Pluto's atmosphere has been a subject of fascination for planetary astronomers since -- well, since astronomers first discovered that it had an atmosphere in the early '90s. The interest is partly because it's fascinating that such a distant and cold world is capable of supporting an atmosphere, and partly because the presence of the atmosphere confounds all attempts to measure Pluto's size precisely. The way you determine diameters of objects that are too small to see as more than a teensy point of light is to watch the object pass in front of a star and time how long the starlight takes to wink back on again. Pluto's atmosphere bends starlight, messing up the apparent timing of that on-and-off winking, making it impossible to know for sure how big the solid part of the object is. You can try to account for this bending mathematically but only if you have a good idea of what's in the atmosphere and how dense it is.
Pluto takes 248 years to orbit the Sun. Since it was discovered in 1930 it hasn't even gone through a third of that orbit, barely more than one of its four seasons. You'd expect seasonal change on Pluto to follow the same stately pace. So it's really quite surprising to find out that Pluto's appearance has changed in the very short time that we've been studying it. I've written before on this blog about how the colors of its surface have been observed to shift over the last couple of decades. Now another team of astronomers, led by Jane Greaves, is reporting that the atmosphere has changed quite a lot over the same time scale. You can read their paper, "Discovery of carbon monoxide in the upper atmosphere of Pluto," on arXiv.
Pluto's tenuous atmosphere
Artist's impression of Pluto's huge atmosphere of carbon monoxide. The source of this gas is erratic evaporation from the mottled icy surface of the dwarf planet. The Sun appears at the top, as seen in the ultra-violet radiation that is thought to force some of the dramatic atmospheric changes. Pluto's largest moon, Charon, is seen to the lower right.
Pluto's atmosphere is very thin, made of gaseous forms of the ices that cover its surface -- nitrogen, methane, carbon monoxide, and carbon dioxide, among others. The discovery of carbon monoxide in Pluto's atmosphere is actually new, part of Greaves' work; but that's not the most interesting part of their story. The most interesting part is that their detection of carbon monoxide was at a signal strength that was much higher than the sensitivity of a search for the same gas a decade previously -- that is, there was an earlier non-detection that put a more stringent limit on the amount of carbon monoxide in Pluto's atmosphere than the amount that Greaves et al. actually measured. That, in turn, means that Pluto's atmosphere (or, at least, the carbon monoxide in Pluto's atmosphere) has gotten substatially denser in the last decade. The authors hypothesize that the molecules could be coming off a new bright patch of ice on the surface, which was proposed to be made of carbon monoxide ice by its discoverers. This is speculation but it's a story that's "consistent with the observations," as scientists like to say when they are spinning fairy tales about what's going on in the solar system. (I'm not being dismissive here -- such stories are where scientists develop testable hypotheses, and they're the best possible attempt at something like the truth -- but they're based on very few knowns and are much more likely to turn out to be wrong than they are to be right.)
Another interesting part of Greaves' story is that they observed the carbon monoxide out to more than 3,000 kilometers above Pluto, or a quarter of the way to Charon. The molecules of carbon monoxide that float up all that way are very unlikely to return to Pluto, or to land on Charon, for that matter; they're going to be lost to space, pushed away by the solar wind, which is still very strong even at such a great distance from the Sun.
It's just more data to support the expectation that New Horizons will, when it finally gets to Pluto, find something that's not a Callisto-like boring old dead-for-billions-of-years world but instead a place with a changeable, varying surface and a fascinating but complicated history.