Faint sunlight enough to drive weather, clouds on Saturn’s moon Titan

Between the large distance from the Sun and the thick atmosphere, Titan's surface receives about 0.1 percent of the solar energy that Earth does. Temperatures hover around -180 degrees Celsius (approximately -280 degrees Fahrenheit) and there is very little estimated day-to-night variation.

Titan is tidally-locked, presenting the same face to Saturn just as the Moon does to Earth, so its solar day is the same as its orbital period around Saturn: 15.9 days. Due to the slight eccentricity of Saturn's orbit, Titan does experience some seasonal variation over that 29.5-year cycle, which the researchers include in their model. Previous results showed that Titan has a global convection pattern known as a Hadley cell. The equatorial region shows giant dune fields (the largest in the Solar System), indicating the presence of winds.

Charnay and Lebonnois attempt to include all these phenomena in their three-dimensional dynamic model. It shows that the troposphere (the layer of Titan's atmosphere closest to the surface, as on Earth) has distinct boundary layers, something consistent with the temperature gradient observed during Huygens' descent. In the simulation, the Sun warms the surface slightly during the day, so the air layer closest to the ground warms and rises, forming clouds. Because the effect is so slight, this layer is only about 300 meters thick. There's an additional seasonal boundary two kilometers above.

The researchers assume a dry atmosphere, so they do not include a methane cycle, which has been proposed to explain the variations in lake populations between hemispheres. They acknowledge their model must be extended to include this before it can be fully tested against the data. However, their simulations do appear to show that even a slight amount of solar heating can produce both daily and yearly variations, layering the atmosphere in a way not drastically different than our home world.

Nature Geoscience, 2012. DOI: 10.1038/ngeo1374  (About DOIs).