integration of morphometric and morphological investigations suggests that these gullies probably formed by surface runoff and through-flow from snow- or icepack melting on the central peak of Lyot Crater.