Titan as seen from the Voyager spaceprobe in 1979

Scientists speculate that instead of having a water cycle like our planet, a methane cycle would be the key component of Titan's weather.

If Titan's atmosphere is supersaturated, as they believe, the methane would condense into high-flying clouds. Precipitation from the clouds could lead to enormous raindrops that would fall at a snowflake-like pace through the thick atmosphere.

Looked at in most regions of the spectrum, Titan's atmosphere is opaque. But there are some narrow wavebands that allow a deeper view into its atmosphere if not to the surface itself.

Closer view

Caitlin Griffith, of Northern Arizona University, Flagstaff, US, and colleagues have obtained spectra of near-infrared light emitted by Titan. They found variations at certain wavelengths that indicate the presence of small clouds.

Observations made over several days suggest that the clouds form and dissipate on a daily basis. This suggests that they are showering rain down on to Titan's surface.

The authors propose that latent heat radiating from the moon, rather than incoming heat from the Sun, may drive a methane-based cycle on Titan similar to the hydrological cycle on Earth.

The Cassini spacecraft is currently en route to Saturn. When it gets there in 2004, a small probe called Huygens will parachute through Titan's haze with the aim of landing on a methane snowdrift or into a methane ocean.

It will then report back on the conditions it finds.

The latest research is published in the journal Science.