Why is Venus so dry?
The atmosphere of Venus has almost no water in it at all, whereas water is everywhere on Earth. This has been a long standing mystery about out sister planet. Where did all the water go? The culprit may be the magnetic field of Venus, or, more accurately, its lack of one.
The Sun spews out subatomic particles in its solar wind. When they reach the Earth, these particles are snagged by our magnetic field tens of thousands of miles above the surface of our planet, where they are slowed and trapped. But Venus has no such field, so these particles slam directly into the atmosphere. They can impart so much energy to the atoms there that they can escape the planet entirely, like shrapnel accelerated by a bullet.
Venus Express, an ESA probe orbiting Venus now, has finally detected this directly. As it rounds Venus on its elliptical orbit, it can detect particles in space around the planet. Venus Express has found that hydrogen on the day side of Venus -- the side facing the Sun -- is flowing out of the atmosphere. Last year the probe detected both hydrogen and oxygen coming off the night side of Venus like a comet tail, indicating water was being stripped apart and ripped out of the atmosphere. This new day side finding is important in that it gives scientists a better look at the actual mechanism at work.
It's not clear if this is the main process that has stripped the water from Venus over the past few billion years or not. We're seeing the tail end of a very long series of events, and there are other factors. For example, the atmosphere of Venus is pretty different than ours; for some reason Venus' carbon budget is all in its air, while ours is mostly locked up in sediment in the ocean floor. Venus suffered a runaway greenhouse effect, so it's incredibly hot on the surface (400 C or about 900 F) and its atmospheric density is absurdly high. Any ocean on Venus would have boiled away long ago, putting all that water into the atmosphere, where possibly its lack of magnetic field left that water at the mercy of the solar wind.
So Venus may be dry because of its greenhouse effect coupled with having no magnetic field (and who knows, maybe those two things are related as well). But why does Venus have no magnetic field, and we do? Venus and Earth are roughly the same size and density, so why should we enjoy a strong field, and Venus has none?
And there is a remaining question: why has no oxygen been seen escaping the day side of Venus? They found hydrogen, but no oxygen. If it's the last remaining water in the atmosphere of Venus providing the hydrogen, then oxygen should be there too. If it's not, that means we still need to figure out what's going on over there.
And, of course, all this points directly back at us. We know our planet is simultaneously stable (it's had life on it for billions of years) as well as fragile (a lot of things have to be balanced pretty well for complex life to maintain itself).
When we look at Venus, there but for the grace of magnetic fields go us. It's important we understand that, and so it's critical that we continue to study the solar system and the Universe around us. Our water and our lives may depend on it.
Tip o' the Faraday cage to Carl Zimmer.