QUESTION:
What is the boiling point of water on Mars?

ANSWER from Bruce Jakosky on February 8, 2000:
On Earth, water boils when the pressure of water vapor in equilibrium with
liquid water is the same as the total atmospheric pressure.  At sea level,
this happens at a temperature of 100 degrees C.  At this temperature, liquid
water spontaneously turns to vapor, as the pressure of the atmosphere is no
longer able to contain the water in liquid form.  At higher elevations on
Earth, the total atmospheric pressure is lower, and the boiling temperature
is lower.

On Mars, the total atmospheric pressure is about 6.1 millibars, or about
6/1000 of the pressure at the surface of the Earth.  For water vapor in
equilibrium with liquid water to have this pressure, the temperature has to
be very close to 0 degrees C.

Interestingly, at some of the higher elevations on Mars, the total pressure
is lower than this, and water will boil at even lower temperatures.  As
liquid water cannot exist at temperatures below 0 degrees C--it freezes to
ice--the situation can arise in which ice "boils", or goes directly from
solid to vapor.