re: Image from Orbit
Several years ago (c. 2000), we had an interesting discussion about this phenomenon right here at HZ (or was it the MSNBC Space BBS?) I think Nick Hoffman was the first to put the matter of geysering at the south pole into some sort of coherent perspective.
It is well known that C02 changes phase from solid to
vapour at relatively low temperatures at the low martian
ambient pressure. The corollory of this is that if you
can heat it above that temperature, it will develop a
substantial pressure (if you confine it). For instance,
at -57 C the solid has an equilibrium vapour pressure of
5 bars which is enough to physically lift a lid of 100m
of solid CO2 ice or ~ 75 metres of regolith.
The polar icepack is only a few metres thick, so
pressures of ~ 0.1 bars will suffice to blow through the
"roof" and erupt as a geyser. This equates to a
temperature of 170K (~24 K warmer than the main snowpack
which condenses in equilibrium with the ~6.5 millibar
atmosphere). Carbon dioxide has a very steep ramp of
equilibrium vapour pressure with temperature whicj makes
effects like this very powerful. It also has low specific
heat capacity which makes it easy to warm (unlike water)
and low latent heat capacity which makes it easy to
melt/vaporise (unlike water).
This is NOT a sign of internal volcanic heat, but simply
a consequence of spring sunshine shining down through
relatively transparent CO2 snow and block ice and warming
the surface of dark dune material underneath, like Earth
sunshine melting snow on a black bitumen sidewalk from
the bottom up.
What we observe is not so much the active geyser, as the
trail of fine dust that has been jetted out of the
orifice and spread downwind. If the wind direction
changes, so does the streak direction so on different
days you get different trails, or sometimes the winds
swirl noticeably acroll the length of a frame and show
varying orientations - a dead giveaway for wind streaks
rather than shadows.
Nick Hoffman
But at the time, there was quite a dispute over whether there was actually anything like active geysering being shown in orbiter images.
Fascinating.
Here's the image that triggered it:
(If the image works)