Abstract
Martian araneiform terrain, located in the Southern polar regions, consists of features with central pits and radial troughs which are thought to be associated with the solid state greenhouse effect under a CO2 ice sheet. Sublimation at the base of this ice leads to gas buildup, fracturing of the ice and the flow of gas and entrained regolith out of vents and onto the surface. There are two possible pathways for the gas: through the gap between the ice slab and the underlying regolith, as proposed by Kieffer (2007), or through the pores of a permeable regolith layer, which would imply that regolith properties can control the spacing between adjacent spiders, as suggested by Hao et al. (2019). We test this hypothesis quantitatively in order to place constraints on the regolith properties. Based on previously estimated flow rates and thermophysical arguments, we suggest that there is insufficient depth of porous regolith to support the full gas flow through the regolith. By contrast, free gas flow through a regolith–ice gap is capable of supplying the likely flow rates for gap sizes on the order of a centimetre. This size of gap can be opened in the centre of a spider feature by gas pressure bending the overlying ice slab upwards, or by levitating it entirely as suggested in the original Kieffer (2007) model. Our calculations therefore support at least some of the gas flowing through a gap opened between the regolith and ice. Regolith properties most likely still play a role in the evolution of spider morphology, by regolith cohesion controlling the erosion of the central pit and troughs, for example.
Highlights
Araneiform terrain consists of groups of so-called spiders; radial, dendritic arrangements of troughs connecting to central pits, found at southern high latitudes of Mars (Kieffer, 2000)
In the formation of the so-called araneiform features, seen in Mars’ southern polar regions, subsurface flow of CO2 gas below an ice slab can occur in a gap between ice and regolith, and/or in the substrate itself
We investigate the effectiveness of subsurface flow of CO2 and compare it to gas flow through the gap between ice and regolith
Summary
Araneiform terrain consists of groups of so-called spiders; radial, dendritic arrangements of troughs connecting to central pits, found at southern high latitudes of Mars (Kieffer, 2000) Their occurrence was thought to be limited to the South Polar Layered Deposits (SPLD). We derive the depth of porous regolith needed to support the magnitudes of flow rate previously calculated in hydrodynamics simulations (Thomas et al, 2011a,b) and energy balance models (Kieffer, 2007; Pilorget et al, 2011; Pilorget and Forget, 2016) We compare this to the required gap size to support laminar freeflow between the ice and regolith, and to the expected bending of the ice sheet in response to the rise in gas pressure beneath.
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