Pluto is just a large Dwarf Planet or Trans-Neptunian Object, like several others out in the Solar System’s ragged Inner Edge.
The atmosphere is probably an equilibrium mix of nitrogen and methane that have sublimed off the ices that cover the surface. A few kilometres up – about one scale height – the temperature inverts and rises to about 100 K, remaining pretty much isothermal all the way out to its solar windy exosphere.
Charon shows no sign of the N2 or CH4 ices that colour Pluto. It’s a dirty ice surface, darkened by cosmic rays over billennia… except, oddly, it shows signs of water-ice floes that have oozed out on to its surface. Perhaps methanol is mixed in with it, since the water-methanol eutectic stays “liquid”, in a gelid way, down to very low temperatures. Thus the oozing, from wounds caused by volume cracking as water freezes in its once fluid mantle. Giant ice-moon scabs…
The fastest launched probe ever, “New Horizons”, is on its way to Pluto and Charon, set to arrive 14 july 2015. Equipped with a powerful little telescope the probe will be doing useful observations for three months before the closest approach and considerable time afterwards. Then it will cruise on to encounter another TNO or two before it runs out of RCS reaction-mass. It won’t overtake our fastest near-Interstellar scout, “Voyager II”, which is over 111 AU away already, but it will travel on far beyond the Sun’s family.
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