Just in time for a summer holiday, scientists have discovered the solar system's newest beach destination. Too bad there's no way to get there—at least not easily. Researchers report in Nature today that they identified a dark liquid lake, surrounded by a lighter shoreline and a "beach," on the surface of Saturn's moon Titan. The foot-shaped lake is the first verified extraterrestrial body of liquid, and is likely filled with hydrocarbons, simple compounds also common on Earth.
"This is the first definitive evidence for both liquid and liquid hydrocarbons on Titan," says lead study author Robert Brown, a professor of planetary science at the University of Arizona's Lunar and Planetary Laboratory (LPL) in Tucson.
Instruments on NASA's orbiting Cassini spacecraft, which has studied Saturn and its satellites including Titan, its biggest moon, since June 2004, revealed the presence of the organic (carbon-containing compound) liquid ethane in a Lake Ontario–size body in Titan's southern hemisphere. Similar-looking patches abound in the northern hemisphere, indicating that Titan's surface is likely dotted with such hydrocarbon seas.
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The researchers confirmed the presence of liquid ethane with Cassini's Visible and Infrared Mapping Spectrometer (VIMS), which picked up the hydrocarbon's telltale light spectrum signature. The ethane forms when sunlight breaks down methane in Titan's upper atmosphere. It is theorized that this ethane then forms clouds and rains onto the ground, cutting streams via erosion and pooling them into lakes.
Titan has long fascinated astronomers with its hazy, orange-colored atmosphere that extends over 600 miles (965 kilometers) from the moon's surface. Titan is the only moon with such a thick gas cover. The atmosphere contains mostly nitrogen and traces of methane and other hydrocarbons including propane, the fuel of choice for barbecues here on Earth. With a diameter of 3,200 miles (5,150 kilometers), Titan is bigger than Mercury and only about 25 percent smaller than Mars, making it the second-biggest moon in the solar system behind Jupiter's Ganymede.
Liquid ethane or methane has been suspected to exist on the Titanian surface. This finding supports the idea that Titan has a "hydrological cycle" as does Earth, but instead of water (forming clouds, raining out and evaporating into clouds again), there are two substances—methane and ethane—at play, says study co-author Christophe Sotin, a planetary scientist at NASA's Jet Propulsion Laboratory in Pasadena, Calif.
In addition to ending the debate over liquid on Titan, the discovery may help determine the course of future missions. NASA and the European Space Agency have been discussing a return to Titan; this finding bolsters the case for sending a "splashdown lander," or a floating probe, into a lake on Titan to sample its contents, says Ralph Lorenz, a planetary scientist at the Johns Hopkins University Applied Physics Laboratory in Laurel, Md.
The liquid pool, named Ontario Lacus after the North American Great Lake of similar size, absorbs nearly all light, which means it would look dark to the human eye, says LPL's Brown. The data also reveals that the lake's surface is smooth, without any ripples or waves, which Brown says is surprising. He and his team had expected to see some wave action caused by presumed winds there.
It is a long shot that there's life on Titan. "It depends upon how open a mind you have," says Jonathan Lunine, a professor of planetary science and physics at LPL, who did not participate in this study. "If you throw a terrestrial organism onto the surface of Titan, it will die. But there is the chance that liquid hydrocarbons could sustain exotic forms of life."
The confirmed presence of organic compounds, along with nitrogen, means Titan has a very similar chemical makeup to that of early Earth when life developed. "These compounds are the building blocks of complex molecules like DNA," Brown says. The prevailing view is that the chemistry necessary to create information-storing, self-replicating molecules like DNA stopped on Titan due to its frigid temperatures, which average –294 degrees Fahrenheit (–181 degrees Celsius). The lack of liquid water on the surface, a crucial ingredient for life as we know it, is another reason such life stalled, if it ever started at all, on Titan.
But Brown points out that volcanoes and other tectonic activity could add energy and even water into the mix. If that happens, "all bets are off," he says, when it comes to life finding a way to take hold in Titan's decidedly un-Earthly climate.
"If we were to find life on Titan," Lunine says, "it would show that life truly is a fundamental cosmic phenomenon, and that it can happen in conditions that are very different from what we would consider habitable."