Thanks to the development of a high resolution infrared spectrograph at ESO's Very Large Telescope, astronomers have performed new analysis of the atmosphere of Neptune's largest moon Triton, discovering carbon monoxide and making the first ground-based detection of methane.

Triton, at 2,700 kilometres in diameter, is the seventh largest moon in the Solar System. It is geologically active, spewing icy water and ammonia lavas that have carved complex valleys on its surface, which is clad in a mixture of ices including frozen nitrogen ice, water ice and carbon dioxide ice. Carbon monoxide was also already known to exist on the surface, but the new observations show that the moon's upper surface layers are enriched by a factor of ten compared to the deeper layers, and that this sublimates into gas during Triton's summer to supplement the nitrogen-dominant atmosphere.

"Climate and atmospheric models of Triton have to be revisited now, now that we have found carbon monoxide and re-measured the methane," says team member Catherine de Bergh.

Triton's average surface temperature is a frigid minus 235 degrees Celsius but this distant world clearly still feels the effect of the Sun on its 165 year orbit, with each season lasting for just over 40 years. ESO's new Cryogenic High-Resolution Infrared Echelle Spectrograph (CRIRES), which yielded the first infrared analysis of Triton's atmosphere, shows that summer is in full swing in the moon's southern hemisphere.

"We have found real evidence that the Sun still makes its presence felt on Triton, even from so far away," says Emmanuel Lellouch, the lead author of the paper reporting the results in the journal Astronomy & Astrophysics. "This icy moon actually has seasons just as we do on Earth, but they change far more slowly."

Triton as observed by Voyager 2 at a distance of 5.4 million miles from Neptune. Image: NASA/JPL.

CRIRES also enabled the team to refine measurements of the moon's atmospheric pressure. It was once thought that its atmosphere was as thick as Mars', but Voyager 2 measurements in 1989 found it to be 14 microbars, 70,000 times less dense than Earth's atmosphere. Now, the ESO team find it to have risen by four times, to between 40 and 65 microbars, consistent with the idea that sublimating gases feed the atmosphere during summer.

"We needed the sensitivity and capability of CRIRES to take very detailed spectra to look at the very tenuous atmosphere," comments co-author Ulli Käufl.

The new equipment will also afford the astronomers the chance to study Pluto's atmosphere. Since both Triton and Pluto are thought to have been captured from the Kuiper Belt and thus share similar compositions the hunt is on for detecting carbon monoxide on the dwarf planet too. But the team's job is far from complete at Triton: "We can now start monitoring the atmosphere and learn a lot about the seasonal evolution of Triton over decades," adds Lellouch.