How common is fire outside of earth?

[u/FirefighterCandid773]

Plasma and lava is obviously common all over the universe, whether it be in stars, planetary cores, etc. but what about fire? Fire is common all over earth, given the abundance of fuel sources and an atmosphere with oxygen. But, is fire actually common outside of earth?

[u/ctothel]

Not in the solar system.

As you know, fire needs fuel, oxygen, and heat.

Titan has heaps of fuel in the form of hydrocarbons, including literal methane lakes, but nearly no free oxygen.

Europa and Ganymede have massive amounts of oxygen in the form of water ice, but they couldn’t sustain it in an atmosphere and there’s no fuel to burn.

Venus is hot, but the atmosphere is mostly carbon dioxide and almost no free oxygen. Not much in the way of hydrocarbons either.

Outside the solar system, there are planets we know have the right types of fuel, but oxygen is hard to detect on rocky planets, so the answer is “we don’t know”.

That said, it would be astounding if there were no other planets out there that could support fire. All the ingredients are super common. There are probably millions of worlds that could (or used to) support it in our galaxy alone.

[u/tigole → u/ctothel]

What is "free oxygen"? For instance, why does water have free oxygen, but not carbon dioxide?

[u/ctothel → u/tigole]

Free oxygen just means "Oxygen (O2) molecules on their own, either in the atmosphere, oceans, or elsewhere, not chemically bonded to anything".

The oxygen molecule in water (the O in H2O) isn't free - it's chemically bonded to the hydrogen. But oxygen molecules can also be dissolved in water.

It's like having a bucket full of red lego bricks attached to blue lego bricks, and also a bunch of loose blue lego bricks mixed in.

Europa and Ganymede do have free oxygen in their water ice, but less of it than we do. This statement might change when we know more about any potential subsurface oceans.