Hi All,

FYI: I just assembled a two-image animation showing a unique
variety of Martian surface feature known as "spiders" that
are found in the cryptic region of the south pole of Mars:

http://www.geocities.com/iamgoddard/spiders01.gif

The "spiders" seen therein are huge branching radial ravines miles
in length on the Martian surface. The CO2 icecap covers the ravines
and the region for part of the year before evaporating. The looping-
animation contains two satellite images that show the same area at
two different times of the year. The first image shows the naked
surface with spider ravines. The second which is the darker image
shows the same area near the end of the annual icecap recession.

The best theory about the Martian-spider ravines (Kieffer's theory)
is that they were etched into the surface by pressurized gas rushing
to escape holes in the CO2 icecap during seasonal icecap evaporations.
While frozen water melts into liquid, CO2 ice vaporizes into gas that
builds under the icecap as it evaporates during its annual recession.
Spider ravines are slowly etched into the regolith surface over many
years by pressurized CO2 gas rushing to holes in the thinning icecap.
Acting as gas-escape channels, spiders are associated with geysers
of CO2 gas escaping from under the evaporating icecap. In this next
satellite image the fallout (known as "fans") of dark debris carried
by CO2 gas that escaped through geysers can be seen deposited on the
thinning icecap through which underlying spider ravines can be seen:

http://www.msss.com/moc_gallery/m07_...7/M0703150.jpg

According to Kieffer those ravines seen through the ice are acting
as gas-escape channels. Here is a study that tested the Kieffer
CO2-geyser-channel theory (a must-read for all interested):

http://www.gps.caltech.edu/~shane/2002JE002007.pdf

Figure 3 in that study contains a composite of satellite images
showing the same area over the annual transition from total CO2-
icecap coverage (seen in A) to total surface exposure (seen in F):

A: http://www.msss.com/moc_gallery/ab1_.../M0305867.html
B: http://www.msss.com/moc_gallery/ab1_.../M0403488.html
C1: http://www.msss.com/moc_gallery/m07_.../M0702549.html
C2: http://www.msss.com/moc_gallery/m07_.../M0705081.html
D: http://www.msss.com/moc_gallery/m07_.../M0807709.html
E: http://www.msss.com/moc_gallery/m07_.../M0900570.html
F: http://www.msss.com/moc_gallery/m07_.../M1100280.html

Select the hi-resolution images. In C1 and C2, spiders begin to become
visible through the thinning ice as geysers and their wind-blown dark
fans become most prolific. In F, the icecap has totally evaporated
and the spider-ravine-etched surface is fully exposed.


Ian Goddard's Journal: http://iangoddard.net/journal.htm

"When we have lived any time, and have been accustomed to
the uniformity of nature, we acquire a general habit, by
which we always transfer the known to the unknown, and
conceive the latter to resemble the former." David Hume