WASHINGTON -- 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.
"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 Rajani Dhingra, a doctoral student in physics at the University of Idaho in Moscow, and lead author of the new study accepted for publication in Geophysical Research Letters, a journal of the American Geophysical Union. "People called it the curious case of missing clouds."
Dhingra and her colleagues 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.
This press release and accompanying images can be found at: https://news.agu.org/press-release/new-study-finds-evidence-of-changing-seasons-rain-on-titans-north-pole/
AGU Press Contact:
+1 (202) 777-7396
University of Idaho Press Contact:
+1 (208) 885-1048
Contact information for the researchers:
University of Idaho
+1 (401) 225-3329
The American Geophysical Union is dedicated to advancing the Earth and space sciences for the benefit of humanity through its scholarly publications, conferences, and outreach programs. AGU is a not-for-profit, professional, scientific organization representing 60,000 members in 137 countries. Join the conversation on Facebook, Twitter, YouTube, and our other social media channels.
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 http://www.uidaho.edu.
Notes for Journalists
This paper is freely available for 30 days. Journalists and public information officers (PIOs) can download a PDF copy of the article by clicking on this link: https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2018GL080943
Journalists and PIOs may also request a copy of the final paper and multimedia by emailing Lauren Lipuma at email@example.com. Please provide your name, the name of your publication, and your phone number.
Neither the paper nor this press release is under embargo.
"Observational evidence for summer rainfall at Titan's north pole"
Rajani D. Dhingra: Department of Physics, University of Idaho, Moscow, ID, USA;
Jason W. Barnes: Department of Physics, University of Idaho, Moscow, ID, USA;
Robert H. Brown: Dept. of Planetary Sciences, University of Arizona, AZ, USA;
Bonnie J. Buratti and Christophe Sotin: Jet Propulsion Laboratory, Caltech, CA, USA;
Philip D. Nicholson: Cornell University, Astronomy Dept., NY, USA;
Kevin H. Baines: Space Science & Engineering Center, University of Wisconsin-Madison, WI, USA;
Roger N. Clark: U.S.G.S., Denver, CO, USA;
Jason M. Soderblom: Dept. of Earth, Atmospheric and Planetary Sciences, MIT, MA, USA;
Ralf Jaumann: Deutsches Zentrum fr Luft- und Raumfahrt, Germany;
Sebastien Rodriguez: Institut de Physique du Globe de Paris (IPGP);
Stéphane Le Mouélic: Laboratoire de Planetologie et Geodynamique, Universite de Nantes, France;
Elizabeth P. Turtle: John Hopkins Applied Physics Laboratory, MD, USA;
Jason E. Perry: University of Arizona, AZ, USA;
Valeria Cottini: John Hopkins University, MD, USA;
Don E. Jennings: NASA Goddard Space Flight Center, MD, USA.
Geophysical Research Letters