20-EPN2-093: Linear gullies on Mars
Visit by Lonneke Roelofs, Utrecht University (Netherlands) to TA2.20 Open University Mars Chamber (UK).
Dates of visit: 21 November – 16 December 2022
Report Summary: A vast range of different gully morphologies occurs on Mars: from the classical gullies, which resemble gullies on Earth, to linear gullies that do not have an Earth counterpart and are found on Martian dunes. Previous experiments have shown that the sublimation of CO2 ice can fluidise and transport sediment in the classical gullies on Mars. However, the linear gullies are hypothesised to form by a different, although related CO2-driven mechanism. For linear dune gullies, it is hypothesised that they form by a block of CO2-ice sliding down the dune. This process has, however, never been observed in real life.
With our visit to the Mars chamber at the Open University, we aimed at deciphering the triggering and forming mechanisms of linear dune gullies on Mars. We identified the possible triggering mechanisms based on hypotheses presented in the literature. The identified mechanisms are; 1) the breaking off and sliding down of CO2-ice blocks, and 2) wind-blown sand being deposited on CO2 frost. We systematically tested these mechanisms in the Mars Chamber at the Open University by means of experiments. For all identified triggering mechanisms a parameter space was used to test the influence of e.g. CO2-ice block size, surface slope and grain size.
With our experiments, we show that CO2-ice blocks slide downslope and create small narrow gullies when dumped on top of fluvial sand, with a large grain-size distribution. However, when dumped on a finer aeolian sand under Martian atmosphere, they do not slide downslope but they dig themselves into the sand, slowly digging a gully downslope by vigorous sublimation and sediment mobilisation. We also show that when a small amount of warm sand is dumped on top of a CO2-frosted the sand is mobilised by CO2 sublimation, but that this process does not create the typical linear gullies we see on Mars.