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 less energetic (but with more UV) billions of years ago. At current loss rates, atmosphere would last 4 billion Earth years.

Five million cubic kilometers of ice is claimed. At 9.2e11 kg/km³, that is 4.6e18 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 Pa.

So, the big question: if below-surface ice is -60C, and has a vapor pressure of 1.08 Pa, and the atmosphere above is 0.01 Pa, what keeps the ice from sublimating quickly, the vapor seeping to the surface, 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?

The heat of fusion of ice is 330 J/g. Ice density at -60C is 922.4 kg/m³. Average Mars insolation is 586 W/m² (493 to 717), averaged over a day on the equator that is about 185 W/m². If 0.1% of that power made it down to the ice, it would evaporate at the rate of 1 meter per 1.65e9 seconds (52 Earth years). After billions of years, why is there any ice left at all?

Perhaps silly question: Could the water vapor be due to comet impacts? Seems unlikely, I expect the flux of water molecules from the Oort to be way too small.

At higher latitudes, the average insolation is lower and temperatures are colder. Ice will be a lot more stable near the poles; perhaps the temperatures are low enough for the ice to condense from a 0.01 Pa concentration in the atmosphere.

Other stats

Rotational Stability

What (if anything) stabilizes the rotational axis of Mars? How fast do the poles shift?