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Seasonal activity of Polar Dunes on Mars

Final Report Summary - MARSDUNE (Seasonal activity of Polar Dunes on Mars)

There has been a paradigm shift in our understanding of the operation of present-day surface processes on Mars. Particularly important are the discoveries of ripple and dune migration, seasonal-scale dune morphology changes and the importance of the seasonal ice deposit as an agent of geomorphic change.
The detection of dune movement on Mars has in the past been constrained by low resolution images. Previous studies suggest that with the exception of a very few sites any dune activity, if it existed, was below data resolution. Dunes were generally considered immobile because of low shear stresses, cementation or armouring. However recent studies demonstrate that sand on Mars is mobile at a range of locations. Newly observed bedform activity includes ripple migration on dunes and sand patches, slipface avalanches, changes in dune outline and whole dune migration. Studies suggest that not only are dunes migrating on Mars, but once mobilized, they can advance rapidly.
The rates of dune advancement in the thin Martian atmosphere may be assisted by several sediment transport conditions unique to Mars. It has been calculated that the trajectory of Martian grains are 100 times longer and higher than those on Earth. They also found that grain impact velocities are 10 times larger than on Earth. In addition, saltation on Mars can be sustained at wind speeds an order of magnitude less than those required to initiate it.
Unlike dunes on Earth, we still do not fully understand airflow dynamics over and around Martian dunes and dune fields at relevant scales. If aeolian bedforms are to be used as a wind direction proxy, then a better understanding of the controls on bedform morphology and migration are needed. We have undertaken studies that map current dune and ripple migration direction and rate and compare those data with atmospheric model output at the micro and meso-scale. This study has applied numerical modelling to establish the influence of variations in wind direction and duration on resultant dune shape.
1. We have found that large dune ridge shapes result in a complex modification of localized airflow over the surface of Martian dunes. Wind modelling on a microscale (5 m) now provides us with an effective new tool to accompany surface ripple displacement data to understand dune dynamics on Mars.
2. Our data show that regional and local topographic settings are important boundary condition that need to be carefully addressed in order to explain spatial variations in the migration rates and morphologies of dunes and ripples. We find that the topography of a crater is an important control on the wind flow field at the dune field scale.

3. Numerical modelling of asymmetric dune form finds that there are three additional controls on the development of barchan asymmetry beyond a variation in wind direction. Local topography, sediment supply and dune collisions can also generate similar dune forms. We advocate that these additional controls be assessed before wind is inferred from asymmetric dune form.
Collectively our findings advocate for the use of numerical and high-resolution surface modelling of winds to better infer regional wind patterns from contemporary bedforms on Mars.
Aeolian dunes in our solar system are modified by a myriad of processes that operate at the surface-atmosphere boundary. Many are climate dependent and bring with them a suite of sediment transport processes. On Earth sediment transport by wind dominates active dune deposits; however, non-aeolian transport also occurs on dunes such as grain flows, slumping, fluvial and debris flows. Grainflows, slumps and gullies also modify Martian dunes. In addition, there is a newly identified sediment transport system on Mars, that of cryo-venting. During the Martian spring venting of gas and sediment occurs through cracks in the sublimating seasonal CO2 ice. Furrows are a newly discovered landform that occurs seasonally on the surface of Martian polar dunes. Bourke has proposed that they are formed during cryo-venting, illustrating the importance of CO2 sublimation as an agent of geomorphic change. Although furrows occur on a majority of polar dunes, they are more prolific and better developed in some locations. The reason for this is not well understood and is investigated in this research.
Our research aimed to
• investigate the interaction of CO2 ice on the surface of sand
• Examine the role of geochemical processes in inhibiting dune migration on Mars.
1. We have demonstrated that features formed by sublimating CO2 ice in laboratory experiments resemble features on Mars dunes. This suggests that a similar formation mechanisms (CO2 sublimation) can generate features similar in morphology to those forming on Martian dunes today. Seasonal ice sublimation is an important geomorphic process on Martian high latitude dunes.
2. We have determined that there is a deposit of aeolianite in Lucaya crater on Mars. The Mapped features (high and low albedo lineations) suggests that groundwater flow is sustained near the surface as well as in the deeper subsurface following crater formation. Geochemical sedimentation in the vadose zone may inhibit, but does not stop dune migration on Mars.