What's the News: Astronomers have known for many years that Saturn's moon Titan sports lakes of liquid methane. And in the past couple years, scientists have suggested that it also has an underground ocean composed of water and ammonia. Now, based on past observations by NASA's Cassini spacecraft, astronomers are saying that Titan's rotation indeed points to an underground sea---and where there's water, there may also be life. "Our analysis strengthens the possibility that Titan has a subsurface ocean, but it does not prove it undoubtedly," researcher Rose-Marie Baland told Astrobiology Magazine. "So there is still work to do." How the Heck:
The researchers started out with two measurements made by the Cassini spacecraft: Titan's moment of inertia (the moon's resistance to changes in its rotation), and its obliquity (the angle between the axis of rotation and the axis perpendicular to the orbital plane). Titan's obliquity, for example is 0.3 degrees, whereas Earth's is 23 degrees.
These two measurements are useful because the Cassini State Model describes a reliable correlation between a moon's obliquity to its moment of inertia. As Baland told me via email, the model predicted an obliquity of 0.12 degrees, which led her to conclude "that the measured obliquity [of 0.3 degrees] seemed too high."
The astronomers discovered that the best way to reconcile the contradiction between Titan's obliquity and moment of inertia is to assume that Titan has a solid interior surrounded by liquid water, which is itself surrounded by an ice layer. The estimates of the ocean's thickness varied greatly, ranging from 3 miles to 264 miles.
There was actually one other scenario that fit the astronomers' model, but that called for a totally solid Titan that was more dense at the surface than at the center, which contradicts "all we know about other planets and satellites and planetary formation processes," Baland said in a release.
What's the Context:
NASA's Cassini spacecraft has been studying Saturn and its moons since it locked into orbit in 2004.
The first suggestion of an underground ocean of water and ammonia on Titan came in 2008: Comparing the locations of prominent features on Titan's surface with their past locations, astronomers discovered that many of them had shifted upwards of 19 miles in a geological blink of an eye, suggesting that the crust rested on a liquid.
Titan is already a leading candidate for possibly hosting extraterrestrial life; the idea that it may have liquid water may increase its chances even more. If it does sport an ocean of liquid water, Titan would join the ranks of other moons, including Saturn's Enceladus, that are thought to contain water.
Not So Fast: The astronomers admit that Titan's rotation could also be explained by a recent collision with a large asteroid or comet, which would have also upset the relationship between the moon's obliquity and its moment of inertia. The Future Holds: Now, the astronomers want to use this same method to look at whether Jupiter's larger satellites---Io, Europa, Ganymede, and Callisto---might also have subsurface water. Reference: Rose-Marie Baland, Tim Van Hoolst, Marie Yseboodt, Ozgur Karatekin. "Titan's Obliquity as evidence for a subsurface ocean?" arXiv:1104.2741v1Image: In this artist's representation of Titan, the cutaway reveals a light gray layer of ice, a blue layer of ocean underneath, and another light gray layer of ice under that, with a dark gray mixture of rock and ice in the interior. NASA/JPL
