Saturn's icy moon Enceladus may have "rolled over"-- dramatically reoriented relative to its axis of rotation, to put an area of low density at its south pole, astronomers reported on Wednesday.

The tiny moon recently attracted scientists when NASA's Cassini spacecraft observed icy jets and plumes indicating active geysers spewing from its relatively hot south polar region.

How the hot spot could end up at the south pole? Researchers at the University of California, Santa Cruz and NASA's Jet Propulsion Laboratory indicated that warm, low-density material rising to the surface from within Enceladus may have driven the moon to reorient.

Their findings were published in the June 1 issue of the journal Nature. A similar process may have happened on other small moons in the solar system, such as Uranus's satellite Miranda, they said.

The researchers calculated the effects of a low-density blob beneath the surface of Enceladus and showed that this could indeed cause the moon to roll over and put the low-density blob at the pole.

Rotating bodies, including planets and moons, are most stable if most of their mass is close to the equator. Any redistribution of mass within a rotating object causes the axis of spin to become unstable.

"Any redistribution of mass within the object can cause instability with respect to the axis of rotation. A reorientation will tend to position excess mass at the equator and areas of low density at the poles," said Francis Nimmo, lead author of the study at the University of California, Santa Cruz.

"The whole body rolls over, while the spin axis stays fixed," he added.

The rising blob, called a "diapir", may be within either the icy shell or the underlying rocky core of Enceladus. In either case, as the material heats up it expands and becomes less dense, then rises toward the surface.

This rising of warm, low-density material could also help explain the high heat and striking surface features, including the geysers and the so-called "tiger-stripe" surface feature on Enceladus suggesting fault lines caused by tectonic stress.

"The whole area is hotter than the rest of the moon, and the stripes are hotter than the surrounding surface, suggesting that there is a concentration of warm material below the surface," Nimmo said.

Internal heating of Enceladus probably results from its eccentric orbit around Saturn. The moon is squeezed and stretched by tidal forces as it orbits Saturn and that mechanical energy is transformed into heat energy in the moon's interior, according to the researchers.

"The reorientation generates large (approx 10 MPa) tectonic stress patterns that are compatible with the observed deformation of the south polar region," they wrote in the Nature paper.

This scenario also leads to other predictions. For example, the leading hemisphere of a moon as it travels through space should have more impact craters than the trailing hemisphere. But if the moon rolls over, the pattern of impact craters will also be reoriented. A low-density mass may also produce an observable anomaly in the moon's gravitational field, the researchers said.

"We predict that the distribution of impact craters on the surface will not show the usual leading hemisphere-trailing hemisphere asymmetry. A low-density diapir also yields a potentially observable negative gravity anomaly," they wrote in the paper.

Source: Xinhua