17 January 2016

Water ice confirmed on the surface of a comet

This apparently isn't unexpected, as this Reddit thread discusses.  But it still boggles my own mind, since I spent the first half-dozen decades of my life believing that water in any form would sublimate instantly and could therefore not exist in an object traveling through a vacuum.

And it is water ice, btw, not frozen methane of ammonia.


  1. Your intuition about sublimation is good for temperatures humans typically experience. If you look at a phase diagram for water you see near the bottom (where pressure is basically zero) that even water we would normally consider well and truly frozen does indeed sublimate. For instance at extremely low pressures experienced in space ice at -50 F (227 Kelvin) sublimates to gas.

    But at 200 Kelvin (circa -100 F) that changes. Ice stays ice even at extremely low pressures. I just looked up an estimate of the temperatures in the Oort Cloud where comets spend most of their life and they are about 4 Kelvin (-452 F). Which explains (a) the presence of water ice in a near perfect vacuum and (b) why the ice does indeed begin to sublimate and kick out lots of gas forming a tail as it nears the sun and the surface warms up to a relatively balmy -94 F.

  2. That actually makes sense now - because for something to sublimate, there has to be movement. Tx, Dan.

  3. So... what did you think comets were made of?


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