QUESTION:
Does the expected cooling of Pluto to the temperature of the March 14
(or thereabouts) Pluto images correspond to the freezing point of the
known gases on the planet? Is that why Marc Buie expect a freezing of
the atmosphere?

ANSWER from Mark Buie on 26 May, 1996:
The surface and atmosphere of Pluto is pretty complicated.  We don't know
the exact surface temperature but we have an idea of the range.  For the
temperature and atmospheric pressure on Pluto, most everything is frozen
out.  Even at those cold temperatures, some gas can exist above the surface.
The amount of gas depends on the temperature of the frost.  If you pump
energy (heat) into the frost, you will create more gas.  The really neat
thing about this balance is that as long as you have gas and frost together,
the temperature doesn't change as you add heat.  The extra heat goes into
making more gas, not into warming the surface unless you got rid of all
the frost.  So, the simplest expectation is for the atmosphere to begin to
slowly collapse as Pluto moves away from the Sun, it's only source of
warmth.

We now expect the surface of Pluto to be quite a bit more complicated.
The amount of heat that Pluto gets from the Sun is very, very small.
That means that the rocks and ice below the surface could store enough
heat to smooth out the temperature changes during an orbit.  Depending on
how important this reservoir of heat is, we could see the atmosphere
collapse completely, or, we could see a nearly constant atmosphere.
It's going to take quite a few year to answer this question, maybe as long
as 250 years!