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

Abstract

Gullies are widespread morphological features on Mars for which current changes have been observed. Liquid water has been one of the potential mechanisms to explain their formation and activity. However, under present-day Martian conditions, liquid water is unstable and should only be transiently present in small amounts at the surface. Yet little attention has been paid to the mechanisms by which unstable water transports sediment under low atmospheric pressure. Here we present the results of laboratory experiments studying the interaction between liquid water flowing over a sand bed under Mars-like atmospheric pressure (c. 9 mbar). The experiments were performed in a Mars Simulation Chamber (at the Open University, UK), in which we placed a test bed of fine sand at a 25°slope. We chose to investigate the influence of two parameters: the temperature of the water and the temperature of the sand. We performed 27 experiments with nine different combinations of water and sand temperatures ranging from 278 to 297 K. Under all experimental conditions, the water was boiling. We investigated and compared the types and timing of sediment transport events, and the shapes, characteristics and volumes of the resulting morphologies. In agreement with previous laboratory studies we found that more intense boiling increased the volume of sediment transported for a given volume of water. We found four main types of sediment transport: entrainment by overland flow; grain ejection; grain avalanches; and levitation of saturated sand pellets. Our results showed that increasing sand temperature was the main driving parameter in increasing the sand transport and in modifying the dominant sediment transport mechanism. The temperature of the water played a negligible or minor role, apart from the duration of sand ejection and avalanches, which lasted longer at low water temperature. At low sand temperature the majority of the sand was transported by overland flow of the liquid water. At higher sand temperatures the transport was dominated by processes triggered by the boiling behaviour of the water. At the highest temperatures, sediment transport was dominated by the formation of levitating pellets, dry avalanches and ejection of the sand grains. This resulted in a transport volume about nine times greater at a sand temperature of 297 K compared with 278 K. Our heat transfer scaling shows that the boiling behaviour will be enhanced under Martian low gravity, resulting in more efficient transport of sediment by levitating sand pellets even at temperatures close to the triple point. Our results showed that the boiling intensity played an important role in the physics of sediment transport by liquid water. This implied that the amount of water required to produce morphological changes at the surface of Mars could be lower than previously estimated by assuming stable liquid water. Boiling is a critical process to be considered when assessing gully formation and modification mechanisms mobilized by liquid water. Our work could ha...

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

“…Top of the slope is 55 cm across and the experiments were performed under terrestrial pressure. (d) Sediment transport engendered by liquid water over fine sand at 20° slope under low pressure (~9mbar) and elevated sediment temperature (~25°C) reported in Raack et al (2017) and Herny et al (2018). White arrow points to sediment displaced by dry avalanches triggered by grain saltation at the flow front and black arrow saturated pellets levitated on cushions of gas released by boiling.…”

“…Frozen soil conditions lead to reduced infiltration, which can lead to both overland flow and debris flow processes at much lower discharge than if the soil was above freezing (Conway et al, 2011a;Gabet, 2000;Jouannic et al, 2015;Védie et al, 2008) (Figure 19a-c). Laboratory simulation experiments have shown that boiling leads to three processes that are not experienced by water flows on Earth (Herny et al, 2018;Massé et al, 2016;Raack et al, 2017): grain saltation at the flow boundary, granular avalanches triggered by the saltation and gas production and finally sediment levitation (Figure 19d). All three processes can act together to lead to much more efficient sediment transport than the equivalent for stable water and no terrestrial field analogues exist for these sediment transport mechanisms.…”

Morphological characterization of landforms produced by springtime seasonal activity on Russell Crater megadune, Mars

“…The most significant problem comes in explaining the source of this liquid, when water is only transiently metastable on Mars (e.g., Hecht 2002) and latent heat loss is a significant barrier to melting. Recent experimental work has highlighted that even boiling of unstable water can have significant effects on the surface topography (Massé et al, 2016;Raack et al, 2017;Herny et al 2018). Sand can be ejected and accumulated at the front of the flow, which in our scenario could produce an obstacle which deviates the flow, but this was not observed in these experiments (as only individual flows were simulated).…”

“…The bright halos represent a re-condensed transient surface frost (e.g., Kereszturi et al, 2011a,b). It is, however, difficult to explain the formation of pits associated with active linear dune gullies with liquid water, but recent laboratory simulations have highlighted some unusual morphologies produced by boiling liquid water (Raack et al, 2017;Herny et al 2018) that could address this issue.…”

“…Although melting of the first few centimetres-decimetres of ground ice could explain the rills, the recharge of this system is an outstanding issue. This problem is somewhat ameliorated by the results of recent experiments (Raack et al, 2017;Herny et al 2018) which have revealed efficient sediment transport by transient boiling water under martian conditions. These results indicate that less water is required to produce morphologic changes at the surface compared to nonboiling flowing water (which forms the channels in our Icelandic analogue).…”