Saturn’s Rings May Have Come From Remnants of Large Kuiper Belt Objects, Scientists Say

A team of scientists has come up with a new study about where Saturn's rings originated. The researchers based the new model on results of computer simulations, which is also applicable to other planets with rings and explains why Saturn's rings are different from Uranus.

The study, led by Hyodo Ryuki of Kobe University Graduate School of Science, focused on a period called the Late Heavy Bombardment. This period is said to have occurred 4 billion years ago, a time when giant planets underwent orbital migration.

Scientists believe that during this period, Pluto-sized bodies existed in orbit in the Kuiper Belt, a region in the outer solar system just beyond Neptune. Because of gravitational interactions between giant planets, the orbits of these objects became unstable and several of them found their way in the solar system and collided with existing planets.

According to the researchers, Saturn, Uranus and Neptune had encountered these large objects several times during their passage in the solar system. The giant planets' tidal force could have destroyed the objects.

The group also conducted computer simulations to determine whether these Kuiper Belt objects had been disrupted by the giant planets' tidal force. Results varied in terms of the objects' rotation and their distance to the planet. But they discovered that in many cases, fragments comprising about 0.1 to 10 percent of the initial mass of the passing objects had been captured into orbits around the planets.

The researchers also found that the combined mass of these captured fragments is sufficient to explain the mass of the current rings around Saturn and Uranus.

Using supercomputers at the National Astronomical Observatory of Japan, the researchers also simulated the long-term evolution of the captured fragments. They found that, with a size of several kilometers, they are expected to undergo high-speed collisions multiple times and gradually shatter into smaller pieces, and the collisions between fragments circularized their orbits to form rings that are seen today.

According to the researchers, the new model also explains the compositional difference between the icy rings of Saturn and rocky rings of Uranus. Uranus (and Neptune), has higher density, which means that if the Kuiper Belt objects - assuming they are composed of a rocky core and an icy mantle -- passed within close vicinity of the planet, both the rocky core and the icy mantle will be destroyed and captured.

But Saturn, which has less density, will only shatter the icy mantle, as evidenced by the 95 percent icy particles on its rings.