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

Herny, Clémence Emilie Lucile; Conway, Susan J.; Raack, Jan; Carpy, Sabrina; Colleu-Banse, Tanguy; Patel, Manish R. (2018). Downslope sediment transport by boiling liquid water under Mars-like conditions: experiments and potential implications for Martian gullies. Geological Society Special Publications, 467(1), pp. 373-410. Geological Society 10.1144/sp467.10

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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 have similar implications for any water-formed landform
on Mars, which could include recurring slope lineae, dark dune flows and slope streaks.

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