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New computer model explains Titan’s lakes and atmosphere

Dr Amanda Doyle, Feature writer
Jan 7, 2012, 8:00 UTC

Sen— Scientists have developed a computer model that can explain several puzzling aspects about Saturn’s moon Titan.

The gas giant Saturn is most famous for the bright, extensive rings that surround the planet. But also orbiting the planet are many moons, the largest of which is Titan. Titan is embraced by a substantial atmosphere, and it is the only moon in the Solar System that can boast such an attribute. Surface features are obscured by the 200 kilometre thick atmosphere, which is composed mainly of nitrogen and methane, and that can make it difficult for scientific observations. 

Since NASA’s Cassini spacecraft entered into orbit around Saturn in 2004 and the Huygens probe landed on Titan in 2005, much has been learnt about the moon. But some of this new knowledge is puzzling, and up until now couldn’t be properly explained by computer models. However researchers at the California Institute of Technology (Caltech) have developed a new computer model that successfully explains some of these puzzling features.

There are three aspects in particular that have caused confusion. The first is the overabundance of methane lakes at the poles of the moon compared to other regions. There are also more lakes in the northern hemisphere than the southern hemisphere. The second is that the area around the equator is known to be dry as evidenced by the lack of lakes and precipitation. But the discovery of channels carved by liquid flows and occasional storms in the area contradicts this evidence. Thirdly, clouds have been observed to cluster in middle and high latitudes during the southern hemisphere summer.

Previous computer models have attempted to explain these phenomena, but none have been successful in explaining all three of these puzzles, at least not without resorting to complicated processes. Once such process is cryovolcanism, where volatile material such as methane erupts instead of molten rock. But there is no need for such an elaborate explanation in this new computer model. So what makes this model so different? Tapio Schneider from Caltech explains.

“Our atmosphere model is three-dimensional, whereas most relatively successful other models were two-dimensional. Three-dimensionality of the atmosphere allows us to resolve waves and instabilities in the atmosphere, which are crucial, for example, for generating the rare but intense low-latitude rain storms. Our simulations were run for more than 135 Titan years, or about 4,000 Earth years -much longer than previous simulations. This allows us to study steady-state balances in the methane cycle and so to identify unambiguously, for example, how the surface methane distribution is maintained.”

The model also connects the atmosphere with a reservoir of methane on the surface, which allows the scientists to visualise how exactly methane is transported around Titan. It takes into account both the fact that methane can evaporate from this reservoir, as well as the fact that methane can be transported along the surface. “Other models did not take in particular the along-surface transport of methane into account, which is crucial for obtaining a closed methane cycle with more methane lakes in the northern hemisphere polar region,” Schneider tells Sen.

So how does the Caltech model solve the trio of mysteries? The accumulation of lakes in the polar regions is explained because less sunlight reaches these areas. As such, the solar energy needed to evaporate liquid methane doesn’t exist, so the methane can exist in liquid form more easily. Saturn also has a slightly elliptical orbit, which influences the methane lakes. As the northern summer occurs when Titan is furthest from the Sun, it lasts longer than the southern summer. And as summer is the rainy season, it means that more rain fails during the northern summer and thus there are more lakes in the northern hemisphere.

The channels carved out by liquid flows at low latitudes are also explained by the model. Storms have been observed on Titan, but previous models could create nothing beyond a mild drizzle and thus failed to explain this weather. However the Caltech model can easily describe the formation of rare but extremely intense storms which cause rain at low latitudes around the time of the equinoxes. The amount of rain predicted is enough to form the channels that are observed. Finally, the computer model has also been able to reproduce the distribution of clouds around the moon, which matches observations.

“Many of the refinements and developments of the model were prompted by Cassini and Huygens data as they became available,” says Schneider. “For example, as the distribution of lakes became better known thanks to Cassini, we realised we had to refine how we represented the methane surface reservoir. And clouds observations from ground-based telescopes and Cassini were an important target for us to hit with our model.”

The computer model not only allows scientists to understand what they are currently seeing on Titan, but it can predict what should happen in the near future. It is expected that the next two Earth years will see clouds form around the north pole and that the northern lake levels will rise in the next 15 years. Making testable predictions is an important part of science, and astronomers will be keeping a close eye on Titan to see if this model is correct.