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Titan’s Ocean?

Posted: March 25, 2008

NASA’s Cassini spacecraft has discovered evidence that points to the existence of an underground ocean of water and ammonia on Saturn’s moon Titan, giving us a window into the satellite’s interior for the first time.

With its organic dunes, lakes, channels and mountains, Titan has one of the most varied, active and Earth-like surfaces in the Solar System, and now radar data suggests that beneath a thin shell of 100 kilometres lies an internal ocean of liquid water mixed with ammonia. Data collected over several years show that Titan’s rotational period (the time it takes to complete one spin about its axis) is changing, leading to an apparent shift in longitude of various surface features. From radar imaging, used to reveal surface features beneath Titan’s otherwise impenetrable dense and methane-rich atmosphere, the location of fifty unique landmarks, such as lakes, canyons and mountains were recorded, but in later flybys these prominent surface features appeared to be shifted from their expected positions by up to 30 kilometres. “In reality the features haven’t moved at all,” explains Dr Ralph Lorenz, Cassini radar scientist at the Johns Hopkins Applied Physics Laboratory, “It is only the coordinate system that is different, but relative to the old coordinates, some of the features appear to have moved.”

This false-colour view of Titan is a composite of images captured by Cassini's infrared camera, which has penetrated some of Titan's clouds to reveal light and dark regions across the surface. New research suggests that an ocean may exist below the surface.  Image: NASA.

Since Titan has an incredibly thick, super-rotating atmosphere, 1.5 times more dense than the Earth, it is possible that powerful winds are affecting the rotation of the satellite, causing it to speed up at some points during its rotation and slowing down at other times. This scenario relies on an internal ocean that decouples the crust from the core, the only current explanation for the systematic displacement of surface features. "Further study of Titan's rotation will let us understand the watery interior better, and because the spin of the crust and the winds in the atmosphere are linked, we might see seasonal variation in the spin in the next few years,” says Lorenz. The situation is similar to what is believed to be the case on Europa, Callisto and possibly Ganymede, “We expected, and now have some evidence of, an internal ocean,” adds Lorenz. “We have also seen some evidence of cryovolcanic features, although it’s too early to say whether those represent local melting, or instead represent places where the ocean has been able to ascend through cracks in the surface.”

The Cassini radar team hope that future measurements of Titan’s gravity field may confirm the ocean by showing how the crust can be flexed by tidal forces, and it is possible magnetic measurements might detect an induced field of a conductive interior. Although the precise details of the subsurface ocean are still to be confirmed, one robust conclusion of the new findings is that Titan’s spin period can be expected to vary substantially over the next few years, peaking during Cassini’s proposed extended mission (2008-2010), allowing scientists to delve deeper into the rotation effects driven by atmospheric dynamics that are more profound than those observed on any terrestrial planet. A follow-on lander mission to Titan would be an obvious next step, which would include a seismometer that could detect an ocean directly, and would also satisfy the astrobiological appeal for the search for life in a watery ocean on an organic-rich icy satellite, a unique environment in our Solar System that is thought to represent conditions similar to those which prevailed on the early, pre-biotic Earth.

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