Methane rain fills

Titan’s lakes

BY DR EMILY BALDWIN

ASTRONOMY NOW

Posted: 30 January, 2009

Changes in the appearance of hydrocarbon lakes and the presence of extensive cloud systems provide strong evidence for rainfall and changing seasons on Saturn’s largest moon Titan.

Titan is the only satellite in the Solar System with a thick
atmosphere in which complex organic chemistry occurs, but how long this atmosphere has existed or can continue to exist is still an open question. With Cassini this dense veil was lifted, and with it one of the mission’s most significant discoveries was made: the presence of hydrocarbon lakes in the moon’s polar regions. But where these lakes came from, how they are maintained or even replenished, and their relationship with the dense atmosphere has long been speculated.

These mosaics of the south pole of Saturn’s moon Titan, made from images taken almost one year apart, show changes in dark areas that may be lakes filled by seasonal rains of liquid hydrocarbons. Image: NASA/JPL/Space Science Institute.

In a paper published this week in the journal Geophysical Research Letters, Cassini Imaging Team scientists present evidence, in the form of images taken a year apart, of changes in the appearance and presence of these lakes, apparently due to extensive cloud systems covering the area in the intervening year and producing methane rainfall.

And in concert with the paper, having now surveyed nearly all of the satellite’s surface at a global scale, the Cassini Imaging Team have released the latest maps of Titan, including the first near-infrared images of the leading hemisphere of Titan's northern "lake district" captured in August 2008, which complement existing high-resolution data from Cassini's Visible and Infrared Mapping Spectrometer (VIMS) and RADAR instruments.

Such observations show that there are greater stores of liquid methane in the northern hemisphere than in the southern hemisphere. But as the northern hemisphere moves toward summer, Cassini scientists predict large convective cloud systems will form there and precipitation greater than that inferred in the south could further fill the northern lakes with hydrocarbons. Lakes may also begin to appear in the equatorial regions where they are currently absent.

But there is still a missing link in the way in which methane is cycled through the atmosphere. Even evaporation from the surface area of all the lakes combined - some 510,000 square kilometres - is not great enough to replenish the methane lost from the atmosphere by rainfall and by the formation and eventual deposition on the surface of methane-derived haze particles.

These updated maps of Saturn’s moon Titan, consisting of data from the Cassini Imaging Science Subsystem, include Cassini's August 2008 images of the moon's north polar region. Image: NASA/JPL/Space Science Institute.

"A recent study suggested that there's not enough liquid methane on Titan's surface to resupply the atmosphere over long geologic timescales," says Dr. Elizabeth Turtle, Cassini Imaging Team associate at the Johns Hopkins University Applied Physics Laboratory, and lead author of the paper. "Our new map provides more coverage of Titan's poles, but even if all of the features we see there were filled with liquid methane, there's still not enough to sustain the atmosphere for more than 10 million years."

Combined with previous analyses, the new observations suggest that underground methane reservoirs must exist. But as to why the liquids apparently collect at the poles rather than low latitudes, where dunes are common instead, is another mystery, but one that could also be found in seasonal changes of the moon's meteorology.

"Titan's tropics may be fairly dry because they only experience brief episodes of rainfall in the spring and fall as peak sunlight shifts between the hemispheres," says Dr. Tony DelGenio of NASA's Goddard Institute for Space Studies, and a member of the Cassini imaging team. "It will be interesting to find out whether or not clouds and temporary lakes form near the equator in the next few years."

The intimate relationship between transformations at the surface and the weather on Titan will be closely watched throughout the remainder of Cassini's Equinox mission.

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