Is there any material that can exist as a liquid in vacuum?

[szputnyik]

For example when lava covered the early Earth, above that lava ocean was there the vacuum of space or some kind of rock vapour atmosphere?

Is there anything that is a liquid in vacuum?

[Holo]

From what I recall, Earth had an atmosphere made mainly of Hydrogen during its early formation.

[K^2]

Ionic liquids. I mean, absolutely anything will eventually evaporate in absolute vacuum, technically even the solids, but conventional liquids evaporate a lot faster because you have to be pretty close to binding energy for things to behave as liquids in the first place. So if you want a fluid to stay fluid something has to prevent particles from leaving the mix, and ionic liquids achieve that by having ions instead of molecules in the mix. A single ion can't break off and leave, because that results in an overall charge strong enough to pull that ion back. And pair of ions getting enough energy to leave together is far, far less likely, resulting in very low vapor pressures and slowest evaporation in vacuum.

Typical use of ionic liquids is lubrication in vacuum pumps, so that should give you some idea.

[Kryten]

I know for sure that sodium-potassium alloy is liquid in space (at least over reasonable timescales-it'll boil away, but over thousands of years)-the soviets used it as coolant in some space-based nuclear reactors, and to this day there's droplets of it whizzing around in LEO after a couple developed leaks.

[lajoswinkler]

From what I recall, Earth had an atmosphere made mainly of Hydrogen during its early formation.

That hydrogen was not in vacuum.

[Holo]

That hydrogen was not in vacuum.

For example when lava covered the early Earth, above that lava ocean was there the vacuum of space or some kind of rock vapour atmosphere?

I was answering this question.