Titan’s atmosphere has much in common with our own: it consists mainly of nitrogen, contains organic material, and exerts a pressure one-and-a half times what we feel on Earth. But Titan receives just 1% of the power that the Earth receives from the Sun. Griffith and co-workers believe that at such low temperatures, the methane in Titan’s atmosphere plays a similar role to the water in the Earth’s atmosphere. The team analysed variations in infrared spectra collected during September 1999 and found changes in intensity over periods of only a few hours. Griffith’s team proposes that a cycle of methane condensation and evaporation would produce short-lived clouds that could explain these fast-changing patterns.

By comparing spectra from clear and cloudy days on Titan, Griffith and colleagues also established that all of the clouds are at a similar altitude, suggesting that a common process drives cloud formation. Because Titan rotates slowly, it has a less turbulent atmosphere than Earth, and its cool atmosphere lacks the thermal gradients that produce our weather systems. But Griffith and colleagues believe that Titan’s methane-rich atmosphere produces large quantities of latent heat as the methane condenses into “rain”, making pockets of air very buoyant. This means that convection, which plays only a minor role in our weather, is a likely mechanism for cloud formation on Titan.

Titan’s weather may bear some of the hallmarks of ours, but Titan’s clouds cover less than 1% of its surface compared with around 50% on Earth. Clouds also cover a huge altitude range on Earth due to the complex effects of solar radiation.