Summer is coming: Cassini images show methane rain on Titan
Titan, the largest moon of Saturn, is a surprisingly dynamic world, with a thick atmosphere and surface oceans of liquid methane. Like the fictional land of Westeros in Game of Thrones, seasons on Titan last about seven years, and summer was due to hit its northern hemisphere in 2016. But strangely enough, it didn't seem to arrive as expected. Now, astronomers have analyzed Cassini images and found evidence of rainfall, indicating that finally, summer is coming.
When Cassini first arrived at Titan in 2004, the moon's southern hemisphere was enjoying summer, and the spacecraft observed cloud cover and rainfall over that half. Astronomers developed climate models of Titan from Cassini data, which predicted that similar weather would be seen across the northern hemisphere as the seasons changed.
But Titan didn't play ball. According to the climate models, the northern summer solstice was due in 2017, but as late as 2016 no clouds were seen, puzzling scientists.
"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, lead author of the study. "People called it the curious case of missing clouds."
But clouds may not be the only indicator of a seasonal change in weather. Dhingra and her team found a strange anomaly in an image taken by Cassini on June 7, 2016, which didn't show up in images taken during earlier or later flybys. A large section of the moon's surface, measuring about 46,332 sq mi (120,000 sq km), had suddenly become reflective.
This newfound reflectiveness was chalked up to the "wet sidewalk" effect after rainfall. But keep in mind this wouldn't have been an Earthly shower of boring old water – on Titan, it rains liquid methane. The team also says the shape of the pattern suggests the rain fell on pebbly, rough terrain rather than a smooth surface, which would have produced a more circular reflective spot.
While the predictions may have been a little off, it looks like summer has arrived in Titan's north after all. The next step, the researchers say, is to investigate why it was late, which may require tweaking the climate models.
"We want our model predictions to match our observations," says Dhingra. "This rainfall detection proves Cassini's climate follows the theoretical climate models we know of. Summer is happening. It was delayed, but it's happening. We will have to figure out what caused the delay, though."
The research is due to be published in the journal Geophysical Research Letters.
Source: American Geophysical Union