# Can liquid water exist on a planet without atmosphere?

One way, I think, is if the gravity is exactly correct. However, while I think the water might be in a unstable equilibrium, I have been unable to work out the mathematics. Let's assume that such a planet exist, will tides/ high waves result in spontaneous boiling of water; meaning that the water will soon or later disappear in space?

• Does underground water count? – Ville Niemi Dec 9 '14 at 7:57
• Also if you have liquid water exposed to vacuum, it will lose water as individual molecules in any case (since the molecule energies at a specific temperature are a distribution, not a single value) and a trace atmosphere of water vapor will always exist. Can this be ignored? – Ville Niemi Dec 9 '14 at 8:03

In vacuum water has no liquid state--it behaves like dry ice does here on Earth.

You could still have underground water and you could have an atmosphere that was just water vapor.

Depends on how you look at it. Europa doesn't have much of an atmosphere (and apparently it is mostly oxygen). It has a lot of liquid water, however, it is all under a crust of ice. Protecting it from being carried away. At least really slowing down the process. Gravitational forces from Jupiter generate enough energy to keep the water liquid beneath the surface.

Define atmosphere. It may consist solely of a water vapours. If the planet gravity cannot sustain that, then yes all the water will eventually evaporate into the outer space. I see nothing unstable.

The atmosphere is stable, I believe, when $\gamma \frac{mM}{R} > kT$; with higher temperatures it will dissipate.

• I meant unstable equilibrium more in the mechanical sense, with a slight change in height of water resulting in boiling. I'll work the maths once again; but as far as I can tell by rough calculations, the gravity required to achieve earth like oceans will not be able to hold water vapour without the already existing atmosphere ($v_{rms}$ seems to be higher than permitted). That was what I wanted to make sure. Thanks though – strNOcat Dec 9 '14 at 6:42
• That will require a precise gravity vs water depth. As soon as enough water evaporates, hence the depth lessens, there be no more accidental boiling. Yet again, nothing unstable. – user58697 Dec 9 '14 at 6:50

The surface water will evaporate to form a pure water-vapor atmosphere giving you the much needed atmospheric pressure you need to maintain liquid state water. Gravity will make sure the water vapor does not escape into space. But the problem here is 1) the temperature(mostly due to distance from nearest star and partially presence of greenhouse gases and volcanoes) and 2) the amount/ratio of water present on the planet with respect to the size of the planet.

In an extreme hypothetical scenario, you can have a planet made entirely of water. The core will be ice due to the compression of surface water, liquid water on the surface closer to equator and ice landmass closer to the pole, and an atmosphere of water vapor.

Not in any stable sense, you could have liquid water on a world that had a very thin atmosphere but the boiling temperature of that water would be next to freezing, if the world is massive and cold enough you could have a semi-liquid slush of seawater with very little atmosphere but there's always going to be out-gassing from it where the sunlight heats it directly. In theory you could alter this further with chemistry, saltier water will be more thermally stable with a higher melting and boiling point, the more thermally stable the liquid the less you'll lose at any given time. But you're basically right eventually this system is going to result in any and all surface water boiling off into space over a long geological time. The last puddles of the ocean are going to take anywhere between hundreds of thousands and tens of millions of years to finally evaporate or freeze. All of this is assuming that the primary doesn't undergo catastrophic change first, like a nova or late phase red giant expansion, if that sort of thing happens all bets are off.

• Water evaporates. – AlexP Jul 31 '17 at 14:33
• @AlexP Not in a vacuum, then it just Sublimes. – Ash Jul 31 '17 at 14:46