  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 CO$_{2}$ 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 et al (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. 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 et al (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.
