A new instrument that lets us look deeper into things almost always changes the game. Such an instrument is CRIRES, the Cryogenic High-Resolution Infrared Echelle Spectrograph. Now operational at the Very Large Telescope, CRIRES has already done yeoman work on Pluto, and has now been used to study the atmosphere of Neptune’s large moon Triton in more detail than ever before. The result: A new understanding of Triton’s carbon monoxide, whose existence in its upper surface layer is now confirmed and shown to be an icy ‘film’ that, over time, adds to the atmosphere.

Image: Artist’s impression of how Triton, Neptune’s largest moon, might look from high above its surface. The distant Sun appears at the upper-left and the blue crescent of Neptune right of centre. Using the CRIRES instrument on ESO’s Very Large Telescope, a team of astronomers has been able to see that the summer is in full swing in Triton’s southern hemisphere. Credit: ESO/L. Calçada.

It should come as no surprise that astronomers have also identified seasonal variation in the atmosphere, given that we see similar changes on Pluto. The team, led by Emmanuel Lellouch, estimates that the atmospheric pressure on the distant moon may have risen by a factor of four compared to what Voyager found during its 1989 flyby. The spacecraft found a nitrogen and methane atmosphere at a pressure of 14 microbars, roughly 70,000 times less dense than the atmosphere of Earth. The pressure is now measured at between 40 and 65 microbars, or 20,000 times less than on Earth.

“We have found real evidence that the Sun still makes its presence felt on Triton, even from so far away,” says Lellouch. “This icy moon actually has seasons just as we do on Earth, but they change far more slowly.”

Slowly indeed. It’s hard to talk about summer on a place where the average surface temperature is minus 235 C, but Triton’s southern hemisphere is currently enjoying that season, causing a layer of frozen nitrogen, methane and carbon monoxide to sublimate into gas. It’s a long, slow process, with seasons on Triton lasting for forty years. The new findings will doubtless lead to a reassessment of atmospheric models on Triton.

You may remember a 2009 ESO study using CRIRES that revealed a temperature inversion on Pluto. The instrument helped scientists find more methane than anticipated in the atmosphere of the dwarf planet, a place where the thin envelope of nitrogen, methane and (most likely) carbon monoxide freezes out when Pluto moves away from the Sun during its 248-year orbit. Pluto’s atmospheric pressure is five times less than what is currently found on Triton. The Triton news means that the hunt for carbon monoxide on Pluto will now intensify.

The paper is Lellouch et al., “Detection of CO in Triton’s atmosphere and the nature of surface-atmosphere interactions,” Astronomy & Astrophysics Vol. 512, L8 (March/April 2010). Abstract available.