Why does Venus have an atmosphere?

We put this to astronomer Matt Middleton, from the University of Cambridge...

Matt - So, a magnetosphere is incredibly important - the Earth has one. The Earth has one because we have a molten core that rotates, generates a magnetic field, a dynamo, and this magnetic field deflects particles that are coming to us from the Sun's highly charged magnetic particles. Now, if we didn't have a magnetosphere, the interaction would start to remove ions from the ionosphere, so our atmosphere would get leaky.

Now Mars doesn't have a magnetosphere; it used to probably around 4 billion years ago and we're not entirely clear why it went away. Now it's not just because it lost it's magnetosphere and everything just 'woo' off it went into space - that's the noise it makes actually. But, actually Mars is much smaller than the Earth and it has a gravitational field of roughly around 36% of Earth's, so holding onto stuff is actually very, very difficult.

Now if you go to Venus I would have to correct one of the things that was said. You're right, it doesn't have a magnetosphere but it has got something. It has an induced magnetosphere because the magnetic field from the Sun wraps around it, so it has some protection from the.

Kat - Is that because it's so close to the Sun?

Matt - Yes.

Kat - OK. So it's sort of hogging a bit of magnetic field from the Sun?

Matt - Yeah, yeah. Because magnetic fields are slamming into Venus and wrapping around so you end up with a sort of quasi magnetosphere.

Kat - Like a magnetic cuddle?

Matt - A big magnetic cuddle. That is the cutest thing I've ever heard in terms of planetary astrophysics.

Kat - And its holding some gas onto it like an atmosphere.

Matt - That's cute! Ah, I see a brand of toys emerging here. And Venus is very similar to Earth - it's essentially our twin planet. So it's heavy and that means it holds onto its gas so that is why it has more of an atmosphere.

Kat - Because of the magnetic cuddle.

Matt - Because of the magnetic cuddle.