Triggering Mechanisms and Formation Processes of Linear Dune Gullies on Mars

Abstract

Linear dune gullies are landforms seen on Martian dunes, for which no terrestrial analogue exists. These gullies are characterised by their narrow channel, uniform in width, which can be sinuous and frequently terminates in a pit. The morphology of linear dune gullies is considerably different from the alcove-channel-apron gullies, also seen on Martian dunes. At first it was believed that the linear dune gullies were formed by flowing water, however, as liquid water is not stable for current climatic conditions on Mars, other hypotheses are arising. The leading hypothesis is that the linear dune gullies are formed by CO2 blocks that break off in early spring and slide down slope, however, experimental data in a Mars-like environment is lacking. In this thesis processes that form linear dune gullies were studied. This was carried out by analysing morphometric parameters and topographic conditions with satellite imagery (HiRISE) in the Kaiser and Proctor dune fields on Mars and by testing two triggering mechanisms and boundary conditions in an experimental set-up. Experiments were carried out a flume inside the Mars chamber located at the Open University, with the capability to lower the atmospheric pressure.

For triggering mechanism 1 sand was deposited on a mixture of granular CO2 ice, which resulted in a morphology with wider and shallow form. For triggering mechanism 2 blocks of CO2 were dropped on a sand bed, which created long and narrow gullies. The lower atmospheric pressure highly influenced the amount of sublimation and therefore the entrainment of sediment. Different modes of transport of the blocks were observed under Martian pressure: sliding and digging. Sliding blocks created narrower and shallower gullies, whereas digging blocks formed wider and deeper gullies. The mode of transport is determined in the experiments by the grain size, permeability, slope and block size.

This study shows that gullies, similar to linear dune gullies can be recreated under Martian pressure in the experiments using CO2 blocks. The length of the linear dune gullies on Mars is determined by the size of the dunes. The width varied from 2 to 9 meter and is related to the size of the block and the angle of the slope. The sinuosity can still not be fully explained. The distinct orientation of the linear dune gullies on Mars towards the south shows that insolation differences are likely involved in the formation process and that alcove-channel-apron gullies are caused by a different triggering mechanism. It can be concluded that CO2 sublimation plays an important role the present-day observed activity on Mars and we should further investigate the role of CO2 sublimation on landscape formation.