Cassini Sees Seasonal Rains Transform Titan's Surface

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Jia-Rui C. Cook 818-354-0850
Jet Propulsion Laboratory, Pasadena, Calif.
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Joe Mason 720-974-5859
Space Science Institute, Boulder, Colo.
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Michael Buckley 240-228-7536
Johns Hopkins University Applied Physics Laboratory, Laurel, Md.
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News Release: 2011-084 March 17, 2011

Cassini Sees Seasonal Rains Transform Titan's Surface

The full version of this story with accompanying images is at:
http://www.jpl.nasa.gov/news/news.cfm?release=2011-084&cid=release_2011-084

PASADENA, Calif. -- As spring continues to unfold at Saturn, April showers on the planet's
largest moon, Titan, have brought methane rain to its equatorial deserts, as revealed in images
captured by NASA's Cassini spacecraft. This is the first time scientists have obtained current
evidence of rain soaking Titan's surface at low latitudes.

Extensive rain from large cloud systems, spotted by Cassini's cameras in late 2010, has
apparently darkened the surface of the moon. The best explanation is these areas remained wet
after methane rainstorms. The observations released today in the journal Science, combined with
earlier results in Geophysical Research Letters last month, show the weather systems of Titan's
thick atmosphere and the changes wrought on its surface are affected by the changing seasons.

"It's amazing to be watching such familiar activity as rainstorms and seasonal changes in weather
patterns on a distant, icy satellite," said Elizabeth Turtle, a Cassini imaging team associate at the
Johns Hopkins University Applied Physics Lab in Laurel, Md., and lead author of today's
publication. "These observations are helping us to understand how Titan works as a system, as
well as similar processes on our own planet."

The Saturn system experienced equinox, when the sun lies directly over a planet's equator and
seasons change, in August 2009. (A full Saturn "year" is almost 30 Earth years.) Years of Cassini
observations suggest Titan's global atmospheric circulation pattern responds to the changes in
solar illumination, influenced by the atmosphere and the surface, as detailed in the Geophysical
Research Letters paper. Cassini found the surface temperature responds more rapidly to sunlight
changes than does the thick atmosphere. The changing circulation pattern produced clouds in
Titan's equatorial region.

Clouds on Titan are formed of methane as part of an Earth-like cycle that uses methane instead
of water. On Titan, methane fills lakes on the surface, saturates clouds in the atmosphere, and
falls as rain. Though there is evidence that liquids have flowed on the surface at Titan's equator in
the past, liquid hydrocarbons, such as methane and ethane, had only been observed on the
surface in lakes at polar latitudes. The vast expanses of dunes that dominate Titan's equatorial
regions require a predominantly arid climate. Scientists suspected that clouds might appear at
Titan's equatorial latitudes as spring in the northern hemisphere progressed. But they were not
sure if dry channels previously observed were cut by seasonal rains or remained from an earlier,
wetter climate.

An arrow-shaped storm appeared in the equatorial regions on Sept. 27, 2010 -- the equivalent of
early April in Titan's "year" -- and a broad band of clouds appeared the next month. As described
in the Science paper, over the next few months, Cassini's imaging science subsystem captured
short-lived surface changes visible in images of Titan's surface. A 193,000-square-mile (500,000-
square-kilometer) region along the southern boundary of Titan's Belet dune field, as well as
smaller areas nearby, had become darker. Scientists compared the imaging data to data obtained
by other instruments and ruled out other possible causes for surface changes. They concluded this
change in brightness is most likely the result of surface wetting by methane rain.

These observations suggest that recent weather on Titan is similar to that over Earth's tropics. In
tropical regions, Earth receives its most direct sunlight, creating a band of rising motion and rain
clouds that encircle the planet.

"These outbreaks may be the Titan equivalent of what creates Earth's tropical rainforest climates,
even though the delayed reaction to the change of seasons and the apparently sudden shift is
more reminiscent of Earth's behavior over the tropical oceans than over tropical land areas," said
Tony Del Genio of NASA's Goddard Institute for Space Studies, New York, a co-author and a
member of the Cassini imaging team.

On Earth, the tropical bands of rain clouds shift slightly with the seasons but are present within
the tropics year-round. On Titan, such extensive bands of clouds may only be prevalent in the
tropics near the equinoxes and move to much higher latitudes as the planet approaches the
solstices. The imaging team intends to watch whether Titan evolves in this fashion as the seasons
progress from spring toward northern summer.

"It is patently clear that there is so much more to learn from Cassini about seasonal forcing of a
complex surface-atmosphere system like Titan's and, in turn, how it is similar to, or differs from,
the Earth's," said Carolyn Porco, Cassini imaging team lead at the Space Science Institute,
Boulder, Colo. "We are eager to see what the rest of Cassini's Solstice Mission will bring."

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and
the Italian Space Agency. The Jet Propulsion Laboratory (JPL), a division of the California
Institute of Technology in Pasadena, manages the Cassini-Huygens mission for NASA's Science
Mission Directorate, Washington. The Cassini orbiter and its two onboard cameras were
designed, developed and assembled at JPL. The imaging team is based at the Space Science
Institute in Boulder, Colo. For more information about the Cassini-Huygens mission, visit
http://www.nasa.gov/cassini and http://saturn.jpl.nasa.gov.

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