TWO teams of astronomers have found short-lived clouds of methane on Saturn鈥檚 largest moon, Titan, that last anything from just a few hours to a day. But the teams disagree over whether geological activity is the main source of these clouds.
Methane is an important component of Titan鈥檚 atmosphere. Without some active source, the gas should have been broken down by sunlight and dissipated by now.
Henry Roe of the California Institute of Technology in Pasadena and his colleagues think that their findings hint at a subterranean source of methane. Using the Gemini and Keck telescopes on Mauna Kea, Hawaii, they found clouds at mid-latitudes that lasted about one Earth day, largely over one region. The researchers think that atmospheric circulation alone cannot explain them, and say that geological features such as geysers or icy volcanoes could be responsible (Science, vol 310, p 477).
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But a second team, led by Caitlin Griffith of the Lunar and Planetary Laboratory at the University of Arizona in Tucson, disagrees. The team analysed images taken by the Cassini spacecraft as it orbits Saturn and found four clouds that rose from the middle to the upper troposphere in 30 minutes, and then dissipated within an hour. Only one cloud was in the same region as the clouds seen by the Caltech team, which suggests that the clouds may not be traceable to a single surface source (Science, vol 310, p 474). 鈥淲e say it could be volcanoes. But it could be other processes as well,鈥� says Griffith.
She says more information is needed to pin down the origin of Titan鈥檚 atmospheric methane. One answer would be to use Cassini鈥檚 radar to peer through the atmospheric haze and look for a volcano or vent in the mid-latitude region. Another option would be to wait. If the clouds persist in this area when the seasons change towards the end of this decade, then it could indicate a surface source rather than just a seasonal variation.
