From: Henry Spencer
Newsgroups: sci.space.policy
Subject: Water at Martian south pole

Marvin wrote:
The 'air' on Mars is basically pure CO2.
The ambient temperature is FAR below the freezing point of CO2...
therefore there is a strong force trying to get that gaseous CO2
to sublimate into dry ice.

Bear in mind that "the freezing point" isn't a unique number -- it
depends on pressure. At an ambient pressure of a few millibars, the
freezing point of CO2 is nearly 50K colder than at 1atm.
That would put the freezing point of Martian CO2 at about -130, the
recorded summer south pole temperature, which explains why the cap shrinks
and expands with Mars' seasons. Indeed, if freezing was -80 C at 3 mbars,
then the equilibrium condition would be no measurable atmosphere.

At 3 mbar, what's freezing point of H2O ? That is
quantitatively, how permanent are the s.pole ice fields?

The vapour pressure of solid CO2 at -130celcius is virtually zero.
Actually, it's about 3 millibars according to the Matheson Gas Data
Book (6th ed), which would be insignificant on Earth but is highly
significant on Mars, especially in highland areas like the South
Pole.

The normal atmospheric content of mars is only that of about
4 cm of dry ice, (in gas form of course). Evaporate another 4cm
and you have just doubled the local air pressure.
That would compute were the atmosphere in an inclosed chamber which it
isn't. More ground level pressure, comes only of added CO2 mass above.

Solving Mars's Polar-Ice Puzzle, 2/25/03
SkyandTelescope.com/news/current/article_881_1.asp
While the polar caps likely contain enough water to form a global ocean
about 20 meters deep, there may only be enough dry ice in storage to raise
the atmospheric pressure by a minuscule 0.36 millibar -- just 5% above its
current value.

Which would put Mars atmospheric pressure at about 7 mbar.
Do they mean ground level pressure at some imaginary sea level?

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