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Explaining Titan’s Alien Weather System


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A composite image taken by Cassini on a fly by in 2005. This is roughly what Titan would look like to the human eye. Credit: NASA/JPL/Space Science Institute

Underneath Titan’s dense atmosphere lies something rather unusual, by terrestrial standards. Some features of the Saturnian moon, at first glance, might look similar to some features we have on Earth — it is the only other body in the solar system with lakes, and appears to have an active weather system. But instead of water, it’s methane that rains from the skies to fill Titan’s vast lakes, before it evaporates to form clouds that cover the surface. Curiously similar to the water cycle here on Earth, but at the same time rather alien.

The Cassini spacecraft has been able to take a closer look at this alien weather system and has seen that the distribution of lakes and clouds is not even across the surface of the moon. The lakes tend to cluster around the poles, in particular in the northern hemisphere. Clouds, meanwhile, prefer the south – that is the hemisphere that until recently was experiencing summer. A year on Titan is the equivalent of 30 years on Earth, so summer lasts a long time. Clouds stick around for about 25 out of these 30 years, but vanish for the remainder.

Some scientists at Caltech have come up with an explanation for this uneven distribution of clouds and lakes. They published their findings in Nature recently, and I wrote about their new model for Imperial’s student newspaper this week – head over to Felix Online to read all about it, if you wish.

I spoke to Dr Ingo Mueller-Wodarg, a planetary scientist in the Physics department at Imperial about the paper, and he explained why the new model is better than previous ones.

“What this study does is reproduce reasonably well, better than others before, the observations in terms of lake and cloud distribution. The significance of this is that we have gained a first understanding of what controls these features, namely a complex interplay of global wind transport, microphysical processes such as condensation and evaporation, cloud formation and radiative heating.

As far as I can tell this model has advanced on two fronts, namely being 3D rather than 2D and fully including the coupling between the atmosphere and surface in terms of methane transport and including surface reservoirs of methane. Many [previous] models are 2D, since calculation times otherwise become prohibitive due to the number of years that the models need to be run to assess seasonal trends (given that a Titan year is equivalent to 30 Earth years!). As far as I know, many models have significantly simplified the surface-atmosphere methane transport processes and hence got differing results. Importantly, many studies previously didn’t fully account for surface reservoirs of Methane and how these change over a year in response to the atmosphere.”

The Caltech team’s model has enabled them to make predictions about what the weather will be like on Titan in the next few years – to see if their model is right, all we will have to do is stay tuned (and make sure Cassini is making the observations needed to check their predictions!).

Refererence
Schneider, T., Graves, S., Schaller, E., & Brown, M. (2012). Polar methane accumulation and rainstorms on Titan from simulations of the methane cycle Nature, 481 (7379), 58-61 DOI: 10.1038/nature10666

Kelly Oakes About the Author: Kelly Oakes has a master's in science communication and a physics degree, both from Imperial College London. Now she spends her days writing about science. Follow on Twitter @kahoakes.

The views expressed are those of the author and are not necessarily those of Scientific American.





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