Martian gully activity and the gully sediment transport system

The formation process for Martian gullies is a critical unknown for understanding recent climate conditions. Leading hypotheses include formation by snowmelt in a past climate, or formation via currently active CO₂ frost processes. This paper presents an expanded catalog of >300 recent flows in gullies. The results indicate that sediment transport in current gully flows moves the full range of materials needed for gully formation. New flows are more likely to transport boulders in gullies that have pre-existing boulder-covered aprons, indicating that current flows are transporting the same materials required for gully formation overall. The distribution of gully activity frequencies can be described by a power law and indicates that the recurrence intervals for flows in individual gullies are commonly tens to hundreds of Mars years. Over the last ~300 kyr, climate variations have been modest but individual gullies have had tens to thousands of flow events. This could be sufficient to account for the entirety of gully formation in some cases, although the same processes are likely to have occurred further in the past. For any gullies that may have initiated under higher-obliquity conditions, this level of recent activity indicates that the observable morphology has been shaped by CO₂-driven flows. These observations of sediment transport and the tempo of gully activity are consistent with gully formation entirely by CO₂ frost processes, likely with spatial and temporal variability, but with no role required for liquid water.