About the paper “A 3D general circulation model for Pluto and Triton with fixed volatile abundance and simplified surface forcing” by Zalucha & Michaels
The authors have run a general circulation model on the cases of the atmospheres of Pluto and Triton.
The two have similar atmospheres. Pluto is the prototype of the “dwarf planets” according to the new IAU nomenclature whilst Triton is a satellite of Neptune, thought to be a captured Pluto-like body. Their atmospheres are made primarily of nitrogen, with ground pressure around 10 microbars.
That’s right: microbars. Collapsed to liquid form, the atmospheres would amount to less than one millimetre of liquid nitrogen over the surface of Pluto and Triton. Ten microbars in a lab on Earth qualifies as a very good vacuum.
Amazingly, this minute amount of air suffices to exhibit most of the features of a full-blown atmosphere: the atmospheres of Pluto and Triton extends over dozens of kilometres vertically, they have eastwards jets, photochemical hazes, and condensation clouds.
The main data comes from occultations of background stars by the planets. During ingress and egress the atmosphere shows are a progressive dimming, an occultation lightcurve.
Unlike Earth, temperatures get colder near the ground. The reason, in the words of to ESO press release 0908 (about an occultation measurement with CRIRES on the VLT), is the following:
The reason why Pluto’s surface is so cold is linked to the existence of Pluto’s atmosphere, and is due to the sublimation of the surface ice; much like sweat cools the body as it evaporates from the surface of the skin, this sublimation has a cooling effect on the surface of Pluto. In this respect, Pluto shares some properties with comets, whose coma and tails arise from sublimating ice as they approach the Sun.
Artist’s impression of the surface of Pluto. Credit: ESO/L. Calçada
In the simulations, the day-night temperature contrast is 5 K, and the maximum wind speed 10 m/s.
How small can an atmosphere be before it ceases to be an atmosphere? Io, down to nanobars, has no circulation to speak of, and the structure of its atmosphere depends on the latest volcanic events, so Pluto and Triton are probably near the lower limit of applicability for the usual tools of comparative planetology.