Neptune's largest moon Triton undergoes seasonal variations just like the Earth and is presently experiencing summer in its southern hemisphere, astronomers have found.

In a first-ever infrared analysis of Triton's atmosphere with the help of ESO's Very Large Telescope, the researchers have found presence of frozen nitrogen, carbon monoxide and methane on the moon�s thin surface which turn into gas as the southern hemisphere warms up by the Sun. The thin, icy atmosphere then thickens as the season advances during Neptune's 165-year orbit around the Sun.

�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,� said Emmanuel Lellouch, lead author of the paper reporting the results in Astronomy & Astrophysics journal.

The average surface temperature of Triton is about -235 degree Celsius and a season in the moon lasts a little over 40 years. While it is summer in its southern hemisphere, the northern hemisphere is witnessing winter.

While carbon monoxide was known to be present as ice on Triton's surface, the researchers' team has now found that the moon's upper surface layer is enriched with carbon monoxide ice by about a factor of ten compared to the deeper layers, and that it is this upper �film� that feeds the atmosphere. While the majority of Triton�s atmosphere is nitrogen, much like on Earth, the methane in its atmosphere, first detected by Voyager 2, and only now confirmed in this study from Earth, plays an important role as well.

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

Triton is Neptune's largest of 13 moons and is the seventh largest moon in the Solar System. The moon has fascinated astronomers due to its geologic activity, presence of different types of surface ice, such as frozen nitrogen as well as water and dry ice. Its unique retrograde motion, i.e, a motion in the opposite direction to its planet's rotation, has also caused curiosity among researchers.

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