## Wednesday, September 5, 2012

### atmospheric pressure on Venus

the atmospheric pressure on the surface of Venus is almost 100 times larger than on Earth, despite the fact that the gravitational acceleration is 10% less. how is that possible?

bonus question: why is the pressure on Mars so low? (actual numbers are $p=6\times10^{-3}$ bar at a third of Earth's gravity)

1. Because Venus' atmosphere consists of heavy elemets like carbon dioxide at a high concentration. Therefore you have more mass above you which leads to a high surface pressure.

2. bonus: I would guess because the gravity of Mars is to low to permanently retain an atmosphere. This means that gas can leak away. If now new gas is released (i.e. from volcanos for a planet like Mars). Therefore the same argument as above, for fewer mass in this case.

3. The answer is: Simply because there is much more gas. That's all. Once you have enough gravity to prevent the gas from leaving the planet (i.e. a certain minimum planet mass), the final pressure does mainly depend on the total supply of gas. As long as the atmosphere is thin compared to the planet's radius, the pressure is simply the mass of gas per square meter times the gravitational acceleration on the surface. The former of the two factors can vary by many orders of magnitude, the latter varies only mildly among stony planets.

As correctly guessed by "Tyra", Mars's gravity is just a bit too small to keep the atmosphere, and the Moon's is hopelessly weak.

It is, by the way, not really the level of gravity acceleration on the surface which is responsible for keeping an atmosphere, but more important is the depth of the gravitational potential. And that is proportional to the product of the acceleration and the radius of the surface.

And another side remark: originally, the total supply of gas at Earth and Venus was very similar. The early Earth had an atmosphere of a few hundred bar, similar to Venus now. But then practically all the water rained down into oceans (at Venus it gradually got dissociated by sunlight and escaped into space in the form of hydrogen), which removed about 300 bar of Earth's atmosphere. And practically all of the carbon dioxide was bound into carbonates, which removed another 100 bar or so. That carbon dioxide was retained by Venus, because the surface has always been too hot to allow liquid water, and thus the formation of carbonates.

As I said: "Very simply, because there is much more gas." At a closer look, a slightly more telling reply would thus be: Because Earth lost most of its original atmosphere.

Uli Bastian