Saturn moon rolled to form hot south pole

Saturn's icy moon Enceladus may have rolled, moving a hot spot to its south pole, scientists propose.

The orbiting Cassini spacecraft has photographed active geysers at the moon's south pole spewing ice and water vapour, but researchers have been at a loss to explain why the excessive heat is only at the moon's southern pole.

Now planetary scientists are suggestingthe hot spot didn't start outin this position, and that its source is internal, not external.

The source of the warmth, a hot, low-density blob,called a diapir,might be in the moon's ice shell, its rocky core, or both places, the researchers said.

If the heating were coming from an external source namely Saturn the warmth would be evident at both poles.

They say a plume of warm ice in the moon's icy shell could have caused the moon to roll over by up to 30 degreesso the warm part became the new south pole.

Francis Nimmo, a professor of Earth sciences at the University of California,Santa Cruz and colleagues came up with the idea after studying the Cassini images and other data.

"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 in a release.

Robert Pappalardo, co-author and planetary scientist at NASA's Jet Propulsion Laboratory in Pasadena, Calif., used an analogy to explain the rollover.

"A spinning bowling ball will tend to roll over to put its holes the axis with the least mass vertically along the spin axis," Pappalardo said. "Similarly, Enceladus apparently rolled over to place the portion of the moon with the least mass along its vertical spin axis."

The team hopes more observations of the moon's impact craterswill help them to test their predictions of the reorientation scenarios.

Similar effects could be happening elsewhere in the solar system, such as onUranus's moon Miranda, they said.

The study appears in Thursday's issue of the journal Nature.