DRY ICE–DRIVEN EROSION: A UNIQUELY MARTIAN PROCESS

Every autumn ~25% of Mars’ CO₂ atmosphere condenses, forming a seasonal polar cap on the winter pole. Over the winter the ice anneals, becoming a translucent and impermeable layer of ice [1]. In the spring, sublimation of the dry ice polar cap drives an effective erosional process that creates uniquely martian landforms [2].

We have used the High Resolution Imaging Science Experiment (HiRISE) on the Mars Reconnaissance Orbiter (MRO) to image spring activity in the north and in the south for 6 Mars years. Ice-free summer images allow us to detect changes from one year to the next, driven by CO₂ gas erosion. Basal sublimation of the seasonal layer of ice traps gas under pressure; when the gas escapes loose material from the surface is entrained, eroding channels in the surface [1].

In the north, dry ice deposition on the dunes of the north polar erg triggers formation of new alcoves [3] that become sites for enhanced seasonal sublimation activity in the spring [4]. Shallow channels termed furrows form in mobile dune sand to allow gas to escape from below the seasonal ice layer [5]. The furrows form along seasonal cracks in the ice on the stoss side of the dunes, and terminate at the crest and around the margins of dunes, where material is transported by the escaping gas and deposited in fans. These furrows are largely ephemeral.

In contrast, in the south, channels eroded in the ground by escaping gas form radially-organized channels known as araneiforms or araneiform terrain, colloquially “spiders”, that widen and branch in dendritic patterns as the years go by [2]. Several new spiders have been observed to form over the years that HiRISE has been imaging [6].

Due to Mars’ elliptical orbit the seasons in the north and south are not symmetric. Erosional features in the northern and southern hemispheres will be compared and contrasted.

[1] Kieffer, H. H. (2007), JGR 112, E08005; [2] Piqueux, S. et al. (2003) JGR 108, (E8):3-1; [3] Hansen, C. J. et al. (2011) Science, 311, 575-578; [4] Dinega, S. et al., (2017) in: Conway, S. J., Carrivick, J. L., Carling, P. A., De Haas, T. & Harrison, T. N. (eds) Martian Gullies and their Earth Analogues. Geological Society, London, Special Publications, 467, doi:10.1144/SP467.6; [5] Bourke, M. C. (2013) 44th LPSC, abs.# 2919; [6] Portyankina, G. et al. (2017) Icarus 282:93-103.