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RGClark
2006-Aug-18, 02:09 AM
Greg Orme has a site where he collected several images containing the spiders:

Martian Spiders.
http://www.martianspiders.com/

The Kieffer et.al. theory of geysering sounds plausible. The only problem I have with it is the dendritic patterns (like a tree branching) seen with the channels that form the "legs" of the spiders. On Earth this has been taken as a signature of channels cut by liquid water. This was part of the reason why the Malin/Edgett gullies were attributed to flowing water.
Here's an image showing the spiders with the dendritic patterns:

MOC narrow-angle image M11-00280.
http://www.msss.com/moc_gallery/m07_m12/images/M11/M1100280.html

Robert Bodnar has argued that CO2-water clathrates could be stable at Mars surface conditions and these could have caused the Malin/Edgett gullies. However, he also notes the transition to liquid water brines at higher temperatures 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

He states the transition to liquid water brines would occur at 262K if they contain NaCl salt, but this could be down to 230K if they contain calcium chloride salts.
This report discusses THEMIS observations showing temperatures reaching 220K at the south pole during southern Summer:

Exposed Water Ice Discovered near the South Pole of Mars.
Science 14 February 2003: Vol. 299. no. 5609, pp. 1048 - 1051
http://www.sciencemag.org/cgi/content/full/299/5609/1048

Given the limitations in the accuracy and resolution of the THEMIS instrument, there could be localized spots at 230K and above.
Burt and Knauth argue that calcium chloride brines are more likely to exist on Mars than NaCl brines:

HIGHLY CONDUCTIVE EUTECTIC BRINES RATHER THAN WATER EXPECTED IN THE MARTIAN SUBSURFACE.
L. P. Knauth, D. M.Burt and J. A. Tyburczy, Department of Geological Sciences, Box 871404, Arizona State University, 85287-1404; ***@asu.edu, ****@asu.edu, ****@asu.edu.
http://www.lpi.usra.edu/meetings/geomars2001/pdf/7043.pdf

Electrically conducting, Ca-rich brines, rather than water, expected in the Martian subsurface.
JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 108, NO. E4, 8026, doi:10.1029/2002JE001862, 2003
http://www.agu.org/pubs/crossref/2003/2002JE001862.shtml

Key is that such brines are expected to be highly electrically conducting and
should therefore be easier to detect than pure water with radar instruments. Then confirmation that brines exist might come with a seasonal change in the radar reflectivity during local Summer.
The Marsis radar instrument on Mars Express has already been able to detect
subsurface water ice at the Martian South Pole:

Big new reservoir of water ice suspected under Mars.
13:54 16 March 2006
NewScientist.com news service
Maggie McKee, Houston
http://www.newscientistspace.com/article/dn8857-big-new-reservoir-of-water-ice-suspected-under-mars.html

MARSIS SUBSURFACE SOUNDING OBSERVATIONS OF THE SOUTH POLAR LAYERED DEPOSITS.
Lunar and Planetary Science XXXVII (2006) 1212.pdf
http://www.lpi.usra.edu/meetings/lpsc2006/pdf/1212.pdf

However, Marsis has very limited vertical resolution. It might be difficult for it
to be able to detect near surface brines. The SHARAD radar instrument on Mars Reconnaissance Orbiter might be better able to detect them.


Bob Clark

RGClark
2006-Aug-19, 01:44 PM
Article describing the Kieffer et.al. theory that the spiders are due to CO2 gas venting:

Spiders Invade Mars.
"Summary (Aug 17, 2006): As the Sun peeks above the horizon at the martian south polar icecap, powerful jets of carbon-dioxide gas erupt through the icecap's topmost layer. If you were there, you'd feel a vibration through your spacesuit boots as, all around you, roaring jets of CO2 gas threw sand and dust hundreds of feet into the thin cold air."
http://www.astrobio.net/news/article2055.html

On uplink.space.com, Alex Blackwell provided a link to a paper that argues that the spiders are due to liquid water brines:

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

It's available for free download. It's a large file so you should download it rather than opening it in a window.


Bob Clark