Is Jupiter made entirely from gas?

Chris asks Caroline Crawford from the University of Cambridge to answer this listener's giant question. 

Caroline - This is a very good question because Jupiter really is giant. It’s got a volume of which is equivalent to 1,300 earths. But the fact that it only weighs just over 300 times the mass of the Earth immediately tells you it’s mainly made of gas. So it’s hydrogen, helium things like that - that’s the predominant component. But we do think there's a rocky core, and maybe a rocky core that could be 10, maybe even up to 30 times the mass of the Earth all compressed down into something slightly less than the size of the Earth, right down into all that atmosphere. We can’t fly through and find out, this isn’t experiment you can do because the trouble is, if you throw a spacecraft into Jupiter, which we have done by the way, in the Galileo mission. A spacecraft goes into Jupiter - a good way to dispose of the spacecraft at the end of it’s mission.

Chris - And a comet because Shoemaker-Levy 9 plunged into Jupiter as well, didn’t it?

Carolin - What happens is when you look at the disc of Jupiter all you see is the cloud tops just in the few hundred kilometres. The gas is molecular for about 1,000 kilometres in but after that it has to carry the weight of all the overlying layers of gas and it starts to get high temperature, high pressure and becomes as incompressible as a liquid. So if you throw any spacecraft in, it’s just going to get crushed, it’s going to get destroyed really quickly, so we can’t fly in and find the rocky core or see it. We think it’s there from everything we understand about how planets form. We think you have to have a rocky core that grows quickly that can then sweep up the gas and accumulate this huge atmosphere.

But the question is how do we measure it’s size? How do we measure its mass? And this is, in fact, what we’re doing now with the Juno spacecraft that’s in orbit around Jupiter, and it’s got this 53 day orbit. On one end of the orbit it skirts to within about 4,000 kilometres of the cloud tops of Jupiter and, as it does that, it’s tracking the form of Jupiter’s gravitational field which, of course, depends on the distribution of mass within the planet. So that’s our way of tracking it and that’s one of the main things of this spacecraft - it’s got a lot of other science - one of the main things it’s trying to determine.

Dani was saying if you Google on thing tonight - go and do “tongue parasites”. Well that doesn’t quite appeal to me, especially when I’m having my dinner, but I would say go and Google “NASA Juno mission”. They put up, just in the last week, some fantastic images of some of these fly pasts of  Jupiter and they are just beautiful, as well as being really interesting science, but just mesmerising.

Chris - Duncan?

Duncan - If there is a rocky core, is there any way of knowing what the rocky core is made of?

Carolin - It’s the same as all the rocky planets. So it’s going to be carbon, nitrogen, oxygen, magnesium, silicon, iron. All the normal things that make up all the rocky planets. It’s just that when Jupiter forms, it forms further out from the Sun and you’ve got all these volatile ices and molecules. That rocky core can then sweep up those and accumulate in an atmosphere in a way that the planets like Earth and Mars can’t because they haven’t got all those lightweight gases around.