It’s Raining at the North Pole of Titan

Saturn’s largest moon is the only place, besides Earth, where rain touches the surface

Astronomers on the Cassini probe launch team have found spots on the surface of the northern hemisphere of Titan, Saturn’s largest moon— the first evidence of precipitation, which marks the beginning of summer in the north pole of the natural satellite. Scientists have been waiting for this event since Cassini arrived at the orbit of Saturn in 2004.

Scientists have discovered spots near the north pole of Titan in an image taken by the Visual and Infrared Mapping Spectrometer spacecraft (VIMS) on 7 June 2016 on a Cassini probe. Cassini's infra-red eyes, the VIMS, can peer through the atmosphere and look at the Titan's surface.  

In the image, a region 120,000 square kilometres in size looks wet – the same as the sun reflects on wet asphalt. No other images display the same phenomena. According to the analysis of short-term reflectivity, the region’s surface is the result of methane deposits on a rough, pebble-like surface, and possibly an evaporation period will follow.

“Based on the overall brightness, spectral characteristics, and geologic context, we attribute this new feature to specular reflections from a rain‐wetted solid surface like those off of a sunlit wet sidewalk,” scientists said in the article published in the Geophysical Research Letters.

Scientists noted that the rainfall observation at the north pole is a major finding for two important reasons.  

“Firstly, this discovery observation heralds the muchawaited arrival of the north polar summer rainstorms on Titan,” leading study author Rajani Dhingra told Asgardia Space New. “Secondly, it is extremely difficult to detect rainfall events on Titan due to its thick atmospheric haze and very limited opportunities to view the surface and its changes. We have used a novel phenomenon –  a wet side walk – as evidence for rainfall events on the surface of Titan.”

Earlier, in the southern hemisphere of Saturn’s satellite, summer rain had been observed, so methane precipitation in the northern hemisphere of Titan was predicted, but scientists have expected to see it at the beginning of the season - years before the 2017 solstice. Their assumptions were based on theoretical models, given the fact that most of the lakes and oceans of Saturn's satellite are in the northern hemisphere.

When scientists’ assumptions have not come to be realised, the absence of methane rain became something of a mystery. But now, when precipitations have been found in the images, the researchers of Titan have received another piece of data that will help them figure out how Titan’s climate works. Nonetheless, the delay of rain in the northern hemisphere remains unexplained. In the future, scientists plan to find out why the beginning of summer on Titan has been delayed.

On 15 October 1997, the Cassini probe was launched into space and arrived in the Saturnian system in 2004, during summer in the southern pole. The Cassini team has observed cloud cover, storms and precipitation on the south pole and were eagerly following signs of seasonal changes.

Photo credit: NASA