It's Always Raining on Titan

Liquid methane drizzles on the surface of Titan, a moon of Saturn, according to a paper by NASA and university scientists in today's issue of Nature.

Data from the European Space Agency's Huygens probe indicates there is a lower, barely visible, liquid methane-nitrogen cloud that drops rain to the surface of Titan. The rain on Titan is just a slight drizzle, but it rains all the time and makes the ground wet and muddy with liquid methane. This is why the Huygens probe landed with a splat. It landed in methane mud.

The rain on Titan is equal to about two inches (about 5 centimeters) a year, about as much rain as Death Valley. But on Titan, this rain is spread out evenly over the entire year.

On Titan, the clouds and rain are formed of liquid methane. On Earth, methane is a flammable gas, but Titan has no oxygen in its atmosphere that could support combustion. Also, the temperatures on Titan are so cold -- minus 300 degrees Fahrenheit (minus 149 degrees Celsius) -- that the methane can form liquid. Titan's landscape includes fluvial, river-like features that may well be formed by methane rain, scientists noted.

A gap separates the liquid methane cloud -- the source of the rain -- from a higher, upper methane ice cloud, according to the scientific study. The upper clouds are methane ice, and the lower clouds are liquid and composed of a combination of methane and nitrogen. Tthese thin liquid methane clouds cover about half of Titan.