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The Curious Case of Titan's Disappeared Clouds

Posted on Jan 17, 2019

Titan in the Rings of Saturn

Saturn's Titan scientific community "was eager to see clouds and rain on Titan's north pole, signaling the beginning of the northern summer, but despite what climate models predicted, we were not even seeing clouds," said Rajani Dhingra at the University of Idaho and lead author of the new study. "People called the curious case of lost clouds."

But that changed with an image taken on June 7, 2016, by the near infrared instrument of the Cassini probe, the infrared and visual mapping spectrometer that provides evidence of rainfall on the north pole of Titan, the largest of Saturn's moons. The rain would be the first indication of the beginning of a summer season in the northern hemisphere of the moon. The reflective feature covered approximately 46,332 square miles, roughly half the size of the Great Lakes, and did not appear in images of previous and subsequent Cassini passages.

The image above shows Titan, Saturn's largest 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 characteristic suggested that they probably resulted from sunlight reflected off a wet surface. The study attributes the reflection to a methane precipitation event, followed by a probable period of evaporation. "It's like looking at a wet sidewalk lit by the sun," Dhingra said.

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This reflecting surface represents the first observations of summer rains in the northern hemisphere of the moon. Compared to the annual four-season Earth cycle, one season on Titan lasts seven years. Cassini arrived on Titan during the southern summer and observed clouds and rainfall in the southern hemisphere. Titan's climate models predicted that a similar climate would occur in the northern hemisphere in the years leading up to the northern summer solstice in 2017. But by 2016, expected cloud cover in the northern hemisphere had not appeared. This observation may help scientists gain a more complete understanding of Titan's seasons.

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"We want our model forecasts to match our observations. This rainfall detection proves that the Cassini climate follows the theoretical climate models we know of, "Dhingra said. "Summer is happening. It's been postponed, but it's happening. We'll have to figure out what caused the delay, though.

Further analysis suggests that methane rain fell on a surface relatively similar to pebbles, Dhingra said. A rougher surface generates an amorphous pattern as the liquid settles into cracks and gullies, while the liquid that falls on a smooth surface accumulates in a relatively circular pattern.

Dhingra is using the wet sidewalk effect to look for additional rain events on Titan as part of his research. The new study was accepted for publication in the journal Geophysical Research Letters, a journal of the American Geophysical Union.

The Daily Galaxy through the American Geophysical Union

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