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U of I-Led Study Finds Evidence of Changing Seasons, Rain on Titan’s North Pole

January 16, 2019

MOSCOW, Idaho — Jan. 16, 2019 —An image from the international Cassini spacecraft provides evidence of rainfall on the north pole of Titan, the largest of Saturn’s moons. The rainfall would be the first indication of the start of a summer season in the moon’s northern hemisphere.

A team of scientists led by University of Idaho doctoral student Rajani Dhingra and Department of Physics Associate Professor Jason Barnes published their findings today in the journal Geophysical Research Letters.

“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,” said Dhingra, who is earning her doctorate in physics at U of I. “People called it the curious case of missing clouds.”

Dhingra and her colleagues, who represent 12 other institutions and universities, identified a reflective feature near Titan’s north pole on an image taken June 7, 2016, by Cassini’s near-infrared instrument, the Visual and Infrared Mapping Spectrometer. The reflective feature covered approximately 46,332 square miles, roughly half the size of the Great Lakes, and did not appear on images from previous and subsequent Cassini passes.

Analyses of the short-term reflective feature suggested it likely resulted from sunlight reflecting off a wet surface. The study attributes the reflection to a methane rainfall event, followed by a probable period of evaporation.

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

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

“We want our model predictions to match our observations. This rainfall detection proves Cassini’s climate follows the theoretical climate models we know of,” Dhingra said. “Summer is happening. It was delayed, but it’s happening. We will have to figure out what caused the delay, though.”

Additional analyses suggest the methane rain fell across a relatively pebble-like surface, Dhingra said. A rougher surface generates an amorphous pattern as the liquid settles in crevasses and gullies, while liquid falling on a smooth surface would puddle in a relatively circular pattern.

Dhingra is using the wet sidewalk effect to search for additional rain events on Titan as part of her research.

Media note: An image associated with the study’s findings is attached to the press release.

Image caption: Top panel: Titan's north pole as seen by the Cassini Visual and Infrared Mapping Spectrometer. The orange box shows the “wet sidewalk” region, what analyses suggests is evidence of changing seasons and rain on Titan’s north pole. The blue box shows the expanded region in the bottom panel. Bottom Panel: Pictured is an expanded view of Titan's north pole. Dark blue arrows mark clouds. Red arrows mark the mirror-like reflection from a lake called Xolotlan Lacus. Pink arrows mark the “wet sidewalk” region. The black dot marks the actual north pole of Titan. Light blue arrows mark the edges of the largest north polar sea, Kraken Mare.

Image credit: NASA/JPL/University of Arizona/University of Idaho.

This project was funded under the National Aeronautics and Space Administration award NNX15AI77G. The total project funding is $287,252 of which 100 percent is the federal share.

Media Contacts

Rajani Dhingra
University of Idaho Doctoral Student
401-225-3329
dhin8109@vandals.uidaho.edu

Leigh Cooper
Science and Content Writer
University of Idaho
208-885-1048
leighc@uidaho.edu

Top panel: Titan's north pole as seen by the Cassini Visual and Infrared Mapping Spectrometer. The orange box shows the “wet sidewalk” region, what analyses suggests is evidence of changing seasons and rain on Titan’s north pole. The blue box shows the expanded region in the bottom panel. Bottom Panel: Pictured is an expanded view of Titan's north pole. Dark blue arrows mark clouds. Red arrows mark the mirror-like reflection from a lake called Xolotlan Lacus. Pink arrows mark the “wet sidewalk” region. The black dot marks the actual north pole of Titan. Light blue arrows mark the edges of the largest north polar sea, Kraken Mare.

About the University of Idaho

The University of Idaho, home of the Vandals, is Idaho’s land-grant, national research university. From its residential campus in Moscow, U of I serves the state of Idaho through educational centers in Boise, Coeur d’Alene and Idaho Falls, nine research and Extension centers, plus Extension offices in 42 counties. Home to nearly 12,000 students statewide, U of I is a leader in student-centered learning and excels at interdisciplinary research, service to businesses and communities, and in advancing diversity, citizenship and global outreach. U of I competes in the Big Sky Conference. Learn more at uidaho.edu