A new take on how Saturn’s rings formed
The striking rings of Saturn are an anomaly in the solar system. While the rings of other planets are dim and composed of nearly equal parts rock and ice, those of Saturn are much brighter and are more than 95% ice, a phenomenon that has defied explanation — until now.
Using sophisticated new computer calculations, planetary scientist Robin M. Canup of the Southwest Research Institute in Boulder, Colo., has shown how the rings could have been formed by the violent destruction of a moon very similar to Saturn’s largest remaining moon, Titan.
Gravitational forces from Saturn could have stripped the moon’s icy mantle away, scattering the water to form not only the rings but many of the dozens of smaller moons now orbiting the planet, and sucked its rocky core into the body of the planet, Canup postulated in a report published online by the journal Nature.
Canup’s idea “offers an attractive solution to the problem that answers several questions at once,” wrote Aurelien Crida of the University of Nice-Sophia Antipolis in France and Sebastien Charnoz of Paris Diderot University in an editorial accompanying the paper. The model, they said, “offers, for the first time, a convincing starting point for a consistent theory of the origin of Saturn’s rings and satellites.”
Planetary scientist Matthew M. Hedman of Cornell University, who was not involved in the research, added, “This makes sense, given what we know about the rings and the history of the outer solar system. It’s a fairly complete story of what might have happened.”
Researchers originally thought that Saturn’s rings were most likely produced by the impact of an asteroid on a moon, which is probably what produced Jupiter’s rings. But had that been the case, the rings would have a much higher content of rocky material.
A variety of evidence indicates that Saturn once had several moons about the size of Titan, which is roughly 50% larger than Earth’s moon. Slowly, one by one, those satellites were drawn into Saturn’s gravity well, crashing into the planet.
Canup’s numerical simulation suggests that ice was stripped from the last moon destroyed, producing an ice ring that was much more massive than the current ring. Over time, collisions in the ring caused it to spread radially and decrease in mass. Inwardly spreading material was lost to the planet, while material spreading outward accumulated into icy moons with masses consistent with the inner moons seen today, leaving behind the rings as they exist now.
Planetary scientist Joseph A. Burns of Cornell noted that the model used data from NASA’s Cassini mission, “and that’s why we are flying spacecraft.”
In its extended mission, the Cassini spacecraft, which has been orbiting Saturn, will move inside the main ring belt, where it will be able to take better measurements of the ring’s density and thus of its mass. Knowing the precise mass of the rings will allow astronomers to say with more surety how they were formed.
