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.)Note that Mars has more carbon dioxide in its atmosphere than does Earth, but Mars has a lot less of everything else.
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?