Summer arrives late on Saturn’s moon Titan

Recently discovered rainfall has heralded the start of summer in the northern hemisphere of Saturn's moon, Titan. Climate scientists have been on the look out for clouds in Cassini images of the moon's north, but failed to find any. Instead, it was the reflection from a recent rainfall that indicated arrival of the summer season.

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Cassini took many images of Titan over its 13 year mission at Saturn. The mission ended on 15 September 2017, when it plunged into Saturn’s atmosphere, but it’s data is still providing scientific insight. Image Credit: NASA/JPL-Caltech/ASI

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Summer has arrived – at least in the northern hemisphere of Saturn’s moon Titan.

Recently examined images of the moon taken by NASA’s Cassini spacecraft show rain has fallen on the moon’s northern hemisphere.

Unlike Earth, Titan’s climate and weather cycle is based on methane rather than water, which means summer is a time of clouds and rain instead of clear skies and sunshine.

Cassini arrived at Saturn in 2004, when the southern hemisphere was in its summer.

With each of Titan’s seasons lasting seven Earth years, scientists were expecting to see signs of summer arriving at the north pole 13 years later, when Cassini was nearing the end of its mission.

“The whole Titan community has been looking forward to seeing clouds and rains on Titan’s north pole, indicating the start of the northern summer, but despite what the climate models had predicted, we weren’t even seeing any clouds,” says Rajani Dhingra, from the University of Idaho in Moscow who led the study.

“People called it the curious case of missing clouds.”

Despite Titan’s summer solstice falling in 2017, by 2016 clouds hadn’t been sighted in the moon’s north.

However, while analysing Cassini images taken on 7 June 2016 (but only recently examined), Dhingra found a reflective patch covering 120,000 square km that wasn’t present at any other time.

Analysis suggested the light was reflecting off a surface made wet by a recent rainfall.

“It’s like looking at a sunlit wet sidewalk,” Dhingra said.

The damp then evaporated away before the next Cassini pass.

This is the first observation that suggests summer rain has begun to fall across Titan’s northern hemisphere.

“Summer is happening,” says Dhingra.

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“It was delayed, but it’s happening. We will have to figure out what caused the delay though.”