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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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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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    $\begingroup$ This is a terrific answer. $\endgroup$ – dotancohen May 5 '14 at 7:09
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    $\begingroup$ @dotancohen Exactly. $\endgroup$ – Praveen Kadambari May 5 '14 at 7:15
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    $\begingroup$ @Mark Terrific explanation. $\endgroup$ – Praveen Kadambari May 5 '14 at 7:17
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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 '14 at 4:25
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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 '14 at 1:20
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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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  • $\begingroup$ Darn, that one blew me away. OF COURSE ! It would change state. PLUS you have the density of air to consider. Therefore, No Gin Tonics in outer-space. Sad... $\endgroup$ – user1556 May 4 '14 at 21:38
  • $\begingroup$ Unless you are in a suit, you wouldn't survive either. $\endgroup$ – LDC3 May 4 '14 at 22:35
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    $\begingroup$ I wouldn't survive without gin & tonic. $\endgroup$ – Caleb May 5 '14 at 14:10
  • $\begingroup$ Why would the ice cube sublimate ? $\endgroup$ – Nicolas Barbulesco May 5 '14 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 '14 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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