1.07.2012

The Lakes and Storms of Titan

A new computer model of Titan's atmosphere and methane cycle is helping scientists explain phenomena such as rainstorms and methane lakes on the intriguing moon.

Titan is the only moon in our Solar System that has a thick atmosphere. Titan's atmosphere is composed mostly of nitrogen, and may resemble the atmosphere of the early Earth. Credit: NASA/JPL/Space Science Institute

Saturn’s largest moon, Titan, is an intriguing, alien world that’s covered in a thick atmosphere with abundant methane. With an average surface temperature of a brisk -300 degrees Fahrenheit (about 90 kelvins) and a diameter just less than half of Earth’s, Titan boasts methane clouds and fog, as well as rainstorms and plentiful lakes of liquid methane. It’s the only place in the Solar System, other than Earth, that has large bodies of liquid on its surface.


The origins of many of these features, however, remain puzzling to scientists. Now, researchers at the California Institute of Technology (Caltech) have developed a computer model of Titan’s atmosphere and methane cycle that, for the first time, explains many of these phenomena in a relatively simple and coherent way.

In particular, the new model explains three baffling observations of Titan. One oddity was discovered in 2009, when researchers led by Caltech professor of planetary science Oded Aharonson found that Titan’s methane lakes tend to cluster around its poles -- and noted that there are more lakes in the northern hemisphere than in the south.




Channel-like landscape from 16 kilometers, at 40 meters per pixel resolution during the descent of the Huygens probe. Image Credit: ESA
 

Secondly, the areas at low latitudes, near Titan’s equator, are known to be dry, lacking lakes and regular precipitation. But when the Huygens probe landed on Titan in 2005, it saw channels carved out by flowing liquid -- possibly runoff from rain. And in 2009, Caltech researchers discovered raging storms that may have brought rain to this supposedly dry region.

Finally, scientists uncovered a third mystery when they noticed that clouds observed over the past decade -- during summer in Titan’s southern hemisphere -- cluster around southern middle and high latitudes.



Fractional cloud coverage in Titan’s atmosphere integrated between July 2004 and April 2010. Black areas are cloud free and yellow are fully covered. Credit: NASA/JPL/University of Arizona/University of Nantes/ University of Paris Diderot


Scientists have proposed various ideas to explain these features, but their models either can’t account for all
of the observations, or do so by requiring exotic processes, such as cryogenic volcanoes that spew methane vapor to form clouds. The Caltech researchers say their new computer model, on the other hand, can explain all these observations -- and does so using relatively straightforward and fundamental principles of atmospheric circulation.

“We have a unified explanation for many of the observed features,” says Tapio Schneider, the Frank J. Gilloon Professor of Environmental Science and Engineering. “It doesn’t require cryovolcanoes or anything esoteric.” Schneider, along with Caltech graduate student Sonja Graves, former Caltech graduate student Emily Schaller (PhD ‘08), and Mike Brown, the Richard and Barbara Rosenberg Professor and professor of planetary astronomy, have published their findings in the January 5 issue of the journal Nature.

Schneider says the team’s simulations were able to reproduce the distribution of clouds that’s been observed -- which was not the case with previous models. The new model also produces the right distribution of lakes. Methane tends to collect in lakes around the poles because the sunlight there is weaker on average, he explains. Energy from the Sun normally evaporates liquid methane on the surface, but since there’s generally less sunlight at the poles, it’s easier for liquid methane there to accumulate into lakes.