Martian Spiders Recreated in the Laboratory

Post contributed by Dr. Lauren Mc Keown, School of Physical Sciences, The Open University, UK.

Spiders are unusual branched landforms found among the high southern latitudes of Mars (Image 1). They have no Earth analogues and are often accompanied by fans and spots that appear in spring. They are proposed to form when sunlight penetrates the Martian south polar seasonal CO2 ice layer, causing ice at its base to change from ice to gas, and eventually crack. Escaping gas then scours the terrain beneath, carving spider-like patterns and depositing material on top of the ice via a plume (Kieffer et al., 2003). However, although this suggested process is well-accepted, it has never been directly observed on Mars. In order to investigate whether spider patterns could form by CO2 sublimation under Martian atmospheric pressure, experiments were performed (Image 2) at the Open University Mars Simulation Chamber, which simulates Martian atmospheric conditions.

Image 1: Examples of spiders on Mars (HiRISE image ESP_014282_0930). Left shows the ‘classic’ spider morphology which consists of a central depression and radial tortuous dendritic troughs emanating from its centre. Right is a context image. Image Credit: NASA/JPL/University of Arizona.

Image 2: Images of spiders formed in the lab on beds of 150-250 µm grains. (a) shows ‘double spiders’, formed when two holes were drilled apart from one another towards the centre of the block. (b) shows a singular spider with particularly dendritic troughs.

The Martian atmosphere is composed predominantly of CO2 gas. In winter, about 30% of the atmosphere deposits on the Martian surface as CO2 frost and ice. In spring, the ice sublimates, causing a host of seasonal surface expressions that are unlike anything seen on Earth. Spiders, more formally known as araneiforms, are an example of such landforms. They are long (up to 1 km in diameter), dendritic, negative topography features that consist of anastomosing and tortuous troughs emanating from a central pit (Image 1) and are confined to the high south polar latitudes on Mars. Spiders are proposed to form via a well-accepted process (Kieffer et al., 2006, Piqueux et al., 2003) whereby springtime insolation penetrates translucent slab ice and thermal wavelengths become trapped via the Solid State Greenhouse Effect (Matson and Brown, 1989). This heats the regolith beneath the seasonal ice layer and the ice will then sublimate from its base. Pressure will then build and the ice will eventually rupture, forming a vent. Gas will rush towards the vent, eroding the surface and depositing the entrained material on top of the ice in the form of a plume of dust and gas (Image 3). Although this process has been well-accepted for almost two decades, it has been framed in theoretical context and has never been directly observed on Mars. In order to investigate whether such a process can form spider-like patterns and to understand the physical parameters that constrain it, we require empirical data.

Image 3: Artist’s impression of Kieffer’s Hypothesis showing plume activity driven by insolation penetrating translucent slab ice on Mars. Spiders are proposed to be eroded by high velocity gas escaping beneath the seasonal CO2 ice layer. Image Credit: Featherwax.

Therefore, a series of experiments were performed at the Open University Mars Simulation Chamber in the UK (Image 2). The experiments modelled the formation of spider-like patterns by CO2 sublimation under Martian pressure. Blocks of CO2 ice, with holes drilled in their centres, were lowered onto beds of granular substrate and the pressure in the chamber was depressed to ~6mbar. In each case, the ice block was allowed to sublimate and a plume was observed to transport material through the central hole from beneath the block. The resulting surface feature in each instance was similar in morphology to spiders on Mars (Image 2); gas conduits had eroded dendritic troughs that converged towards a central depression. The development of 3D models of the features allowed a deeper morphological characterisation. It was determined that these features were more branched for (a) finer grain sizes and (b) smaller vents. These data will allow us to improve upon models of spider formation on Mars, and to better understand the conditions under which they form. The experiments provide the first set of empirical evidence for Kieffer’s Hypothesis.

Further Reading

Mc Keown, L.E., McElwaine, J.N., Bourke, M.C., Sylvest, M.E., Patel, M.R., The formation of araneiforms by carbon dioxide venting and vigorous sublimation dynamics under martian atmospheric pressure. Sci. Rep. 11, 6445 (2021)

Kieffer, H. H., Christensen, P. R. & Titus, T. N. CO2 jets formed by sublimation beneath translucent slab ice in Mars’ seasonal south polar ice cap. Nature 442, 793–796 (2006).

Piqueux, S., Byrne, S. & Richardson, M. I. Sublimation of Mars’s southern seasonal CO2 ice cap and the formation of spiders. J. Geophys. Res. (Planets) 108, E8 (2003).

Matson, D. L. & Brown, R. H. Solid-state greenhouse and their implications for icy satellites. Icarus 77, 67–81 (1989).

Mc Keown, L. E., Bourke, M. C. & McElwaine, J. N. Experiments on sublimating carbon dioxide ice and implications for contemporary surface processes on Mars. Sci. Rep. 7, 14181 (2017).

Portyankina, G., Hansen, C. J. & Aye, K.-M. Present-day erosion of Martian polar terrain by the seasonal CO2 jets. Icarus 282, 93–103 (2017).

Thomas, N., Hansen, C. J., Portyankina, G. & Russell, P. S. HiRISE observations of gas sublimation-driven activity in Mars’s southern polar regions: II. Surficial deposits and their origins. Icarus 205, 296–310 (2010).

Schwamb, M. E. et al. Planet Four: Terrains—Discovery of araneiforms outside of the South Polar layered deposits. Icarus 308, 148–187 (2018).

Hansen, C. J. et al. HiRISE observations of gas sublimation-driven activity in Mars’ southern polar regions: I. Erosion of the surface. Icarus 205, 283–295 (2010).

Thomas, N., Portyankina, G., Hansen, C. J. & Pommerol, A. HiRISE observations of gas sublimation-driven activity in Mars’s southern polar regions: IV. Fluid dynamics models of CO2 jets. Icarus 212, 66–85 (2011).

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2 Comments

  1. Deanne Rogers

     /  June 1, 2021

    Wow. Very nice work!

    Reply
  2. Very interesting work!

    Reply

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