Posted to sci.astro:

From: rgregoryclark@yahoo.com (Robert Clark)
Newsgroups: sci.astro, alt.sci.planetary,sci.geo.geology,sci.geo.meteorology,sci.astro.amateur
Subject: Re: Once Upon a Water Planet

baalke@zagami.jpl.nasa.gov (Ron Baalke) wrote in message news:<a6nuhf$17p$1@nntp1.jpl.nasa.gov>...
>
>http://science.nasa.gov/headlines/y2002/12mar_waterplanet.htm
>
> Once Upon a Water Planet
> NASA Science News
>...
> MOC's findings are corroborated by data from another instrument on the
> spacecraft, the Mars Orbiter Laser Altimeter (MOLA). For 27 months -- longer
> than a Martian year (one Martian year is 687 Earth days) -- MOLA gauged the
> daily height of the Red Planet's polar icecaps, meticulously recording how
> much frozen material accumulated in winter and eroded (sublimed or
> evaporated) in summer in each hemisphere. MOLA documented that each ice cap
> has a volume as great as the Greenland ice cap on Earth.
>
> Although the upper crust of frost is clearly carbon dioxide, scientists are
> now convinced that much of both caps' supporting mass must be frozen
> water--structurally, "dry ice can't stand up two miles high," Garvin
> remarked.
>
> MOLA and MOC measured how the polar caps shrink in each hemisphere's summer.
> They shrink so much, in fact, that if the observed trends were continued for
> just a few centuries, nearly a third of each polar cap could evaporate into
> Mars's atmosphere. That would pump the atmospheric pressure up from 6
> millibars to 30 or 40 mb (the Earth's atmospheric pressure is about 1000 mb)
> -- high enough pressure for liquid water to be stable on the planet's
> surface under certain temperature conditions. Thus, perhaps as recently as
> just a century or two ago, Mars might have been "clement enough for ponds of
> water" to have dotted its surface like desert oases, Garvin said -- and
> current trends suggest it might become so again.
>...

The conclusion that the observed melting of the south polar cap is a
long term phenomenon is still controversial. It may be due to dust
deposition from the recent global dust storms.
However, one can speculate about what happens when the dust from a
global dust storm that covers one of the poles settles out. This might
lead to melting of most or all of the poles CO2 cover due to increased
heat absorption from the dust, perhaps to the extent of the melting
mentioned by Garvin.
I had thought the current global storm covered the southern ice cap,
but Malin on this page seems to say it doesn't:

Mars Global Surveyor
Mars Orbiter Camera
Evidence for Recent Climate Change on Mars
"August, and September 2001, we devoted a large amount of the high
resolution imaging during that time to the south polar cap (the only
part of the planet that wasn't hidden by atmospheric dust). In
mid-August, we began seeing images of places previously viewed a Mars
year earlier. The changes were impressive. Escarpments had retreated
by between 0 and 8 meters, with 3 m being about average."
http://www.msss.com/mars_images/moc/CO2_Science_rel/malin_etal.html

Despite this I'm thinking the dust that blows onto the pole from high
up in the atmosphere could yet cause increased sublimation of the ice
leading to the high rates of CO2 sublimation observed by Malin.

It has often been remarked that the observations of Mars from the
19th century are somewhat different than current observations.
Conceivably there could have been water related features seen then not
observed now. Canals perhaps?

From: Robert Clark (rgclark@my-deja.com)
Subject: Could global dust storms melt the southern polar ice cap on Mars?
Newsgroups: sci.astro, sci.astro.amateur, alt.sci.planetary,
alt.astronomy, sci.geo.meteorology
Date: 2001-07-20 11:36:46 PST
http://groups.google.com/groups?th=eb7f73c9108af924

Bob Clark