[thedoc (OP)]

alan bunker asked the Naked Scientists:































































































































































































































































   It has not been explained to me why Venus has still got an atmosphere. Mars has lost its magnetosphere ,but Venus is the same, so therefore it must be restored each time it is lost!































































































































































































































































Thank you for your interesting programme.  































































































































































































































































What do you think?

[evan_au]

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.

[thedoc (OP)]

We discussed this question on our  show
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.
Click to visit the show page for the podcast in which this question is answered. Alternatively, [chapter podcast=1001410 track=16.08.09/Naked_Scientists_Show_16.08.09_1005548.mp3] listen to the answer now[/chapter] or [download as MP3]

[evan_au]

Mars has lost its magnetosphere ,but Venus is the same, so therefore it must be restored each time it is lost!

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.
https://en.wikipedia.org/wiki/Volcanology_of_Mars#Potential_current_volcanism

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.
See: https://en.wikipedia.org/wiki/Volcanology_of_Venus#Recent_volcanic_activity

Because of its stronger gravity (and magnetic cuddle), Venus would hold onto more of the volcanic gases than Mars.