First author

When the Huygens spacecraft descended into the atmosphere of Saturn's largest moon, Titan, in January 2005, the data it recorded suggested it had encountered a fine mist of rain. But this was no ordinary rain — it was a methane mist. Some researchers thought methane condensation to be unlikely in Titan's frigid environment (its surface temperature is −180 °C). However, according to the data collected by Huygens, which allowed an international team of scientists to calculate a profile of Titan's relative humidity, there is condensation on Titan. Tetsuya Tokano, a geophysicist at the University of Cologne, Germany, and his colleagues used Huygens' data to characterize Titan's clouds (see page 432). He tells Nature about methane rain on a distant moon.

Had you always had a fascination with Titan?

Clouds on Earth were the subject of my master's thesis. I then moved on to study Titan, developing climate and atmospheric-circulation models. I was interested to know whether, if rain arrives at the moon's surface, there would be a hydrological cycle. As it turns out, there is.

How often does it rain on Titan?

We believe that it was raining during the descent. In contrast to sporadic clouds observed near the South Pole, this rainfall is permanent for at least the season, which lasts several years on Titan. There are 30 Earth years to every one Titan year.

What does methane drizzle suggest about the geological features of Titan?

The rivers on Titan's surface were probably not caused by this drizzle because the rainfall is too weak. But the presence of drizzle does not rule out the occurrence of thunderstorms and heavy rain storms at other times.

How do your results compare with those of Hueso and Sánchez-Lavega ( page 428 )?

They believe that rainstorms are rare but violent, whereas we feel that rain is ubiquitous but weak. However, I think both can occur on Titan, although not simultaneously.

Has such drizzle been documented on other planets and moons?

Methane clouds have been identified on some other planets, but, so far, no actual rainfall has been observed.

Do your results have any relevance for the hydrological cycle on Earth?

This paper would definitely be of interest to terrestrial meteorologists. The comparison of methane and water condensation provides a better idea of how condensation works.