'Hey Bill Nye, Why Don't Gas Giants Have Gas Moons?' #TuesdaysWithBill | Big Think

Hi Bill Nye. My name is Aria. My question is where does all moons come from and why does gas giants not have gas moons?

Wow. That is a great question, Aria. Wow. First of all, I was alive when people figured out or satisfied themselves as to where our moon came from. And it's generally agreed that our moon was created when the Earth was hit with another pretty big thing, another asteroid. The impact was very hard, and the energy of the smashing was converted to heat. Both the Earth and the moon were hot and molten, and they both cooled off, and here we are with this other separate thing in orbit around us.

Now, why do gas giants not have gas moons is a great question, and the answer is almost certainly because of gravity. So when you have these little things that spun off of, let's say, Jupiter or Saturn, they were too small to sustain themselves as just balls of gas. The gas shrunk down, gravity pulled it down, and formed a solid thing.

Whereas on Jupiter and Saturn, there's so much spin, so much heat created that they are held in the gaseous state, or what we call gaseous state, by the energy that's coming out of them, whereas the smaller ones cooled off so much that they turned solid.

It is just a fantastic question, Aria. Astrophysicists or planetary scientists speculate about this all the time, but what a fantastic thing that you made this observation. The moons of Jupiter, the big ones Io, Callisto, Europa, and Ganymede—those big moons are cooled off. They're not gaseous like Jupiter, what they orbit. That is fantastic. Wow.

And maybe more will be learned about this. And I got to tell you, Aria, it's very reasonable that in your lifetime, we will send the right spacecraft to Europa and the moon of Saturn, Enceladus and Titan, and look for signs of life. Instead of being a gas giant, in this one example, Europa is the moon of Jupiter that has twice as much seawater as the Earth, and that water is under a layer of ice.

And I mention it because it's evidence of how things cool off when they're in space and they're small or relatively small. The ice forms because the heat of creation, things smash together; that heat is radiated into space, so there's a crust of ice, but water underneath is kept liquid by the gravitational exercising of its orbit around Jupiter.

It's amazing. What an observation. Aria, you're making my day. I hope you will be involved in missions to Europa and you will look for signs of life below the ice. I mean, who knows? If you have an ocean for four and a half billion years, things are going to happen. There could be living things there. There could be Europanians out there, fish people that you'll interact with. Keep us posted. Thank you...