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'Titanic' Discovery: Earth-like Weather & Methane Rain
By Robert Roy Britt
Senior Science Writer
posted: 07:00 am ET
20 October 2000
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titan_weather_001020 In one of the most distant weather reports ever received, clouds and even rain showers seem to have been spotted on Titan, Saturn's largest moon. Along with vast seas and modest mountains, a picture is emerging of a place more like Earth than anywhere else in the solar system. Scientists have already labeled Titan a hot spot in the search for extraterrestrial life, and the new work adds to that enthusiasm.
Titan backlit by the sun Titan is a cold, dark, smog-shrouded world larger than the planet Mercury and nearly half as big as Earth. Known for decades as the only moon in the solar system with an atmosphere, Titan has many of the raw materials for life, including nitrogen, carbon and water.But at 886 million miles (1.4 billion kilometers) away from the Sun, most if not all of Titan's water is locked in ice. So the distant moon's rain is of a different sort, thought to be methane -- the primary component of the common heating fuel, natural gas. Methane is also an important raw material for other organic compounds. | Roving and Living at Titan | | NASA is designing the equivalent of a sports utility vehicle to take on Titan's mountains, plus its air and sea. Want to learn more? | 
At midday, the Titan sky is dim and orange. Clouds soar high into the sky, and it looks like rain. Learn about a day on Titan. | In a new study, researchers looked at non-visible light emitted by Titan, spotting small clouds that developed and disappeared daily, most likely after causing a methane rain shower. The results of the study, which confirm earlier suspicions about Earth-like weather on Titan, appear in the October 20 issue of the journal Science. "Titan is a planet-sized laboratory hosting perhaps the kinds of organic chemical reactions that preceded and initiated life on Earth 4 billion years ago," said Jonathan Lunine, a professor at the Lunar and Planetary Laboratory who was not involved in the study. "To see methane in action as a cloud-forming and rain-forming condensate lends further credence to the view that Titan is a very attractive astrobiology target."The driving force On Earth, weather is largely driven by heat from the Sun, which creates temperature differences that move large masses of wind and foster huge oceanic currents. But Titan receives about 100 times less solar energy; temperatures hover around minus 288 degrees Fahrenheit (minus-178 degrees Celsius). So a different force is thought to drive weather on Titan, weather that is mild by terrestrial standards. "We propose latent heat [released when a gas condenses] plays a large role in driving Titanian weather," said Caitlin Griffith, lead researcher. "These bizarre conditions conspire to bring about strange clouds. On Titan, clouds are rare, usually covering less than 1 percent of the globe, compared to the Earth's 50-percent coverage." Titan's gravity is only about one-seventh that of Earth. The intense chill, however, means a low-energy atmosphere that hangs around, instead of escaping this relatively weak force of gravity. So Titan's atmosphere is denser than Earth's and extends much higher into the sky. Titan's clouds hover about 15 miles (25 kilometers) up, said Griffith, a researcher at Northern Arizona University in Flagstaff. Earth's clouds, by contrast, range from the ground to occasionally as high as 11 miles (17-18 kilometers) in the middle of the worst thunderstorms and hurricanes.While weather is relatively docile on Titan, and rainfall is probably sparse, it may come down in buckets now and then. "Most rivers on Titan may run dry, but river valleys may nevertheless be abundant and deep," writes the Lunar and Planetary Laboratory's Ralph Lorenz in an analysis of the study.
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