Science Questions

Why does Venus have an atmosphere?

Tue, 9th Aug 2016

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Alan Bunker asked:

It has not been explained to me why Venus has still got an atmosphere. Mars has lost its, no magnetosphere, but Venus is the same, so therefore it must be restored each time it is lost! Thank you for your interesting programme.


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

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 Sunís highly charged magnetic particles.  Now, if we didnít have a magnetosphere, the interaction would start to remove ions from the ionosphere, so our atmosphere would get leaky.

Now Mars doesnít have a magnetosphere; it used to probably around 4 billion years ago and weíre not entirely clear why it went away. Now itís not just because it lost itís magnetosphere and everything just Ďwooí off it went into space - thatís 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 theÖ

Kat - Is that because itís so close to the Sun?

Matt - Yes.

Kat - OK. So itís 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 Iíve ever heard in terms of planetary astrophysics.

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

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

Kat - Because of the magnetic cuddleÖ

Matt - Because of the magnetic cuddle.


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There is a significant difference in mass. Wikipedia shows:
- Mars: 6.4171◊1023 kg (0.107 Earths)
- Venus: 4.8675◊1024 kg (0.815 Earths)

The mass makes a big difference to how well the planet can hold on to an atmosphere. This shows itself as a difference in escape velocity:
- Mars: 5.027 km/s
- Venus: 10.36 km/s
- Earth: 11.186 km/s

Slightly counteracting this, Venus is closer to the Sun than Mars, so the solar wind would be a bit stronger, and the atmospheric molecules would be hotter (higher velocity).

Mars is still thought to retain a reservoir of frozen water underground, especially in polar regions. So not all volatiles have been lost. Explorers could potentially break down the water to produce breathable oxygen. evan_au, Thu, 21st Jan 2016

Mars has some volcanoes (including one very large volcano). Some eruptions are thought to have occurred as recently as 10 million years ago. Volcanic eruptions belch gases into the atmosphere.

Venus is much harder to study, because the atmosphere reflects all visible light. However, radar studies show that Venus has a very large number of volcanoes, and some are thought to have erupted in the past 2.5 million years.

Because of its stronger gravity (and magnetic cuddle), Venus would hold onto more of the volcanic gases than Mars. evan_au, Wed, 10th Aug 2016

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