Throughout our solar system, only Earth has water sloshing on its surface. But three large moons of Jupiter have oceans of their own, lying beneath kilometers of globe-encasing ice. Now Saturn's huge moon Titan has joined the deep-sea club. And given the liquid water, astrobiologists are considering the chances of finding traces of Titan life.
These oceans are no great surprise to theorists. Deep-seated heat can keep hundreds of kilometers of water liquid beneath an insulating layer of ice that's tens of kilometers thick. The trick has been confirming their existence. In the case of Titan, planetary scientist Ralph Lorenz of Johns Hopkins University's Applied Physics Laboratory in Laurel, Maryland, and fellow team members working with data from the Cassini spacecraft checked to see if the moon's rotation speed had changed lately. Modelers have predicted that if Titan has a buried sea, the moon's seasonal shifts in wind direction would alternately slow and speed up its spin. That's because an outer ice shell--separated from the moon's rocky innards by liquid water--could slip easily enough to alter Titan's rotation rate noticeably.
When Lorenz and his colleagues checked the position of landmarks on the surface from one pass of Cassini's imaging radar to the next, they found that the landmarks had rotated as much as 30 kilometers farther than they would have if the rotation rate were constant. That means the outer shell has to be slipping, and there has to be an internal ocean, the team reports tomorrow in Science.
Planetary physicist David Stevenson of the California Institute of Technology in Pasadena agrees that there's liquid water--an essential for life--in Titan. He hastens to add, however, that any ocean life may not leave traces on the surface. The outer shell is probably thicker than the authors' rough estimate of 70 kilometers, he says, and unlikely to be cracked through to the surface. Then there's the ocean's poisonous ammonia--presumably lingering from Titan's formation--and a likely dearth of chemical nutrients in the sea, says co-author and cosmochemist Jonathan Lunine of the University of Arizona, Tucson. "It would have to be very simple, primitive life," Lunine says, "and there might not be much of it." Still, it could be down there, he says, and against the odds make its way to the surface in the ice lavas (Science, 8 April 2005, p. 193) first recognized by the Cassini radar.