Space Forum / Space Science / January 2008

The news release from the AGU conference argues that the Mars "spider"
features at the south pole are due to gas release:

HiRISE | Isolated Araneiform Topography (PSP_003087_0930).
http://hirise.lpl.arizona.edu/PSP_003087_0930

Spring at the South Pole of Mars.
AGU Press Conference
C. J. Hansen, A. McEwen and the HiRISE Team.
December 2007
http://hirise.lpl.arizona.edu/pdf/agu_press_conf_dec07.pdf

However, this report shows temperatures above 250K can be reached
seasonally even as far south as the Mars spiders:

Astrobiology
Spiders: Water-Driven Erosive Structures in the Southern Hemisphere of
Mars.
Aug 2006, Vol. 6, No. 4: 651-667
http://www.liebertonline.com/doi/abs/10.1089/ast.2006.6.651 [abstract]

This is within the temperature range of calcium chloride liquid water
brines.
Evidence for a liquid water origin is the dendritic pattern shown by
the spider "legs". On Earth this is taken for evidence of channels cut
by liquid water.
The news release argues that the channels are cut by gas because some
channels travel uphill. But of course water can also travel uphill for
short distances under pressure or simply because it is traveling
quickly as in a flood.

A related possibility is that the origin of the spiders is due to the
transition of CO2-water clathrates to liquid water brines. This report
shows this could occur on heating and would result in explosive gas
release:

PTX PHASE EQUILIBRIA IN THE H2O-CO2-SALT SYSTEM AT MARS NEAR-SURFACE
CONDITIONS.
R.J. Bodnar1, 1Fluids Research Laboratory, Department of Geological
Sciences, Virginia Tech, Blacksburg, VA 24061 email: *...@vt.edu.
Lunar and Planetary Science XXXII (2001) 1689.pdf
"The decomposition of clathrate to produce brine
plus gas would generate considerable mechanical (PV)
energy owing to the large volume change associated
with this reaction. The reaction of clathrate dissociating
to produce gas plus brine is analogous to the process
that occurs in silicic volcanic eruptions on earth,
whereby water dissolved in the melt exsolves to form a
separate fluid phase. The large difference between the
molar volume of the exsolved fluid (water) and the
partial molar volume of water dissolved in the silicate
melt provides the energy for explosive volcanic eruptions.
A similar process associated with the breakdown
of carbon dioxide clathrate on Mars may provide
the energy to move large volumes of regolith material
to produce the characteristic erosional features that are
observed on the martian surface."
http://www.lpi.usra.edu/meetings/lpsc2001/pdf/1689.pdf

The plausibility of either of these explanations is easy to test
experimentally in the lab. For the gas-carved only explanation the key
questions is would such gas carved channels show the dendritic
patterns usually seen only with liquid water carved channels?
For the dissociating clathrate explanation, the key question is would
the clathrates dissolving under a CO2 ice cover result in liquid water
brines at sufficient pressure to carve channels that even travel
uphill for short distances?
Further on the infrared observations of the "fans" seen on the
surface in the vicinity of the spiders appears here:

Carbon-Dioxide Frost Settling from Seasonal Outbursts on Mars (Movie).
http://mars.jpl.nasa.gov/mro/gallery/video/index.html#CarbonDioxideFrost

This page shows both visible light and infrared images of the fans
due to venting gas. The conclusion presented here is that the fans are
due to CO2 gas release because of the infrared signatures. However, it
is notable that there is quite a bit of evidence in the infrared of
water ice in the images as well. Then the infrared images as well are
still consistent with possibility of dissolving CO2-water clathrates.

    Bob Clark