NASA scientists have discovered that one of Saturn's moons, Enceladus, may have a global ocean locked deep beneath its icy surface. Based on years of analysis of images taken by the unmanned Cassini probe, measurements of the moon's rotation indicate a slight wobble that's similar in nature to that which occurs when trying to spin a raw egg.

The discovery of the ocean is the latest in a string of findings that indicate that Saturn's sixth largest moon is geologically active. The existence of subsurface water, perhaps a large lake or sea on Enceladus, was suspected after Cassini photographed geysers of water vapor, icy particles, and simple organic molecules spraying from fractures at the south pole. However, shifts in the moon's gravity measured by Cassini indicate that the body of water might be global in extent.

The latest findings that seem to confirm the existence of the ocean come from independent data based on analysis of Enceladus's wobble or libration, which was measured by the Cassini probe after it entered the Saturnian system and began a series of flybys of the planet's moons in 2004. This wobble was measured by comparing images of features such as craters over a period of several years. By using hundreds of such images, it was possible to measure the moon's slight wobble.

According to NASA, this wobble is due in part to Enceladus's slightly irregular shape and its elliptical orbit around Saturn. This produces an uneven pull, which causes Enceladus to shift irregularly in much the same way as the Moon does as it orbits the Earth. The question was, what was causing it to wobble to the extent that it did, which was slight, yet pronounced for a solid object?

By running a series of computer models, NASA scientists were able to conclude that the most likely arrangement is that the intensity of the wobble is due to the icy crust and the rocky core being separated by a layer of liquid. This imparts a larger irregularity as the moon rotates – similar to the difference between spinning a hard boiled egg, which stands up, and a raw one, which immediately falls over as the yolk in the liquid interior throws it off balance.

"If the surface and core were rigidly connected, the core would provide so much dead weight the wobble would be far smaller than we observe it to be," says Matthew Tiscareno, a Cassini scientist at the SETI Institute, Mountain View, California. "This proves that there must be a global layer of liquid separating the surface from the core."

Scientists are currently trying to determine how it is that Enceladus is so active, rather than being frozen solid. Tidal heating by Saturn is one possibility. To help answer these questions, Cassini is scheduled to make a dive through one of the polar plumes at an altitude of 30 mi (49 km) on October 28.

The Cassini findings were published in journal Icarus.

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