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Boulder boffins reveal secret of gas giants' moons

Neptune still a puzzle, though

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A computer model developed by scientists at the Southwest Research Institute in Boulder, Colorado, sheds new light on the formation of many of the moons in the solar system.

Up until now, according to the research team, it has been known that collectively, the moons of Jupiter, Saturn and Uranus amount to a similar proportion of their parent planet's mass - around 0.01 per cent.

What has not been clear is why that should be, or why the moons are the sizes that they are, or distributed in the orbits that they inhabit. Existing models also fail to account for why some of the moons contain ice, as they suggest moons form too quickly (and therefore at too high a temperature) to acquire a watery component.

According to New Scientist the model developed by Robin Canup and her colleague William Ward produces gas giants with moon sizes and distributions consistent with real-world examples.

The model also explains the apparent upper limit on the mass of the moons.

In the final stages of a gas planet's formation, it is thought to accrete both gases and solids from a disk of such debris orbiting the sun. This then forms a disc around the planet, in the equatorial plane, which in turn give rise to growing satellites.

According to Canup and Ward, as these satellites grow, they induce spiral waves in the gas disk. The interactions between these waves and the proto-moons cause the orbits to contract. Heavier moons' orbits contract more quickly, and collapse into the planet itself, while smaller moons stabilise in their orbits.

The researchers also suggest many cycles of moon formation and loss. The moons we see today are merely the last ones standing, according to Physorg.com.

Uranus, however, remains something of a mystery. The researchers believe that the key to understanding its arrangement lies in working out how it came to be lying on its side. Only then will they know for sure whether this new model will apply. ®

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