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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?

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3 Answers 3

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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.

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  • $\begingroup$ @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? $\endgroup$ May 7, 2014 at 3:52
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    $\begingroup$ @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. $\endgroup$
    – Mark
    May 7, 2014 at 4:25
  • $\begingroup$ 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). $\endgroup$ Jul 11, 2014 at 20:42
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    $\begingroup$ @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. $\endgroup$
    – Mark
    Dec 28, 2014 at 1:20
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    $\begingroup$ @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). $\endgroup$
    – Mark
    Apr 2, 2018 at 21:29
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It would sublimate. The frozen mass of water would decrease in size as the water converts from a solid to a gas (without becoming a liquid) and drifts away.

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  • $\begingroup$ Unless you are in a suit, you wouldn't survive either. $\endgroup$
    – LDC3
    May 4, 2014 at 22:35
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    $\begingroup$ Why would the ice cube sublimate ? $\endgroup$ May 5, 2014 at 20:01
  • $\begingroup$ @NicolasBarbulesco There is very little gas surrounding the ice cube in space. If the ice is above 40 K, as pointed out above, there is enough heat in the ice for some of the molecules to leave the chunk of ice and become a gas. $\endgroup$
    – LDC3
    May 6, 2014 at 1:57
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In the vacuum of space the most important consideration is to consider how much radiation an ice cube would absorb from, for example, nearby stars and how fast the ice cube itself would radiate away energy (using Wien's law), finding what ice cube temperature would produce an equilibrium (the temperature at which the ice cube radiate energy at the same rate it absorbed energy) and then determining if that temperature is above or below the melting point of the ice cube. If it is above the melting point (of water in a vacuum), then as the other answers have said the ice cube would sublimate; if it is below the melting point then the ice cube would stay frozen.

Specifically for an ice cube that is a cube in orbit around the sun with one side facing the sun you would need to calculate how much energy the side facing the sun absorbs from the sun as well as how much energy radiates away from all six sides of the cube and then find the equilibrium temperature.

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