Moon-merge model could explain Saturnian system

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Simulation suggests that the moons of Saturn were once more like Jupiter's.

With widely ranging densities and locations, Saturn's six mid-size satellites are among the strangest in the outer Solar System. Measuring between 300 and 1,500 kilometres in diameter, the moons have varying characteristics: several are made almost entirely of frozen water; one, Enceladus, is rockier and geologically active; and some show evidence of submoons and rings. How did they get this way?

According to a model proposed by Erik Asphaug, a planetary scientist at the University of California, Santa Cruz, and his colleague Andreas Reufer of the University of Bern in Switzerland, Saturn and its satellites initially resembled a miniature version of the Jupiter system, with four large satellites similar in size to Jupiter's Galilean moons. Saturn's satellites then began to merge, eventually forming Titan, the planet's largest moon, says Asphaug, who presented the model on 17 October at the annual meeting of the American Astronomical Society’s Division for Planetary Sciences in Reno, Nevada.

The mid-size satellites would have formed from the scraps left over from building Titan, with the mergers perhaps accounting for Titan's surprisingly elongated orbit. The merging may have been triggered by an instability in the Solar System about 3.8 billion years ago, when theorists think that the orbits of Uranus and Neptune were migrating. Because of Jupiter’s bigger gravitational grip, its moons were relatively impervious to the disturbance, Asphaug speculates.

But Hal Levison, an astronomer who studies planetary dynamics at the Southwest Research Institute in Boulder, Colorado, says that the model is currently too simple to work. As the model is now configured, he says, the mid-size moons would inevitably accrete on to Titan rather than survive in isolation.

In the simulation shown above, the final merger of two of the Jupiter-like proto-moons is producing Titan, and flinging away water-rich material that later forms the icy mid-size satellites. The colours show the satellites' composition, with dark blue indicating an inner core of iron; pink and yellow representing a rocky mantle; and light blue denoting an outer coating of ice.

A giant of a moon appears before a giant of a planet undergoing seasonal changes in this natural color view of Titan and Saturn from NASA's Cassini spacecraft. Titan, Saturn's largest moon, measures 3,200 miles, or 5,150 kilometers, across and is larger than the planet Mercury. Cassini scientists have been watching the moon's south pole since a vortex appeared in its atmosphere in 2012. See PIA14919 and PIA14920 to learn more about this mass of swirling gas around the pole in the atmosphere of the moon. As the seasons have changed in the Saturnian system, and spring has come to the north and autumn to the south, the azure blue in the northern Saturnian hemisphere that greeted Cassini upon its arrival in 2004 is now fading. The southern hemisphere, in its approach to winter, is taking on a bluish hue. This change is likely due to the reduced intensity of ultraviolet light and the haze it produces in the hemisphere approaching winter, and the increasing intensity of ultraviolet light and haze production in the hemisphere approaching summer. (The presence of the ring shadow in the winter hemisphere enhances this effect.) The reduction of haze and the consequent clearing of the atmosphere makes for a bluish hue: the increased opportunity for direct scattering of sunlight by the molecules in the air makes the sky blue, as on Earth. The presence of methane, which generally absorbs in the red part of the spectrum, in a now clearer atmosphere also enhances the blue. This view looks toward the northern, sunlit side of the rings from just above the ring plane. This mosaic combines six images -- two each of red, green and blue spectral filters -- to create this natural color view. The images were obtained with the Cassini spacecraft wide-angle camera on May 6, 2012, at a distance of approximately 483,000 miles (778,000 kilometers) from Titan. Image scale is 29 miles (46 kilometers) per pixel on Titan. The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the mission for NASA's Science Mission Directorate, Washington, D.C. The Cassini orbiter and its two onboard cameras were designed, developed and assembled at JPL. The imaging operations center is based at the Space Science Institute in Boulder, Colo. For more information about the Cassini-Huygens mission visit http://saturn.jpl.nasa.gov. The Cassini imaging team homepage is at http://ciclops.org. Credit: NASA/JPL-Caltech/SSI

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Cowen, R. Moon-merge model could explain Saturnian system. Nature (2012) doi:10.1038/nature.2012.11611

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