Sunny skies on Triton

Triton is a cold, distant world. At 2,700km diameter, it is the largest moon of Neptune and the seventh largest satellite in the solar system, its host planet takes one hundred and sixty five years to complete an orbit of the Sun. Seasons on Triton therefore last forty years.

The surface of the cold world lies at a balmy 235 degrees Celsius below freezing. Its surface is on the move with cryovulcanism (volcanoes spurting out frigid liquids, in this case ammonia and water, rather than hot lava) pushing matter into the upper reaches of the atmosphere, where the winds whip it away. It is covered in a shell of ices including water, nitrogen and carbon dioxide.

The atmosphere of Triton has been a subject of some interest for some time. It is believed to have originated from the same area of the solar system as the dwarf planet Pluto, meaning their compositions are likely to be the same. Looking at Triton, we can start to infer something about the atmosphere of Pluto, which spends most of its time frozen to the tiny world’s surface, except for that little part of the orbit where it comes within the extent of the orbit of Neptune.

Triton’s atmosphere was once thought to be as thick as that of Mars (about 6 millibars), but in fact turned out to be a lot thinner, 14 microbars or around 70,000 times thinner than Earth’s atmosphere (roughly 500 times thinner than expected). The measurements, by the Voyager 2 spacecraft, also revealed the atmosphere was predominantly Nitrogen. Now ground based measurements have been made of the Southern Hemisphere, presently basking in the summer light, revealing some interesting facets of the evolutionary processes of Triton’s atmosphere.

As with its proposed likeness, Pluto and its near namesake Titan, there are gases in the atmosphere of Triton that only exist during the summer, being frozen to the surface during winter months. One of these has turned out to be Carbon Monoxide, which has been detected to be present in roughly ten times the amount on upper surfaces as there is on lower surfaces. The new measurements have also shown that this sublimates (turns from solid to gas with no liquid phase) directly into the atmosphere. The effect of sublimating gases is shown in measurements of atmospheric surface pressure during the summer decades; 40-65 microbars, four or so times as thick as the Voyager 2 measurements.

The measurements were the first ground based investigations of the atmosphere at this high resolution and were made with the Cryogenic High-Resolution Infrared Echelle Spectrograph (CRIRES) instrument on the Very Large Telescope, part of the European Southern Observatory.

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