Saturn’s moon Titan is one of the most intriguing bodies in our solar system. Its dense atmosphere and lakes of liquid methane make it both beautiful and bizarre, as well as a tantalizing target for those seeking extraterrestrial life. To me, though, the most amazing thing (so far) has been the revelation that is Titan’s meteorology. There’s something extraordinary about imagining liquid methane falling as rain on another world—it’s so similar to our experience, yet so very different. Earth has a familiar hydrologic cycle; Titan has an alien methane cycle.
In a letter published in Nature, researchers describe a model that successfully simulates some key aspects of Titan’s weather. The model offers possible explanations for some of the moon's quirky features that have long been puzzling.
Previous attempts to understand the pattern of methane lakes and cloud systems have invoked both the familiar and the bizarre, including surface topography, cryovolcanism, seasonal patterns in atmospheric circulation, and even an underground "methane table." Researchers have tried to simulate Titan's climate system with simple models, but major differences with observations have resisted explanation.
Although there is evidence of stream erosion near the equator, the moon's methane lakes are located near the poles—primarily the northern pole. Clouds, however, have been observed to be common in the southern hemisphere, but rare north of the equator.
To take the next step, researchers at Caltech and NASA built a more sophisticated, three-dimensional global climate model for Titan. It still contains major simplifications—the surface is simulated as a reservoir of liquid methane that varies in thickness, and its flow is governed by simple diffusion. But the model allows for much more complex atmospheric processes.
Like previous work, the new simulation shows that Titan’s lakes are restricted to the poles because of the cold temperatures there. Atmospheric circulation transports methane gas toward the summer pole, where the researchers say it becomes "cold-trapped."
Titan’s high albedo (reflectivity) prevents most incoming solar radiation from reaching the surface. As a result, the atmosphere is pretty stable, as it's primarily heated from above rather than below. Much of the solar energy that does reach the surface at the poles goes into evaporating methane from the lakes, which acts to destabilize the atmosphere and drive convection, creating clouds.
The southern hemisphere is currently tilted toward the sun, and the model shows that this is behind the cloudiness we've seen there. Because the southern region has plentiful methane to evaporate and is receiving the most direct radiation, that’s where the atmospheric action is right now.
Titan’s year is about 30 Earth years long, and it’s currently transitioning to northern hemisphere summer. (If you’re the type who feels like spring can never bring winter to an end soon enough, don’t move to Titan.) The authors predict that we should start to see cloudiness shift to the northern hemisphere within the next couple years, where it will be remain for the following decade. As atmospheric circulation shifts toward the northern pole, lake levels should rise there, as well.
In the past, some have suggested that the methane lakes near the northern pole may only be more abundant during the winter. That would imply that as the seasons change, we’d expect the see-saw to tip toward bigger southern lakes. However, the authors indicate that this asymmetry is persistent in the model. It's a product of the shape of Saturn’s orbit, which makes the northern hemisphere summer longer and cooler than the southern summer. That results in more liquid methane accumulating in the north.
This paper’s predictions will be easy to test— just set an appointment in your calendar for 2016 that reads, "Check Titan’s weather." The forecast is a high of -180° C with clouds in the north and a chance of mind-blowing showers.