Spring may bring methane showers to the deserts of Titan, Saturn’s largest moon. NASA’s Cassini spacecraft recently saw a large, dark puddle appear in the wake of a storm cloud at the moon’s dune-filled equator.

“It’s the only easy way to explain the observations,” said planetary scientist Elizabeth Turtle of Johns Hopkins University Applied Physics Laboratory, lead author of a study March 18 in Science. “We’re pretty confident that it has just rained on Titan.”

Aside from Earth, Titan is the only world known to have liquid lakes, clouds and a weather cycle to move moisture between them. But on chilly Titan, where temperatures plunge to -297 degrees Fahrenheit, the frigid lakes are filled with liquid methane and ethane, not water.

Titan’s lakes are also exclusively confined to the poles. The moon’s dry central regions are covered in rippling dunes and arid deserts.

But the dunes are crisscrossed by a network of dry channels, suggesting a wetter past. In 2006, Cassini observations showed hints of drizzle at the equator, but not enough rain to explain the riverbeds.

“So the question was, ‘When was the last rainfall near the equator of Titan?’” said planetary scientist Tetsuya Tokano of the University of Cologne in Germany, who was not involved in the new work. Some researchers suggested that the rivers were a relic of a bygone era, or carved by things other than rain.

“This observation by Turtle et al. showed for the first time that there is rainfall on present Titan, not merely millions of years ago but at the present Titan,” Tokano said. “This is extraordinary.”

In the new study, Turtle’s team describes a large cloud system moving eastward across Titan’s equator on Sept. 27, 2010. By October, observations show, a dune field called Belet that lies east of the clouds suddenly darkened. The dark patch extended for more than 190,000 square miles, and started fading fast. Some spots that were dark on Oct. 14 were bright again by Oct. 29, and even more bright spots were visible on Jan. 15.

Turtle thinks the shadow is wet ground after rainfall, like a sidewalk darkened by a shower. Titan’s winds aren’t strong enough to wreak such sudden or vast changes, she says, and it’s doubtful that the kind of explosive volcanic activity that could explain the dark patch is possible on Titan.

It’s not clear how much rain fell, she adds. Some areas could have flooded or sustained small puddles, but it may just be that the surface got wet.

The showers were probably prompted by Titan’s changing seasons. Cassini has been orbiting Saturn since 2004, but since a full year on Saturn — and therefore all its moons — lasts 29 Earth years, the spacecraft has only observed one 7-year season on Titan. Astronomers saw storms and rain at Titan’s south pole during the summer, and then the clouds cleared after the spring equinox in August 2009.

“It’s kind of the equivalent on Titan right now of early April, just into northern spring,” Turtle said. “What we think triggered this huge storm is that the weather patterns are seasonal.” Major cloud patterns move north as the southern summer ends, similar to the way they do on Earth, she says. The only difference is, Earth’s tropics sustain rain clouds year round. On Titan, the equator may see rain only a few times a year.

The difference comes, at least in part, from Titan’s leisurely rotation rate, Tokano said. Titan takes 16 Earth days to rotate once, meaning its atmospheric circulation patterns are somewhat more simple. Titan’s clouds shift quickly from north to south, filling the polar lakes with rain but mostly leaving the equator out to dry.

As for whether the spring showers are good news for the possibility life on Titan, Turtle and Tokano are agnostic.

“There’s no liquid water involved in any of the processes we’re describing here, so life as we know it can’t exist,” Turtle said. “But there’s clearly so much scope for prebiotic chemistry on Titan…. Understanding Titan better in general helps us to understand what the possibilities are.”

Images: 1) NASA/JPL/Space Science Institute. 2) P. Huey/Science AAAS

Citation:
“Rapid and Extensive Surface Changes Near Titan’s Equator: Evidence of April Showers.” E.P. Turtle, J.E. Perry, A.G. Hayes, R.D. Lorenz, J.W. Barnes, A.S. McEwen, R.A. West, A.D. Del Genio, J.M. Barbara, J.I. Lunine, E.L. Schaller, T.L. Ray, R.M.C. Lopes, E.R. Stofan. Science, Vol 331, March 18, 2011. DOI: 10.1126/science.1201063.

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