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Planetary News: Titan (2007)Cassini Reveals “Seas” of Methane and Ethane on TitanBy Amir AlexanderMarch 13, 2007
In the far northern reaches of Titan, hundreds of kilometers beneath the thick haze that envelopes the Saturnian moon, a landlocked sea splashes against its lonely shores. Composed not of water but of a mixture of ethane and methane, this alien sea in other ways looks very much like its counterparts on Earth: rivers nearby plow their way through flood plains before pouring into the sea’s great expanse; coastal islands rise above the liquid surface, forming straits and channels. This landscape, both remarkably familiar and strangely alien, was discovered by the radar instrument of the Cassini spacecraft as it swooped by Titan earlier this year. Because of the thick haze, hundreds of kilometers and several layers deep that surrounds Titan, most imaging instruments can only glimpse the top of the moon’s atmosphere and are useless for mapping its surface. The radar, however, uses Cassini’s high gain antenna to bounce radio waves off Titan’s surface, and uses the reflected signals to draw the contours of the landscape below. As Cassini flew by the moon on February 22, the radar revealed a vast smooth surface, larger than North America's Great Lakes. Such a large and smooth region, surrounded by a distinct hilly landscape is almost certainly a liquid sea. It was only 20 years ago that many scientists believed that the Titan might be an ocean world whose entire surface was coated with a liquid mix of ethane and methane 1 kilometer deep. This was because when Voyager 1 flew by Titan in November of 1980, it revealed that its mostly nitrogen atmosphere also contained a few percent methane. This methane, scientists knew, would have been destroyed within 10 million years by ultraviolet radiation from the Sun if it were not regularly replenished from Titan’s surface. A methane and ethane ocean covering the planet would have been quite stable in the -180 degree Celsius (-290 Fahrenheit) temperature that prevails on Titan, and would have regularly supplied the atmosphere with the methane it was perpetually losing. According to Jonathan Lunine, Cassini interdisciplinary scientists from the University of Arizona, the beautiful image of “ocean world” Titan did not survive for long. Already in the 1980s Earth-based instruments revealed hard surface features on the Saturnian moon instead of the smooth surface of a liquid ocean. When the probe Huygens landed on the moon’s surface in January 2005, revealing a complex landscape, rich in methane, but nevertheless largely dry, scientist were not really surprised. “Would I have liked to see it go ‘splash’ in the ocean? Sure,” commented Lunine. “But seeing methane seeping out from underneath the probe, I certainly wasn’t disappointed.”
With the data and images gathered by Huygens, and repeated fly-bys by its “mother ship” Cassini, a clearer image of the surface of Titan is beginning to emerge. Rather than a single large ocean, innumerable smaller lakes are scattered across the high latitudes of the moon. Most range in size from less than a mile to dozens of miles across, but the new “seas” discovered in the far northern regions are of a different order of magnitude: the largest, covering at least 39,00 square miles has a greater surface area than Lake Superior in North America. At the moment Scientists cannot be 100% certain that the smooth regions they are seeing are indeed liquid surfaces. But the evidence is mounting: in addition to the radar images, Cassini also managed to image the area with its camera, capturing a large dark area, 620 miles long and only slightly smaller than the Caspian Sea. The northern end of this dark region corresponds to the outlines of the radar image. Smooth and dark is what one would expect a liquid sea to look like from space, and it is therefore the most obvious and likely explanation. The question, according to Lunine, will probably be settled when in the coming years Cassini’s VIMS instrument (Visual and Infrared Mapping Spectrometer) will be able to determine whether this smooth sea is in fact releasing ethane and methane vapors. The discovery of “seas” of methane on Titan, explained Lunine, greatly furthers our understanding of Titan and its unusual atmosphere. It cannot, however, replace the old hypothetical “Titan ocean” as the ultimate source of methane in the atmosphere. “The amount of methane and ethane in the “seas,” explained Lunine, might regulate the humidity on Titan over a span of millions of years.” But to preserve methane in the atmosphere over billions of years, a different and more plentiful source is needed. And so, the search continues In an article published last year in Nature, Lunine and his collaborators Gabriel Tobie and Christophe Sotin of Nantes University in France offered a solution to the mystery of Titan's missing methane. Large quantitites of methane, they argued, are stored in Titan’s crust in the form of “clathrate hydrates,” which release methane into the atmosphere under certain conditions. Measurements of Titan’s gravitational field, which Cassini will perform in the coming years, will likely demonstrate whether this hypotheses proves true said Lunine. And so, for those of you hoping to some day surf the methane seas of Titan, the news is mixed. Vast oceans, with giant waves such as one finds in California and Hawaii are probably out. But if you are one of those who would enjoy the milder pleasures of surfing in the great lakes (where temperatures may indeed approach a Titan-like freeze…), or perhaps “methane-skiing” on a smooth lake surface, then Titan may yet be the place for you. |
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