Abstract Martian gullies are widespread landforms in the mid-latitudes of Mars. When the first reports of these kilometre-scale features were published in 2000, they were controversially hailed as a sign of recent flows of liquid water on the surface of Mars. This supposition was contrary to our understanding of recent environmental conditions on Mars, under which water should not exist in its liquid form. In response to their discovery, researchers proposed a wide range of scenarios to explain this apparent paradox, including scenarios driven by CO 2 , climate change or the presence of a liquid water aquifer. This Special Publication is a collection of papers arising from the topics discussed at the Second International Workshop on Martian Gullies held at the Geological Society, London. A review paper opens the Special Publication and thereafter the papers are presented under three themes: Martian remote sensing, Earth analogues and laboratory simulations. This Special Publication establishes the state of the art in Martian gully research, presents the latest observations and interpretations of the present-day activity and long-term evolution of Martian gullies, explores the role of Earth analogues, highlights novel experimental work and identifies future avenues of research. The importance of gullies as a potential marker of habitable environments on Mars underlines their importance in framing space exploration programmes.

Abstract Upon their discovery in 2000, Martian gullies were hailed as the first proof of recent (i.e. less than a few million years) flowing liquid water on the surface of a dry desert planet. Many processes have been proposed to have formed Martian gullies, ranging from liquid-water seepage from aquifers, melting of snow, ice and frost, to dry granular flows, potentially lubricated by CO 2 . Terrestrial analogues have played a pivotal role in the conception and validation of gully-formation mechanisms. Comparison with the terrestrial landscape argues for gully formation by liquid-water debris flows originating from surface melting. However, limited knowledge of sediment transport by sublimation is a critical factor in impeding progress on the CO 2 -sublimation hypothesis. We propose avenues towards resolving the debate: (a) laboratory simulations targeting variables that can be measured from orbit; (b) applications of landscape-evolution models; (c) incorporation of the concept of sediment connectivity; (d) using 3D fluid-dynamic models to link deposit morphology and flow rheology; and (e) a more intense exchange of techniques between terrestrial and planetary geomorphology, including quantitative and temporal approaches. Finally, we emphasize that the present may not accurately represent the past and that Martian gullies likely formed by a combination of processes.