ONLY two worlds in the solar system have rain that falls all the way to the ground. We live on one; the other is Saturn's giant moon Titan. Now findings from the Cassini mission and the Huygens probe that landed on Titan last year are starting to reveal the patterns of rainfall on the moon.
Unmistakable river channels cut into Titan's icy terrain. So there must be some kind of rainfall, almost certainly drops of liquid methane, to feed the rivers.
”Some of this methane rain on Titan could come from cloudbursts comparable to the fiercest storms on Earth
Some of this rain could come from fierce storms generated by clouds that cluster around Titan's south pole, according to Ricardo Hueso Alonso at the University of the Basque Country in Bilbao, Spain. Hueso and his colleague Agustín Sánchez-Lavega modelled Titan's atmosphere and then varied the methane "humidity" and temperature until their model recreated the moon's polar clouds, which sometimes reach as high as 25 kilometres. To climb so high, the clouds must be lifted by powerful convection currents driven by the heat energy that sudden condensation releases. Raindrops would condense around the aerosol particles that the Huygens lander detected in large quantities when it descended through Titan's atmosphere in January 2005.
In the model, cloudbursts over the south pole would dump about 150 millimetres of rain in just 2 hours - comparable to some of the fiercest storms on Earth (Nature, vol 442, p 428). Methane droplets, up to 5 millimetres across, could fall at more than 100 kilometres per hour.
Most of the moon does not see storm clouds like this, however; Titan's run-of-the-mill rainfall may be much less dramatic. A team led by Tetsuya Tokano of the University of Cologne in Germany has used Huygens data to work out the methane humidity in the atmosphere (Nature, vol 442, p 432). They calculate that the methane should condense into small ice crystals at altitudes of about 20 to 30 kilometres, forming the hazy layer of clouds that Huygens saw above Titan. The ice crystals should then drift down, melt and eventually drizzle to the surface.
Such drizzle and polar storms can't be the whole story, however. Most of Titan's river systems are far from the pole, and the drizzle is much too light to account for them - it only adds up to about 50 millimetres per year, less than the average rainfall in some deserts on Earth. "Maybe at middle latitudes there are storms in a different season," says Hueso.
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18:00 07 October 2008
17:56 07 October 2008
