Atmospheric 'pulse' may spread rain clouds across Titan

时间:2019-03-08 05:11:04166网络整理admin

By Rachel Courtland A pulse in the atmosphere of Saturn’s moon Titan can spawn methane clouds across the moon, new observations suggest. Although the cause of such atmospheric events is still unknown, it could explain some puzzling features seen by the only probe ever to land on Titan’s surface. Titan boasts clouds of methane that are thought to rain methane down to the surface in a cycle similar to the water cycle on Earth. This process is thought to occur near the moon’s poles, where astronomers have seen lakes on the surface and evidence of clouds that could produce light rain or perhaps monsoons. But the picture is blurrier closer to the equator. Pictures taken by the Huygens probe, which descended through Titan’s atmosphere in 2005, revealed features that resemble stream-carved valleys and shorelines, but the region is thought to be too dry to produce clouds that can form rain. “People have been scratching their heads for a long time about how to put those two observations together,” says Mike Brown of Caltech. Now Brown and colleagues have found a possible explanation. The team spotted a vast cloud below Titan’s equator that triggered cloud formation in regions across the southern hemisphere. Such events might be a way to deliver methane rain to arid regions like the Huygens landing site, which sits some 10° south of the equator. “We think that this is an important part of the story,” Brown says. On average, just 1 per cent of Titan’s surface is obscured by methane clouds, but the moon has boasted at least five times that much cover on several occasions. To search for new weather events, a programme at NASA’s 3-metre Infrared Telescope Facility on Mauna Kea in Hawaii has been monitoring the moon for evidence of brightenings that might suggest new methane clouds. The team found that Titan appeared brighter on 13 April 2008, and on the following night, Mauna Kea’s 8-metre Gemini North telescope revealed a large cloud centred some 30° south of Titan’s equator. Over the course of a month, clouds formed across the southern half of the moon, some close to the south pole and other stretching as far north as 12° south. The clouds seemed to have been created by an atmospheric wave that caused changes in pressure, temperature and wind velocity. Eventually the wave wrapped around the moon to cause the original site to brighten. We “saw an enormous event occur that kicked off cloud activity all over the southern hemisphere,” says Henry Roe of Lowell Observatory in Flagstaff, Arizona. Similar atmospheric pulses might be responsible for creating clouds that could deliver methane rain to Titan’s desert-like equator, the team says. The event “showed how cloud activity in one part of the planet can kick off activity anywhere else in the same hemisphere, including over areas that are usually thought of as arid and dry.” The team is not sure what might have caused the pulse. Odd effects that result from the circulation of Titan’s atmosphere may be directly responsible for the outburst. “There’s also a good chance that surface activity could generate an event like this,” Roe says. The pulse may have been created by changes on the surface that could have heated the lower atmosphere or jettisoned more methane into the air. “Cassini has been returning evidence that Titan’s surface is geologically active, but nobody’s really found the smoking geyser or the new mountain range forming to say what that activity is yet,” says Roe. “These are a fascinating set of observations,” says Caitlin Griffith, who studies planetary atmospheres at the University of Arizona in Tucson and was not involved in the study. But she adds that it is not yet clear whether these sorts of events can make rain clouds over Titan’s tropics. “I think that the jury is still out as to whether these clouds can cause the fluvial features seen at the landing site,” Griffith told New Scientist. Journal reference: Nature (vol 460, p 873) More on these topics: