Comets, Ice, and Vacuum of Space
I understand that the process of "freeze drying",
used to make instant coffee, salvage water-damaged papers, etc.
uses a vacuum to turn ice directly into vapor without going
through the liquid stage. How can comets, being mostly ice,
exist for very long in the vacuum of space?
The temperature of those comets with long lifetimes must have, for
most of their lifetime, an average temperature such that the vapor
pressure of water, on average, is so low that they do not sublime a
significant amount of water vapor. Those that did not are dust, and
are not around to tell their history.
In the details the story may be more complicated because the
sublimation of water and other volatile components may evaporate
leaving a shell of minerals that slow the evaporation of volatile components.
You are correct in your understanding of the freeze drying process
in that water is made to sublime rather than evaporate.
If you look at a water phase diagram
-- I've found this site that has a nice one:
and look at the 1st graph, you can see that at 273K (freezing point
of water) the vapor pressure of water is around 4mm Hg. This means
that any sample of water will only be giving off enough vapor to
provide around 4mm Hg of pressure.
Now look at the 2nd graph which shows the low-end values. If the
temperature drops to 190K, the vapor pressure of water is now down
to 0.0002mm Hg -or a drop of 4 orders of magnitude. The temperature
of space varies from place to place, but in deep space with only the
background radiation as heat source, the temperature is estimated to
be around 5K, imagine what the water vapor pressure would be then.
So, the reason we can freeze dry goods is that we keep the
temperature near 273K (0 degC) and apply a strong vacuum. In space,
even though there is a strong vacuum being applied to water, the
temperature is so low that the water does not sublime very much.
(This, of course, changes when comets get close to the sun - hence the tail.)
Greg (Roberto Gregorius)
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Update: June 2012