The Lakes and Storms of Titan
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.
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.
“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.
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 Schaller, Brown, Schneider, and then-postdoctoral scholar Henry Roe discovered storms in this same, supposedly rainless, area.
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,” he adds. “We now have a tool to do new science with, and there’s a lot we can do and will do.”