Why doesn't space suck away our atmosphere?

This question from listener Nigel is out of this world. Chris Smith asked physicist Adam Murphy to explain why space doesn't suck up our oceans and atmosphere...

Adam - There's two reasons for this. The first one is that Earth is really big. There's a lot of atmosphere but there's a lot more earth and gravity means it will just pull all that air in and pull it in so strongly that it doesn't escape off. Little bits might bounce off the very top of the atmosphere or fly away but so few that we're gonna be fine breathing. The other one is that it doesn't actually hold that much. Space is roughly defined by the Kármán line which is about a hundred kilometres straight up. So that's an hour at motorway speeds in a car. That seems like a lot and we say it's a lot: things like "the sky's the limit". But, if you think that the Earth is about twelve thousand kilometres from one end to the other, but space is only a hundred kilometres up, it actually isn't holding that much atmosphere.The atmosphere is a tiny little shell. So a giant Earth can hold a tiny shell of atmosphere with all its gravity.

Chris - And that's the reason why it doesn’t all get pulled off into space, because there is sufficient gravitational pull on those molecules to hold them against the Earth's surface? And so they don't just whip off into space?

Adam - Exactly, there's too much gravity pulling them down, compressing them against us.

Chris - Because one other argument was that Mars used to have a nice atmosphere, rather like we've got, but that having lost its magnetic field, possibly because it cooled down because it's a much smaller planet, that magnetic field loss meant that the wind coming from the sun, the solar wind, was then able to pluck away, slowly eroding away the atmosphere of Mars. So it turned into the prune of a planet that we've got today. We, luckily, have our magnetic field, so we're protected.

Adam - Yeah that's it. We've still got our shield holding our air to us.

Chris - So we've got gravity hanging onto it and the magnetic shield protecting it...

Adam - And all that together means we still get to breathe.