An article just published in the journal “Icarus” describes a research that used data collected by the Cassini space probe to determine that Enceladus, one of Saturn’s moons, has a global underground ocean. That there was an ocean beneath the icy surface of Enceladus was a well-established fact from previous research but it remained to be seen whether the water was liquid all over beneath the surface or only in some warm enough areas.

A study carried out last year by analyzing the gravitational data collected by the Cassini space probe allowed to establish the existence of an ocean beneath the south polar region of Enceladus. This new research used an approach someway similar but with a bigger scope. The gravitational data collected over many steps over the south polar region supported the possibility that the ocean was global, now new results derived from independent evidence based on Cassini’s images confirm that hypothesis.

The researchers examined the images of Enceladus collected by the Cassini spacecraft in more than seven years. They mapped with great care the position of the various features – which are especially craters – present in hundreds of images. This analysis made it possible to measure the changes in this moon’s rotation with extreme accuracy. This allowed to find a very small wobble, called libration, due in part to the fact that Enceladus isn’t perfectly spherical and that its speed in various portions of the orbit change.

If the surface and the core of Enceladus were rigidly connected, the core would provide so much dead weight that its wobble would be much smaller than what was observed. The consequence is that there must be a global ocean separating the surface from the core. This finding is surprising because it means that this moon is heated more than expected. Either the tidal forces generated by the Saturn’s gravity are greater than expected or there’s some other force involved.

One of the possibilities is that the core of Enceladus doesn’t consist of a single block but of more blocks that haven’t merged because there isn’t enough gravity. The consequence would be its increased friction generated by tidal forces and therefore more heat to maintain a global layer of liquid water.

This and other hypotheses will be examined to try to explain the conditions on Enceladus, which are the big surprise of this research. A global ocean means a greater chance that there are life forms, especially considering the presence of hydrothermal vents, another very interesting discovery announced a few months ago. The Cassini space probe is already scheduled to make a close flyby of Enceladus on October 28, 2015: it will be an opportunity to gather more data.

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