Originally posted by cantsee4looking
reply to post by Phage
so,how can water freeze in space if the ambient temperature in space is above freezing??
(cue spoon girl from matrix 1)
It's not that the ambient temperature in space is above freezing. There is no ambient temperature in space.
Ambient temperature is something you get on the ground from atmosphere. You don't really have any in space. Space isn't hot or cold. Since there's no
ambient temperature because there's no ambiance to have one, you lack a few heat exchange mechanisms you have on the ground. For one, convection's
right out. For another, conduction is gone too. They both require some sort of working fluid, you got none.
That leaves radiation. Not the nuclear sort, the IR sort. Also, you've still got evaporation/sublimation in the case of water.
So, your heat budget is - energy from sunshine + Earthshine in, energy to near-absolute-zero infinite heat sink out. The ice chunk is absorbing solar
energy, if it's in the sun, and IR/visible shine from Earth. The high albedo of ice sort of reduces the input from the sun. In addition, if you DO
start to warm the surface toward a liquid state, the more energetic molecules will leave the ice chunk, cooling it by sublimation.
On the other hand, you've got the rest of the ice chunk not being irradiated by Earth or Sun shedding its heat to the background temperature of space.
That's near absolute zero. The high albedo slows that down too, so just how warm the thing gets depends on a lot of unknowns - how clean the ice is,
whether it's tumbling, the orbit etc.
In general, though, the sublimation term will dominate until the ice chunk is individual water molecules.
edit to add:
Dang you, OP, now I've got "Warmth of the Sun" stuck in my head.
edit on 14-7-2012 by Bedlam because: (no reason given)