### What would happen if an ice cube is left in space?

• Recently I boarded a flight and noticed outside air temperature as -53°C at an altitude of 36860ft (11.23km). I don't know what causes such a freezing temperature in that altitude but was wondering higher altitudes (space) may have even freezing temperatures. Here I got a doubt i.e what happens if an ice cube is left in space? Would it be melting or stay as it is?

8 years ago

It depends on where in outer space you are.

If you simply stick it in orbit around the Earth, it'll sublimate: the mean surface temperature of something at Earth's distance from the Sun is about 220K, which is solidly in the vapor phase for water in a vacuum, and the solid-vapor transition at that temperature doesn't pass through the liquid phase. On the other hand, if you stick your ice cube out in the Oort Cloud, it'll grow: the mean surface temperature is 40K or below, well into the solid phase, so it'll pick up (or be picked up by) gas and other objects in space.

A comet is a rough approximation to an ice cube. If you think of what happens to a comet at various places, that's about what would happen to your ice cube.

@dotancohen Exactly.

@Mark Terrific explanation.

@Mark You said anything at earth's distance from the sun is about 220k which is -53.15°C.How can water be in vapor phase in such a freezing temperature?

@PraveenKadambari Because there's no atmospheric pressure. See Wikipedia's phase diagram of water: the phase depends on both temperature and pressure, and as the pressure drops, so does the freezing point. In non-scientific terms, you can think of it as there being no air to help hold the ice together, so it takes more cold to do so.

In the Oort cloud, I don't think there's a whole lot of gas and other objects for it to pick up (though we probably don't have a good estimate for its density).

What if the ice cube was shaded from the Sun and wedged against the metal side of a spacecraft such as the Gaia orbital observatory?

@steveOw, the shading doesn't matter over the long run: the shade will reach thermal equilibrium with the Sun and in turn heat the ice cube. The spacecraft makes things too difficult to figure out, because it actively controls the temperature of its environment.

Gaia has a continuing problem with unexpected icing over a year after launch and presumably any deliberate spacecraft activities would be intended to increase temperature in the affected area. Presumably the "long run" for equilibration in this case is more than a few months?

@steveOw, Gaia has active thermal management (both heating and cooling), and is not in equilibrium with its surroundings. The "long run" for equilibration hasn't even started yet.

@Mark I need some education here. At 15 PSI, 270K, ice is in the solid phase i.e. an ice cube in your freezer. Yet over time, ice cubes in the freezer get smaller, presumably due to sublimation. What changes going from solid at 15 PSI 270K to solid at vacuum (or close to) and 40K?

@dgnuff, in the freezer, the ice cube sublimates and the water vapor re-freezes on the other surfaces of the freezer (hence the need to periodically defrost it). In space, the only available surface for re-freezing is the ice cube (and at 40K, the sublimation is a lot slower than at 270K).