Clouds gather on Titan
Oct 19, 2000
The weather on Saturn's largest moon, Titan, is similar to the weather on Earth, according to astronomers. Caitlin Griffith of Northern Arizona University and co-workers studied daily variations in Titan's infrared spectra and found evidence for an earth-like system of clouds and rain on Titan, characterised by short-lived, high-altitude clouds (C A Griffith et al 2000 Science 290 509).
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.