The formation of gullies on Mars today

Abstract

A decade of high-resolution monitoring has revealed extensive activity in fresh Martian gullies. Flows within the gullies are diverse: they can be relatively light, neutral, or dark, colorful or bland, and range from superficial deposits to 10-meter-scale topographic changes. We observed erosion and transport of material within gullies, new terraces, freshly eroded channel segments, migrating sinuous curves, channel abandonment, and lobate deposits. We also observed early stages of gully initiation, demonstrating that these processes are not merely modifying pre-existing landforms. The timing of activity closely correlates with the presence of seasonal CO2 frost, so the current changes must be part of ongoing gully formation that is driven largely by its presence. We suggest that the cumulative effect of many flows erodes alcoves and channels and builds lobate aprons, with no involvement of liquid water. Instead, flows may be fluidized by sublimation of entrained CO2 ice or other mechanisms. The frequent activity has likely erased any features dating from high-obliquity periods, so fresh gully geomorphology at middle and high latitudes is not evidence for past liquid water. CO2 ice-driven processes may have been important throughout Martian geologic history, and their deposits could exist in the rock record, perhaps resembling debris-flow sediments. Gully landforms on Mars resemble water-formed features on Earth, with channels transporting material from an alcove to a depositional apron. From their discovery (Malin and Edgett, 2000), they have generally been interpreted as evidence for wet debris flows or flowing liquid water (e.g., Carr, 2006). Such liquid would have major implications for Martian climate, geology, the possibility of life, and the definition of Special Regions for planetary protection (Rummel et al., 2014). Understanding the formation of gullies has thus been a major focus of recent Mars science, as shown by the work in this volume. Numerous hypotheses for gully formation have been considered. Martian surface conditions are not favorable for the existence of liquid water, so initial models focused on release of groundwater from shallow or deep aquifers (Malin and Edgett, 2000; Mellon and Phillips, 2001; Gaidos, 2001), possibly aided by geothermal heating melting permafrost (Hartmann, 2001; Hartmann et al., 2003) or the occurrence of brines (Knauth and Burt, 2002). However, the occurrence of gullies on sand dunes and isolated peaks argued against significant input from aquifers, and led to the development of models based on insolation-driven melting of snow or shallow permafrost at times when the

CO 2 sublimation in Martian gullies: laboratory experiments at varied slope angle and regolith grain sizes

“…As reported in Vincendon (2015) and Dundas et al (2017), activity in mid-latitude classic gullies is limited to periods when thin (microns to centimetres) and patchy CO 2 or H 2 O frost is present during, or just prior to the noted activity. So far, c. 67 such sites have been catalogued as active (Dundas et al 2017), which is c. 18% of monitored sites in the southern hemisphere, but activity is sporadic, rather than annual. As reported in Reiss & Jaumann (2003), Reiss et al (2010), Pasquon et al (2016;this volume, in review and Jouannic et al (this volume, in review), activity in linear dune gullies happens when the last CO 2 ice is disappearing from the pole-facing crest of the dune.…”

“…The remaining active gully sites comprise 'classic', mid-latitude gullies and linear dune gullies (Auld & Dixon 2016;Conway et al 2017). As reported in Vincendon (2015) and Dundas et al (2017), activity in mid-latitude classic gullies is limited to periods when thin (microns to centimetres) and patchy CO 2 or H 2 O frost is present during, or just prior to the noted activity. So far, c. 67 such sites have been catalogued as active (Dundas et al 2017), which is c. 18% of monitored sites in the southern hemisphere, but activity is sporadic, rather than annual.…”

“…For classic gullies, we must rely on orbital observations, with thermal inertia measurements suggesting that gullies reside in materials classed as unconsolidated 'sand-to pebble-sized grains' (Reiss et al 2009;Harrison et al 2014). Source areas for recent motions in classic gullies are usually diffuse, suggesting remobilization of a loose surface cover (Dundas et al 2017) and source areas of classic gullies range upwards from 20°( median 25°; Conway et al 2015). Results from our laboratory work and application of our analytical model reveal that these grain sizes and slope angles should be able to sustain the CO 2 sublimationtriggered failures that we have investigated in our experiments.…”

“…However, other features of recent (and past) activity, including the transport of metre-scale boulders, the formation of levees and lobate termini (e.g. Dundas et al 2010Dundas et al , 2014Dundas et al , 2017Johnsson et al 2014;de Haas et al 2015b) require some viscosity or fluidization of the flow beyond that of a simple granular flow. Further, in linear gullies the new morphologies are complex, encompassing albedo changes, formation of pits, channels with levees and complex distributary networks on relatively low slopes (Diniega et al 2013;Pasquon et al 2016;this volume, in review;McKeown et al 2017;Jouannic et al, this volume, in review).…”

Downslope sediment transport by boiling liquid water under Mars-like conditions: experiments and potential implications for Martian gullies

“…1a, b; e.g. Malin & Edgett 2000;2006;Balme et al 2006;Dickson et al 2007;Dundas et al 2010Dundas et al , 2012Dundas et al , 2015Dundas et al , 2018Harrison et al 2015;Raack et al 2015;Conway et al 2015Conway et al , 2018; (2) 'dune gullies' ( Fig. 1c; e.g.…”