QUESTION:
The air pressure on Mars is about 6 millibars which is about 4.5 mm/mercury.
At this pressure, the boiling point of water is 0 C. The average temperature
range on Mars is -63 (the range is -125C to about 15 C) - so usually on Mars
the temperature is well below the boiling point. So why can't liquid water
exist at this pressure and temperature? Also, water ice exists in clouds and
at the Poles. So how is it that it gets cold enough for water to be a solid,
but not a liquid? It seems like H20 on Mars skips the liquid stage. Any
ideas?

ANSWER from Bob Haberle on November 23, 1999:
Your question is very timely since I have recently been looking into the
possibility for liquid water on Mars.

To have stable liquid water on the surface of Mars the temperature must be
above 273K, and the surface pressure must be above 6.1 mb. This is the
triple point of water, i.e., the temperature and pressure at which all
three phases can exist in equilibrium. Based on the results of a model I
have developed, there are five regions on Mars where the temperature and
pressure is at or above the triple point: the low lying near-equatorial
regions of Amazonis, Chryse, and Elysium, and the impact basins of Hellas
and Argyre in the southern hemisphere.

According to the model, there are as many as 90 days in a Martian year
where liquid water could be stable. However, it is not continuous in time.
Temperature get above freezing only during the day when the sun is high in
the sky. Thus, liquid water is potentially stable only for a few hours in
the middle of the day.

Boiling occurs when the saturation vapor pressure of liquid water is
greater than or equal to the overlying surface pressure. The saturation
vapor pressure depends only on temperature. According to the model, water
would frequently boil on Mars since the surface pressure never exceeds 12
or 13 mb. To get the saturation vapor pressure of liquid water to equal
these pressures only requires a ground temperature of about 283 K, i.e.,
only about 10 degrees Celsius higher than the melting point. Ground
temperatures in excess of 283 K frequently occur on Mars (only during the
day) so this greatly limits the amount of time liquid water could be
stable.