The Rain on Titan

Ever since 1980, when the Voyager fly-bys revealed that Titan has a dense methane atmosphere, planetary geologists have speculated about the weather on this distant world. The temperature there is close to the triple point of methane which therefore it ought to exist as a solid, a liquid and a gas. That raises all kinds of exciting meteorological possibilities.

One of these is methane rain. Various models of the atmosphere predict that it should rain on Titan although some have suggested that any rain drops would evaporate before hitting the ground. images of Titan’s surface taken by Cassini and Huygens show river-like structures, indicating that liquid must have flowed across the surface in the recent passed. This liquid may well have come from rain as it does on Earth.

But nobody had actually seen rain on Titan until 2007 when Mate Adamkovics at the University of California and a few buddies spotted a thin cloud of methane snow that envelopes the entire moon. Amazingly, they were able to watch as the opacity of that cloud increased early each day, suggesting that as this world warms up, a light morning drizzle of liquid methane begins to fall.

That was a terrific result but some researchers questioned the observation, saying that the increase in opacity might also be the result of surface reflectivity artifacts.

Now Adamkovics and pals have hit back with a new set of observations from the Very Large Telescope in northern Chile. They’ve studied Titan’s atmosphere on a series of consecutive nights at wavelengths not absorbed by methane. They say their new observations show that surface reflectivity artifacts are not present and so cannot account for the increase in opacity.

This would tend to confirm their claim that what they are seeing is methane drizzle. This seems to fall over a region of the surface called Xanadu, a large plateau of water ice about the size of Australia. The new observations show that it drizzles not only in the morning but also in the afternoon too. So more like Seattle than Sydney.

But there is still an open question: the new images do not show whether this drizzle reaches the ground. So nobody yet knows whether umbrellas will be needed on Titan after all.

Ref: arxiv.org/abs/0907.2255: Evidence for Condensed-Phase Methane Enhancement Over Xanadu on Titan