Saturn鈥檚 tiny moon Enceladus may have tilted over, stabilising itself as warm ice or rock welled up from beneath its surface, a new study suggests. The analysis may explain why there is a 鈥渉otspot鈥� that spews water vapour and ice situated exactly on its south pole, while there is nothing comparable on its north pole.
The researchers say their tilting theory could be tested by observations of the moon鈥檚 craters and gravity field by the Cassini probe. Similar tests could reveal signs of past or present geothermal activity on other moons, which could help determine whether they are potential havens for life.
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When NASA鈥檚 Cassini spacecraft swooped to within 173 kilometres of Enceladus in July 2005, it discovered
the moon鈥檚 south pole was 15掳C warmer than the rest of the surface and was spurting geysers of ice and water vapour.
The heating is probably caused by Saturn鈥檚 gravity 鈥� or tidal effect 鈥� which causes Enceladus to stretch and compress slightly as it moves around in its orbit. But the observation was surprising because not all theoretical models can account for the high degree of tidal heating apparently taking place.
Researchers were also perplexed at the location of the hotspot and geysers. 鈥淭hat was a puzzle as soon as we found all this activity 鈥� it鈥檚 absolutely perfectly centred around the south pole,鈥� says Cassini team member John Spencer of the Southwest Research Institute in Boulder, Colorado, US.
Gradual tilt
Researchers expect polar regions to be tidally heated more than equatorial regions, says Francis Nimmo of the University of California in Santa Cruz, US. That is because tidal stresses are spread over a larger area at the equator, diluting the heat they produce there. But any extra heating should warm up both poles, and the effect on Enceladus is only seen at the south pole.
Now, Nimmo and colleague Robert Pappalardo of the Jet Propulsion Laboratory in Pasadena, California, US, say they can explain the single hotspot. They say it could have originated in the southern hemisphere鈥檚 mid-latitudes before moving to the south pole as the moon gradually tilted.
The idea is that the warm hotspot material would be less dense than its surroundings and would well up from within the moon. This low-density 鈥渂ubble鈥� would put the moon off kilter, causing it to tilt over and regain its balance, says Nimmo.
鈥淎 spinning sphere is most stable when excess mass 鈥� denser regions 鈥� are at the equator, and less dense regions, such as the hotspot, are at the pole,鈥� he told New 杏吧原创.
Oceans apart
The researchers came to this conclusion after modelling the extent and density of the lighter material in Enceladus. But they say it is not clear whether the low-density bubble lies within Enceladus鈥檚 rocky core or its icy crust.
If it is a bubble of low-density rock within the moon鈥檚 rocky core, which spans nearly two-thirds of the 500-kilometre-wide world, that would imply Enceladus has no liquid ocean beneath its icy crust. 鈥淚f you want the rocky layer and the icy layer to tilt over together, then they have to be in direct contact 鈥� and that means you can鈥檛 have something like an ocean between them,鈥� he says.
But if the low density material lies within the icy crust, the surface alone could have spun around if there were a submerged liquid ocean beneath it, he says. 鈥淧eople would be very excited if there were an ocean down there because it has lots of implications for life.鈥�
Tilt test
Nimmo says Cassini could reveal whether Enceladus has indeed tilted over time. The moon always keeps the same side facing Saturn. So if it had never tilted over, its 鈥渓eading鈥� side, which faces the direction of its movement, should be pockmarked with many craters, while its 鈥渢railing鈥� side should have markedly less. Cassini would reveal a tilt if this simple picture were not borne out, he says.
But Paul Schenk of the Lunar and Planetary Institute in Houston, Texas, US, disagrees. He says the moon鈥檚 own geological activity will have erased any such imprint. Lavas, geysers, and plate tectonics have all 鈥渞esurfaced鈥� the moon, leaving few craters behind, he says: 鈥淵ou won鈥檛 find any record of reorientation in the cratering.鈥�
Another way to look for evidence of a tilt is to look for the effect of a low-density bubble in the moon鈥檚 local gravity field. 鈥淐assini would feel less acceleration than expected鈥� over the south pole, says Nimmo.
Spencer says such a gravitational clue could be detected if Cassini were to fly within 200 km of the south pole 鈥� a possibility that may arise after the spacecraft鈥檚 main mission is finished in mid-2008.
The same tests could also reveal whether other moons in the solar system have tilted in the past, says Nimmo. 鈥淚f they鈥檝e rolled over, that suggests the interiors at some time were quite geologically active and warm,鈥� he told New 杏吧原创. 鈥淭hat makes the presence of oceans more likely, which increases the potential for habitability.鈥�
Journal reference: Nature (vol 441, p 614)
