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The curious case of Titan's missing clouds



Posted on Jan 17, 2019

  Titan in Saturn's Rings

Saturn's Titan Science Community "is looking forward to seeing clouds and rain on Titan's North Pole, indicating the start of the northern summer, but what the climate models had predicted, so we didn't even see any clouds, "Rajani Dhingra of Idaho University said, leading the author of the new study. "People called the curious case of missing clouds."

But it changed with a picture taken on June 7, 2016 by the Cassini spacecraft's infrared instrument, the visual and infrared mapping spectrometer, which provides proof of precipitation on the northern pole of Titan, the largest of Saturn's moons. The rain would be the first indication of the start of a summer season on the moon's northern hemisphere. The reflective function covered approx. 46,332 square kilometers, about half the size of the Great Lakes, and was not shown on images from previous and subsequent Cassini passes.

The picture above shows Titan, Saturn's greatest moon, behind the planet's rings. The much smaller moon Epimetheus is visible in the foreground. Image via NASA / JPL / Space Science Institute.

Analyzes of the short-term reflective function suggested that it was probably due to sunlight reflecting a wet surface. The study attributes the reflection of a methane precipitation event followed by a probable evaporation period. "It's like looking at a sunlit wet pavement," Dhingra said.

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This reflective surface represents the first observations of the summer rainfall on the northern hemisphere of the moon. If compared to Earth's annual cycle of four seasons, a season of Titan lasts seven Earth years. Cassini arrived at Titan during the southern summer and observed clouds and precipitation in the southern hemisphere. Climate models of the Titan predicted that similar weather would occur in the northern hemisphere in the years leading up to the northern summer solstice in 2017. But in 2016 the expected cloud cover in the northern hemisphere had not emerged. This observation can help scientists gain a more complete understanding of Titan's seasons.

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"We want our model predictions to match our observations. This method of precipitation shows that Cassini's climate follows the theoretical climate models we know of," Dhingra said. "The summer is happening. It was delayed, but it happens. We need to find out what caused the delay."

Further analysis suggests that methane rain fell across a relatively pebbled surface, Dhingra said. A harder surface generates an amorphous pattern, as the liquid lies in crevices and joists, while liquid which falls on a smooth surface will poodle in a relatively circular pattern.

Dhingra uses the wet indulgence effect to seek additional rain events on Titan as part of her research. The new study is accepted for publication in geophysical research letters, a journal of the US geophysical union.

The Daily Galaxy via the American Geophysical Union


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