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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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