Atmosphere of Mars

Here is a comparison of the atmospheric composition of Earth and Venus and Mars. I list the number of molecules per m² of surface area of the planet in each planet's atmosphere relative to the total number of molecules per m² in Earth's atmosphere.

          Earth       Venus        Mars
N2        0.79         2         3 x 10-4
O2        0.20       < 0.001     10-7
Ar        0.01         0.005     2 x 10-4
CO2       0.0003      64         0.009
H2O     ~ 0.02       ~ 0.01      ~10-6

Total     1.00        90         0.01

--------------------------
H2O      3 km        0.5 mm        small
liquid 
+ vapor

(A technical note: This is based on a pressure ratio of 0.0056/1,
a surface gravity ratio of 0.38/1.0, and a mass per molecule
ratio of 1.5/1.0 for Mars/Earth.)

In cold weather, find H₂O frost on the surface.

There is CO₂ ice in polar caps. Most of it evaporates in summer.

Water ice in the polar caps. Maybe some under the surface. But how much?

How about liquid water?

Really, it can't exist?

Where did the atmosphere go?

• Presumably it started out something like Venus.
• Put most of CO₂ in rocks (using rainfall), as on Earth.
• Then the weaker gravity of Mars wasn't able to hold the rest of the atmosphere.
  • Ultraviolet light helps by breaking up molecules so that they are light enough to escape.
• Not enough volcanos to replenish the atmosphere.
  • No plate tectonics.