By Ron Cowen
Just like Mickey’s dog, the former planet Pluto has a tail — or at least a hint of one.
Scientists have detected a wisp of carbon monoxide in Pluto’s thin upper atmosphere, extending a quarter of the way to its largest moon, Charon, or about 3,400 kilometers above Pluto’s surface. To the astronomers who detected it using the James Clerk Maxwell Telescope on Hawaii’s Mauna Kea, the cloud appears to have properties resembling a comet’s gas tail.
“Whether Pluto’s atmosphere forms a tail is just a suggestion on our part,” says Jane Greaves of the University of St. Andrews in Scotland. The gas is too thin to image directly. But a tiny shift in the wavelength of microwaves emitted by the carbon monoxide hints that the gas is receding from Earth and the sun, with ions and some neutral atoms in the gas swept up by the solar wind — just as a comet’s gas tail would be.
Planetary scientist Mike Brown of Caltech says that a future array of radio telescopes called ALMA “will be able to do a fabulous job of detecting the [carbon monoxide emission] and measuring it precisely.”
Greaves and her colleagues report in an upcoming Monthly Notices of the Royal Astronomical Society that the carbon monoxide emission is brighter than a tentative detection made by another group 11 years ago, indicating that the amount of gas in Pluto’s upper atmosphere has increased since then. At first glance, that seems odd, because Pluto has been receding from the sun since 1989; its atmosphere would have been expected to freeze and collapse over that period.
But observations over the past decade suggest that the solar heat absorbed by Pluto when it was closer to the sun is being released slowly (SN: 8/23/03, p. 126).
“Pluto’s atmosphere continues to change and continues to be different from what we expect and predict,” says Brown.
