Below Haze, Saturn's Biggest Moon Has Lakes
By JOHN NOBLE WILFORD
Published: January 4, 2007
As scientists have predicted but have had a hard time proving, the surface of Titan, Saturn's largest moon, appears to be dotted with lakes of liquid methane. The lakes are more intriguing evidence of the active phenomena at play on the only moon in the solar system with a dense atmosphere.
The discovery, reported yesterday by an international team of researchers, was made by a radar survey of Titan's high northern latitudes by the Cassini spacecraft, which has been orbiting Saturn and its retinue of satellites since July 2004. One of the mission's major objectives is the investigation of Titan's environment, thought to be a frigid version of conditions on the primordial Earth.
The radar imaging system detected more than 75 dark patches in the landscape near Titan's northern polar region, the scientists said in a detailed description of the find published today in the journal Nature.
The patches, they said, indicated smooth surfaces in an otherwise rugged topography, suggesting lake beds either partly dry or filled with liquid. These smooth surfaces, more or less circular and with diameters ranging from 2 miles to 40 miles, are associated with channels that appear to have been formed by flowing liquids, presumably tributaries to the lakes.
Methane exists in Titan's atmosphere and, in the extreme cold of high latitudes, is expected to rain on the surface and be present as liquids in subsurface reservoirs.
The radar images, made on a close pass of Saturn's largest moon by the Cassini spacecraft in July, ''provide definitive evidence for the presence of lakes on the surface of Titan,'' the discovery team concluded.
When the spacecraft conducted its first radar search above 70 degrees north latitude, Ellen R. Stofan, leader of the team, said in an interview, ''We saw a huge swath of the surface just covered with lakes, like Minnesota.''
Dr. Stofan is a planetary scientist at Proxemy Research, a contractor in Virginia to the National Aeronautics and Space Administration, and also at University College London. Other team members include researchers at the Jet Propulsion Laboratory in Pasadena, Calif., which directs the Cassini mission; the University of Arizona; and several European institutions.
In an accompanying article in the journal, Christophe Sotin, a planetary scientist at the University of Nantes in France who was not involved in the research, wrote that the lakes and other mission findings were strong evidence that ''methane on Titan plays the role of water on Earth.'' It is the second-most abundant component of Titan's atmosphere, after nitrogen.
The discovery, Dr. Sotin concluded, adds to the ''weight of evidence'' that processes on and beneath Titan's surface must be similar to those dominating the early evolution of any Earth-like planet.
''As far as we know, there is only one planetary body that displays more dynamism than Titan,'' he wrote. ''Its name is Earth.''
Finding large bodies of liquid methane and probably ethane on Titan, in lakes or perhaps vast seas, had long been hypothesized, based on telescope observations of that moon's smoggy methane-rich atmosphere and by the two Voyager spacecraft that passed close some 30 years ago. But the Cassini remote-sensing instruments had failed to detect an ocean, though they and the European Space Agency's Huygens lander did find traces of the channels where liquids had apparently flowed across the surface.
On Cassini's previous encounters with Titan, the smog-penetrating radar found no smaller liquid bodies, until it observed the region from 70 degrees north latitude to 83 degrees north latitude. At present, it is winter in Titan's north. Methane is one of the few molecules to exist as a liquid in such cold conditions, scientists said, and is the most plausible explanation for the smooth, dark surfaces in the radar images.
Dr. Stofan said that the lake depressions could be volcanic craters or sinkholes; very few craters gouged out by meteorite impacts have been detected on Titan. Their roughly circular shapes and steep-sided rims are typical of volcanic depressions. Surface collapses, as in sinkholes, could fill the lakes with liquids from interior chambers, like aquifers on Earth.
In both cases, scientists said, methane vapor condensing and raining out of the cold Titan sky was probably a significant source of the liquid in the lakes.
Jonathan I. Lunine of the University of Arizona, another member of the discovery team, described in an interview what the lakes probably look like. The methane liquid would be transparent, enough to see the dark hydrocarbon sediments on the floor of shallow lakes. The liquid would be less viscous than water, perhaps like gasoline. Overhead, aerosols, minute particles in the upper atmosphere, presumably cast a dim orange light on the lake. In the dark of winter, one would need a flashlight to walk the shore.
Photo: Titan's methane lakes, in a radar image from the Cassini spacecraft. (Photo by NASA)