A striking similarity between the moons of Jupiter, Saturn and Uranus could be explained by a new model of how they formed. The model could also explain why some of the moons have ice, something previous models could not do.
Jupiter, Saturn and Uranus each have several dozen moons. But while the planets vary in size, each collection of moons contains the equivalent of 0.01% of its host planet's mass.
"This has been a puzzle, since each of the planet's individual growth histories would presumably have been quite different," says Robin Canup, at the Southwest Research Institute in Boulder, Colorado, US.
Canup and colleague William Ward have now figured out a way to explain this curiosity. For the first time, the computer simulations produced planet-moon systems consistent with those seen around Jupiter, Saturn and Uranus in terms of the number, masses and spacing of the large moons.
Moons are thought to form near the end of a planet's formation process, and starts when gas and dust from a disc around the Sun flows toward the planet.
An interaction between the gravity of each nascent moon and the gas disc causes the moon to spiral slowly in toward the planet. In the model, the bigger the moon is, the bigger the effect and the faster it approaches the planet. This means that moons above a certain mass swirl into the planet and are destroyed, leading to the limit seen.
The model also solves another problem. Previous models had moons forming too quickly and too violently, making them too hot to contain the ice observed today. In the new model, the moons take about a million years to form.
The solar system's fourth gas giant, Neptune, is an exception. Its large moon, Triton, was captured by Neptune's gravity after the formation process. But Canup and Ward note that Triton's mass – about 0.02% that of Neptune – is nevertheless similar to the 0.01% limit.
Solid planets like Earth and Pluto have relatively large satellites with up to 10% of their mass. These were probably formed by cataclysmic impacts that threw up material into orbit. Since it is the gas that causes the moons of giant planets to spiral in to their death, there is no equivalent upper size limit for the moons of rocky planets.
Whilst the model is an improvement, Jonathan Lunine, a planetary scientist at the University of Arizona, US, notes that there are some details it does not explain. For example, why the largest moons – Titan, Ganymede and Callisto – came to be that size. "The answer, I think, lies in the behaviour of water during the final stages of accretion of these largest moons," he says.
Journal reference: Nature (vol 441, p 834)
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