Neptune’s big moon is heating up

Artist’s impression of how Triton, Neptune’s largest moon, might look from high above its surface

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.

Summer is in full swing in the southern hemisphere of Neptune’s moon Triton, say astronomers who performed the first-ever infrared analysis of its atmosphere.

The European observing team used the European Southern Observatory’s (ESO) Very Large Telescope to detect carbon monoxide and make the first ground-based detection of methane in Triton’s thin atmosphere.

These observations revealed that the thin atmosphere varies seasonally, thickening when warmed.

“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,” says Emmanuel Lellouch, the lead author of the scientific paper reporting the results.

On Triton, where the average surface temperature is about minus 235 degrees Celsius, it is currently summer in the southern hemisphere and winter in the northern.

A Voyager 2 image of Triton, with Neptune in the distance

Combined images of Neptune's moon Triton (foreground), and Neptune.

As Triton’s southern hemisphere warms up, a thin layer of frozen nitrogen, methane, and carbon monoxide on Triton’s surface turns into gas, thickening the icy atmosphere as the season progresses during Neptune’s 165-year orbit around the Sun.

A season on Triton lasts a little over 40 years, and Triton passed the southern summer solstice in 2000.

Triton’s thickening atmosphere

Based on the amount of gas measured, Lellouch and his colleagues estimate that Triton’s atmospheric pressure may have risen by a factor of four compared to the measurements made by Voyager 2 in 1989, when it was still spring on the giant moon.

But despite that increase, the atmospheric pressure on Triton is still only between 40 and 65 microbars — that’s 20,000 times less than on Earth.

Carbon monoxide was known to be present as ice on the surface, but Lellouch and his team discovered that Triton’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 the atmosphere, first detected by Voyager 2, and only now confirmed in this study from Earth, plays an important role as well.

Is Triton a twin of Pluto?

Of Neptune’s 13 moons, Triton is by far the largest, and, at 2,700 kilometres in diameter (or three quarters the Earth’s Moon), is the seventh largest moon in the Solar System.

Since its discovery in 1846, Triton has fascinated astronomers thanks to its geologic activity, and its many different types of surface ices, such as frozen nitrogen as well as water and dry ice (frozen carbon dioxide).

Triton is also the only large moon in the Solar System with a retrograde motion, which is an orbital direction in the opposite direction to its planet’s rotation. This is one of the reasons why Triton is thought to have been captured from the Kuiper Belt, and thus shares many features with the dwarf planets, such as Pluto.

Pluto, often considered a cousin of Triton and with similar conditions, is receiving renewed interest in the light of the carbon monoxide discovery, and astronomers are racing to find this chemical on the even more distant dwarf planet.

Adapted from information issued by ESO / L. Calçada.

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