Long streaky methane clouds at Titan鈥檚 mid latitudes may come from a geyser or ice volcano on the surface, new observations suggest. But other scientists say the clouds may simply form due to atmospheric effects 鈥� leaving the source of Titan鈥檚 atmospheric methane an unsolved mystery.
Methane is a component of the atmosphere of Saturn鈥檚 largest moon and the gas is broken down by the ultraviolet light from the Sun. Without a renewable source, like a series of ground vents or volcanoes, Titan鈥檚 methane would disappear after about 10 million years 鈥� a short time relative to the moon鈥檚 age. But so far, astronomers have not been able to pinpoint a source for the gas.
Now, astronomers at Caltech say they have evidence suggesting the source could come from geologic activity. They used the Gemini Observatory in Hawaii and found mid-latitude clouds on 15 nights.
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Titan produces skinny methane clouds at a latitude of 40掳 south. The new observations found they also appear to be clustered over 0掳 longitude, with a second group of clouds at 150掳 west longitude.
Having methane clouds concentrated over one region indicates that there could be something below on Titan鈥檚 surface releasing the gas.
Rising air
But another team of researchers using the Cassini spacecraft have a different interpretation of the cloud formation. During flybys of Titan, Cassini was able to measure the height and thickness of four streaky mid-latitude clouds.
They expected to see clouds rising from one spot if a volcano or other surface activity was spawning them. But only one of the four clouds they looked at was at the same longitude as the Caltech team鈥檚 clouds. That suggests that the clouds may not be traceable to a single surface source.
鈥淲e say it could be volcanoes. But it could be other processes as well,鈥� says Caitlin Griffith, with the Lunar and Planetary Laboratory at the University of Arizona, who led the Cassini research.
The first clouds ever spotted on Titan were over the south pole. They form a cap over the moon up to 40掳 south latitude 鈥� the same latitude these long, thin clouds are found.
鈥淲e argue that it鈥檚 not a coincidence that they both occur at that latitude,鈥� Griffith says. 鈥淵ou could explain both of them by having a rising branch of air there.鈥�
Cutting through the haze
On Earth, one of these rising air currents occurs at the equator as the Sun warms Earth鈥檚 surface, which then heats up the lower atmosphere, causing air to rise.
Griffith鈥檚 team found that the mid-latitude clouds on Titan rose 40 kilometres in altitude within several hours, a finding consistent with an upwelling of air due to global circulation. But she says her group does not have enough information from observations of Titan鈥檚 clouds to address the origin of Titan鈥檚 atmospheric methane.
Anthony Del Genio, one of Cassini鈥檚 Imaging Science Subsystem team at the NASA Goddard Institute for Space Studies in New York City, US, published an earlier paper with colleagues suggesting that the generation of mid-latitude clouds from a surface source seemed likely. However, he told New 杏吧原创, the current evidence still does not prove it.
One way to confirm the clouds鈥� origin would be for Cassini to peer down at this region with its radar instrument, which can cut through the haze, during a close fly-by. Then it could see whether a volcano or vent exists. But no such radar opportunities are currently scheduled.
Another alternative would be patience. If the clouds persist in this area when the seasons change 鈥� in the latter part of this decade 鈥� then it could indicate a surface source rather than just a seasonal variation.
Journal reference: Science (v 310, p 474)
