Martian gullies: a comprehensive review of observations, mechanisms and insights from Earth analogues

Abstract

Upon their discovery in 2000, martian gullies were hailed as the first proof of recent (<a few Ma) 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 CO2. 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 CO2-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) 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 martian gullies may have formed by a combination of processes. Recent scaled physical models under Mars pressures have revealed that the metastable nature of water on Mars means that more sediment transport could occur than might be expected from stable water (Herny et al., 2018; Massé et al., 2016; Raack et al., 2017). Therefore, the main argument against the meltwater hypothesis-that melting surface frost cannot produce enough water for surface flow-may be somewhat ameliorated if these processes are indeed active.

Glacial and gully erosion on Mars: A terrestrial perspective

“…Does this suggest a role of water in the formation of martian gullies? Unfortunately, the findings reported in this paper do not provide any resolution to the ongoing debate between CO2 sublimation (e.g., Dundas et al, 2017, and references therein) and meltwater generation (e.g., Conway et al, 2018bConway et al, , 2018a, and references therein) for forming martian gullies. A meltwater hypothesis for this recent glacial erosion event on Mars leads to the reasonable assumption that gullies can be produced via a similar yet less widespread mechanism, which has been occurring periodically since this glacial erosion event.…”

“…A meltwater hypothesis for this recent glacial erosion event on Mars leads to the reasonable assumption that gullies can be produced via a similar yet less widespread mechanism, which has been occurring periodically since this glacial erosion event. However, CO2 and H2O ices are often found together (Vincendon et al, 2010;e.g., Vincendon, 2015), and without more detailed knowledge on how CO2 sublimation might engender sediment transport assessing whether the morphological evidence fits with this transport mechanism remains difficult (Conway et al, 2018b).…”

Initiation and Flow Conditions of Contemporary Flows in Martian Gullies

“…Mixing within the falling material will allow the generated heat to be transferred to the CO 2 frost, causing sublimation, enhancing the gas pressure within the pores of the flowing mass leading to fluidization of the flow. Complete fluidization is not necessary to explain all recent Martian gully flows, as the gully fans are moderately steep, roughly ranging between 10 • and 15 • (Conway & Balme, 2016;Gulick et al, 2019), and sometimes consistent with no fluidization (Kolb et al, 2010;Conway & Balme, 2016), and it is likely that a spectrum of behaviors occurs even within individual gullies (e.g., Conway et al, 2018b;De Haas et al, 2017).…”

Periglacial gully erosion on the east European plain and its recent analog at the Yamal peninsula

“…Interest in the formation of such ravine-like features formed in the absence of water has been renewed due to planetary studies. Numerous attempts to find terrestrial analogs for the gullies on Mars (see, for example, review by Conway et al 2019) showed that water-lacking processes (including ground ice melting) can produce only small gully-like features, which cannot be compared with large gully nets formed by water erosion. A general degradation of permafrost on the East European Plain with former periglacial conditions left numerous remnants of thermokarst features in recent relief morphology (Velichko 1965).…”