Before the Cassini spacecraft began observing Saturn’s largest moon, Titan, researchers had suggested that a vast ocean of methane and ethane covered the hydrocarbon-shrouded body. But the craft’s penetrating radar, along with a probe that descended to the moon’s surface in 2005, revealed a different portrait. Icy Titan appears to contain small hydrocarbon lakes, not oceans. Now, Cassini researchers have evidence that Titan may have a global ocean after all—100 kilometers below the surface and consisting of water and ammonia.
Ralph Lorenz, of the Johns Hopkins University Applied Physics Laboratory in Laurel, Md., and his colleagues base their findings on Cassini radar observations recorded from 2005 to 2007. During that time, hydrocarbon mountains and other prominent features on Titan shifted position by up to 30 km, the team reports in the March 21 Science. That displacement wasn’t in sync with the moon’s expected rotation because winds in Titan’s dense atmosphere rocked the crust back and forth, the researchers propose. But they say the winds could do that only if the moon has an underground ocean, decoupling the icy crust from the core.
If so, Titan would be the fourth known solar system object—after three of Jupiter’s moons—with an internal ocean. “Large reservoirs of water, a condition for life to form and develop,” would therefore be common in the solar system, note Christophe Sotin of NASA’s Jet Propulsion Laboratory in Pasadena and Gabriel Tobie of the University of Nantes in France in an accompanying commentary.
To test their hypothesis, researchers will look for seasonal changes in the shift in coming years, as the winds change, Lorenz says.