January 5, 2012
Computer Model Explains Lakes, Storms On Titan
Researchers studying Saturn´s moon Titan -- a natural satellite roughly half of Earth´s diameter, with a thick atmosphere of methane and a surface temperature of nearly -300 degrees Fahrenheit and the only other body in our solar system that has large bodies of liquid on its surface -- have created a computer model of its atmosphere and methane cycle that, for the first time, explains how lakes and storms form and exist on the distant moon.
The new model, created by researchers led by Caltech professor of planetary science Oded Aharonson, particularly explains three baffling observations of Titan. They first found that Titan´s methane lakes tend to cluster around its poles -- and noted there are more lakes in the northern hemisphere than in the south.
They also found that areas at low altitudes, near the equator, are more arid, lacking lakes and regular precipitation. When Huygens probe landed on Saturn´s moon in 2005, however, it saw channels carved out by flowing liquid -- possibly runoff from rain. In 2009, researchers from Caltech also discovered raging storms that may have brought rain to this otherwise dry region.
The third mystery they uncovered was finding that clouds observed over the past decade -- during the summer phases in the southern hemisphere -- cluster around southern middle and high latitudes.
Scientists have tried to explain the features in various ways before this, but their models either cannot 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 team said their new computer model, however, can explain all of these observations using relatively fundamental principles of atmospheric circulation.
“We have a unified explanation for many of the observed features,” said Tapio Schneider, the Frank J. Gilloon Professor of Environmental Science and Engineering. “It doesn´t require cryovolcanoes or anything esoteric.”
Schneider and his colleagues published the findings in the 5 January issue of the journal Nature. He said their simulations were able to reproduce the distribution of clouds that has been observed. Their model also produces the right distribution of lakes. Methane tends to collect in lakes around the poles because sunlight there is weaker on average, he explained.
The sun´s energy usually evaporates liquid methane on the surface, but since there is generally less sunlight at the poles, it is easier for liquid methane to accumulate into lakes. To explain why the northern hemisphere has more lakes than the south, Schneider pointed out that Saturn´s slightly elongated orbit means that Titan is farther from the sun when it´s summer in the northern hemisphere.
Titan spends more time at the far end of its elliptical orbit, when it´s summer in the north. As a result, the northern summer is longer than the southern summer. And since summer is the rainy season in Titan´s polar regions, the rainy season is longer in the north.
Despite the rainy summer seasons on Titan, the weather is generally bland, with regions near the equator even more so dull. Years can go by without a drop of rain, leaving the lower latitudes of Titan parched. But it was shocking when Huygens probe found the evidence of rain runoff in the equatorial regions -- a surprise that increased in 2009 when Schneider and colleagues discovered storms in the same, supposedly rainless, areas.
Nobody really understood how those storms arose, because previous models failed to generate anything more than a drizzle. The new models, however, produced downpours. It generally rains very rarely at low latitudes, but when it does rain, it really pours, said Schneider.
Schneider added that the new model reproduces what scientists have already been seeing on Titan, but perhaps what is most exciting is that it also can predict what scientists will see in the next few years. Based on simulations in the model, the researchers predict that the changing seasons will cause lake levels in the north to rise over the next 15 years. They also predict that clouds will form around the north pole in the next two years.
Making testable predictions is “a rare and beautiful opportunity in the planetary sciences,” Schneider said. “In a few years, we´ll know how right or wrong they are.”
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