European astronomers have identified carbon monoxide in the atmosphere of Triton, Neptune's largest moon. They owe their discovery to new technology, including the first ever infrared analysis of Triton's atmosphere. The researchers also made the first ground-based detection of methane in Triton's atmosphere. The results of the study, published in the journal Astronomy & Astrophysics, will enable astronomers to begin an extensive monitoring of Triton over the coming decades.
Astronomers have been fascinated by Triton since its discovery in 1846. It is the largest of Neptune's 13 moons and the 7th largest moon in our solar system (being nearly three-quarters as big as the Earth's moon). Astronomers are particularly fascinated by the different types of ice found on Triton's surface, which include frozen nitrogen, water and dry ice (frozen carbon dioxide), as well as its geological activity and its unique retrograde motion (it rotates in the opposite direction to its planet, the only moon in our Solar System to do so).
Neptune takes about 165 Earth years to orbit the Sun, which means each season on Triton lasts about 40 of our years. The researchers' infrared analysis revealed that it is currently summer in Triton's southern hemisphere. As the southern hemisphere warms up, a layer of frozen nitrogen, methane and carbon monoxide turns into gas, which thickens the moon's otherwise thin atmosphere.
The team made their discoveries by using the European Southern Observatory's (ESO) newly-developed Cryogenic High-Resolution Infrared Echelle Spectrograph (CRIRES) at the Very Large Telescope (VLT), the world's largest optical telescope. The infrared analysis proved that the Sun's effects are still felt on Triton, even if the average surface temperature stands at a chilly -235ºC.
Lead author of the study Emmanuel Lellouch of LESIA (Laboratoire d'Études Spatiales et d'Instrumentation en Astrophysique) at the Observatoire de Paris, in France, said, 'We have found real evidence that the Sun still makes its presence felt on Triton, even from so far away. This icy moon actually has seasons just as we do on Earth, but they change far more slowly.'
Astronomers already knew that carbon monoxide was present on Triton, but the team learned that Triton's upper surface is rich in carbon monoxide ice which feeds the atmosphere. Triton's atmosphere is mostly composed of nitrogen, but the methane content (which was first perceived by the spacecraft Voyager 2 and confirmed in this study) plays an important part too.
'Climate and atmospheric models of Triton have to be revisited now that we have found carbon monoxide and re-measured the methane,' commented co-author of the study Catherine de Bergh from LESIA.
It is not easy for astronomers to measure Triton's atmosphere as it is approximately 30 times further from the Sun than Earth. Back in the 1980s, astronomical theory held that Triton's atmosphere might be as thick as that of Mars (7 millibars). But when Voyager 2 passed it in 1989, it recorded an atmosphere of nitrogen and methane at a pressure of 14 microbars (70 000 times less dense than the atmosphere on Earth). Since then few other ground-based observations have been recorded until the development of CRIRES) offered a chance of more detailed study.
'We needed the sensitivity and capability of CRIRES to take very detailed spectra to look at the very tenuous atmosphere,' said co-author of the study Ulli Käufl from ESO.
The development of CRIRES is the first step for astronomers to begin measuring distant bodies in the Solar System. 'We can now start monitoring the atmosphere and learn a lot about the seasonal evolution of Triton over decades,' concluded Emmanuel Lellouch.