Abstract We reanalyse the global distribution of gullies in order to provide a set of observational constraints that models of gully formation must explain. We validate our results derived from the global data with four detailed case studies. We show that the availability of steep slopes is an essential factor to consider when assessing the spatial distribution and abundance of gullies. When the availability of steep slopes is taken into account, it reveals, with a few exceptions, that gullies are found almost uniformly across the whole 30°–90° latitude band. Our analysis also reveals that massive ice deposits are anti-correlated with gullies, and that the undulations in the equatorwards limits of the gully distribution could be explained by longitudinal variations in maximum surface temperatures (controlled by variations in surface properties, including thermal inertia and albedo). We find a sharp transition in both hemispheres between pole-facing gullies, which extend from 30° to 40°, to a more mixed, but dominantly equator-facing orientation of gullies polewards of 40°. We have no definitive explanation for this transition but, based on previous studies, we suggest it could be linked to the availability of near-surface ice deposits.

Abstract Gasa crater has been the most active site observed on Mars to date, making it of particular interest for studying the process(es) behind gully formation and activity. In this study, we investigate whether differences in thermal inertia across different segments of gully systems, combined with morphological and colour observations with High-Resolution Imaging Science Experiment (HiRISE), can provide some constraints on the physical characteristics associated with recent activity within gullies in Gasa. We also investigate thermophysical differences between slopes in Gasa dominated by gully activity compared to those predominantly modified by dry mass-wasting processes. We find that Gasa crater exhibits clear variations in thermal inertia across its walls, controlled by the material properties and the types of dominant mass movement processes occurring on each wall. The youthful gully-fan lobes display thermal inertia values c. 20–40 J m −2 K −1 s −1/2 higher than adjacent older eroded and dust-covered lobes. The talus aprons from mass wasting in Gasa have thermal inertia values c. 60–80 J m −2 K −1 s −1/2 higher than gully aprons. The results of this study thus suggest that thermal imaging can inform us on surface change detection on Mars.