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Summary
The climate of planets is often described in terms of surface temperature only. In contrast, the widely used climate classification of Köppen (1) considers not only the surface temperature but also precipitation. Saturn's moon Titan is a rare extraterrestrial body where a similar concept could be applied to describe the climate because there is a methane hydrological cycle with rivers, methane vapor, clouds, and rainfall. But information on Titan's rain has been scarce. Although bright methane clouds can be monitored from Earth-based telescopes, rainfall is more difficult to observe from space, and there are no rain gauges or weather radars on the surface. However, precipitation can change the surface appearance in ways that can be observed by the Cassini spacecraft. Just as countrywide brightening of Earth's surface seen on weather satellite images can be interpreted as recent snowfall if a similar-sized cloud system was observed beforehand, similar ideas can be applied to Titan to monitor rainfall. On page 1414 of this issue, Turtle et al. (2) report the first observational evidence of substantial rainfall in the equatorial region of Titan. The appearance of a large cloud system was soon followed by surface darkening of an area east of this cloud, most likely by methane rainfall.
