Computer model explains Titan mystery

CALTECH (US) — A new computer model may explain the mysterious polar lakes, rainstorms, and clouds on Titan, Saturn’s largest moon.

With an average surface temperature of -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 model of Titan’s atmosphere and methane cycle explains unanswered phenomena in a relatively simple and coherent way, says Tapio Schneider, professor of environmental science and engineering at California Institute of Technology (Caltech).

The first oddity, discovered in 2009, found that Titan’s methane lakes tend to cluster around its poles—and that there are more lakes in the northern hemisphere than in the south.

Seasonal changes in the atmosphere of Saturn’s largest moon are captured in this natural color image, which shows Titan with a slightly darker top half and a slightly lighter bottom half. Titan’s atmosphere has a seasonal hemispheric dichotomy, and this image was taken shortly after Saturn’s August 2009 equinox. (Credit: NASA/JPL/Space Science Institute)


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, researchers discovered raging storms that may have brought rain to the 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.

Scientists have proposed various ideas to explain these features, but their models either couldn’t account for all of the observations, or were only able to do so by requiring exotic processes, such as cryogenic volcanoes that spew methane vapor to form clouds.

The new computer model 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,”  Schneider says. “It doesn’t require cryovolcanoes or anything esoteric.”

The team’s simulations, reported in the journal Nature, were able to reproduce for the first time the distribution of clouds that’s been observed. 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, Schneider 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.

But then why are there more lakes in the northern hemisphere? Schneider points out that Saturn’s slightly elongated orbit means that Titan is farther from the sun when it’s summer in the northern hemisphere.

Kepler’s second law says that a planet orbits more slowly the farther it is from the sun, which means that 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.

Even though the summer rains in the southern hemisphere are more intense—triggered by stronger sunlight, since Titan is closer to the sun during southern summer—there is more rain over the course of a year in the north, filling more lakes.

In general, however, Titan’s weather is bland, and the regions near the equator are particularly dull, the researchers say. Years can go by without a drop of rain, leaving the lower latitudes of Titan parched.

It was a surprise, then, when the Huygens probe saw evidence of rain runoff in the terrain. That surprise only increased in 2009 when storms were discovered in this same, supposedly rainless, area.

No one really understood how those storms arose, and previous models failed to generate anything more than a drizzle. But the new model was able to produce intense downpours during Titan’s vernal and autumnal equinoxes, enough liquid to carve out the type of channels that Huygens found. With the model, the researchers can now explain the storms. “It rains very rarely at low latitudes,” Schneider says. “But when it rains, it pours.”

The new model differs from previous ones in that it’s three-dimensional and simulates Titan’s atmosphere for 135 Titan years, equivalent to 3,000 years on Earth—so that it reaches a steady state. The model also couples the atmosphere to a methane reservoir on the surface, simulating how methane is transported throughout the moon.

The model successfully reproduces what scientists have already seen on Titan, but perhaps what’s most exciting, Schneider says, is that it also can predict what scientists will see in the next few years. For instance, based on the simulations, the researchers predict that the changing seasons will cause the 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 says. “In a few years, we’ll know how right or wrong they are. “This is just the beginning. We now have a tool to do new science with, and there’s a lot we can do and will do.”

The research was supported by a NASA Earth and Space Science Fellowship and a David and Lucile Packard Fellowship.

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