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The Question

(Submitted May 25, 1997)

This question is about comets in space. How can ice, which is a form of water and which has a finite vapor pressure, exist in the near vacuum of space?

The Answer

Comets are usually in the outermost regions of the Solar system (the Oort cloud), where it is extremely cold. Water ice can survive billions of years in the Oort cloud.

However, the comets we observe -- those that come into the inner Solar system --- do lose a lot of volatiles. This is a part of the process that creates the tails, the signature we associate with comets. Comets that are trapped in the inner Solar system will soon (astronomically speaking) exhaust all their volatiles and become extinct (i.e., rocks with no cometary activities).

Koji Mukai for
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