Mars

Water Vapor in the Martian Atmosphere

0.021% or 210 ppm, presumably by weight.

Atmosphere is 64 Pa. About 20 g/m³ (Earth is 1290 g/m³ at sea level. Equatorial wind speed up to 150 m/s, which produces a dynamic pressure equivalent to a 19 m/s (42 mph) wind on Earth. Scale height 11 km, Total mass 25e15 kg.

Air stripped away (about 200 g/s) by solar wind. Sun more energetic (with more UV) billions of years ago. At current loss rates, atmosphere would last 4 billion years.

Five million cubic kilometers of ice. At 1e12 kg/km³, that is 5e18 kg of frozen water. About 5e12 kg in the atmosphere, perhaps 0.01 Pa vapor pressure.

Mars global average temperature is -60C or 210 K (surface?) kT = 0.018 eV or 2.9e-21 J. 3.71 m/s² surface gravity, escape velocity 5.03 km/s.

Viking lander temperature (at 22°N) ranges from -89C to -31C, average -60C. Corresponding ice/water vapor pressures of 0.1 Pa, 34 Pa, and 1.08 Pa.

So, the big question: if below-surface ice is -60K, and has a vapor pressure of 1.08 Pa, and the atmosphere above is 0.01 Pa, what keeps the ice from sublimating very quickly and raising the vapor pressure? There must be a cap layer of rock that is impermeable to water vapor, but how can that be, over an entire planet with significant surface topology?