Atmosphere of Venus

Not a nice place to visit, and you wouldn't want to live there.
Venus has a very thick atmosphere:

air pressure at surface ~ 90 atmospheres.

Here is a comparison of the atmospheric composition of Earth and Venus. 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
N₂       0.79         2
O₂       0.20       < 0.001
Ar        0.01         0.005
CO₂      0.0003      64
H₂O    ~ 0.02        0.003

Total     1.00        66

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

(A technical note: This is based on a pressure ratio of 92/1,
a surface gravity ratio of 0.91/1.0, and a mass per molecule
ratio of 1.5/1.0 for Venus/Earth.)

Note that the amount of nitrogen in Venus's atmosphere is a little larger than in Earth's atmosphere, but the amount of oxygen is much less and the amount of carbon dioxide is much more. Also, if you count the oceans on Earth, Venus has a lot less water than Earth.

Venus has clouds 50 to 70 km above surface containing H₂SO₄ (sulfuric acid).

Where did Earth's CO₂ go?

On Earth it was cool enough for water vapor to condense.
CO₂ dissolves in water.
At the Earth's surface it could combine with chemicals in the ocean to make rocks. (``chemistry'' in the picture.)
Now living things help get rid of CO₂.
Venus doesn't have any living things and it is too hot for water vapor to condense.

Where did Venus's water go?

H₂O vapor in the atmosphere can be dissociated into H₂ and O.
H₂ escapes.
Oxygen is very reactive and combines with other gasses or with iron on the surface.
On Earth, plants make O₂ for us.

ASTR 121 Home

Davison E. Soper, Institute of Theoretical Science, University of Oregon, Eugene OR 97403 USA soper@physics.uoregon.edu