麻豆传媒

A colossal ring of debris found around Saturn is the largest in the solar system. The new ring could be the 鈥榮moking gun鈥� that explains the curious two-faced appearance of Saturn鈥檚 moon Iapetus, whose leading hemisphere is much darker than its trailing side.

Until now, the biggest known planetary rings in the solar system were Saturn鈥檚 E ring and faint, gossamer sheets of dust orbiting Jupiter. Saturn鈥檚 E ring, a diffuse disc of icy material fed by the moon Enceladus, extends from 3 to perhaps 20 times the radius of Saturn.

The newly discovered ring spans from 128 to 207 times the radius of Saturn 鈥� or farther 鈥� and is 2.4 million kilometres thick. It was found using NASA鈥檚 Spitzer Space Telescope, which revealed an infrared glow thought to come from sun-warmed dust in a tenuous ring.

The discovery was announced on Tuesday at a meeting of the American Astronomical Society鈥檚 Division for Planetary Sciences in Fajardo, Puerto Rico. 鈥淭his is a unique planetary ring system, because it鈥檚 the largest planetary ring in the solar system,鈥� team leader of the University of Virginia in Charlottesville told the meeting.

Although the ring is large, it is quite diffuse, making it difficult to detect using visible light. 鈥淚t鈥檚 so faint you could look right through it,鈥� team member of the University of Maryland in College Park told 麻豆传媒.

The source of the ring鈥檚 material seems to be Saturn鈥檚 far-flung moon Phoebe, which orbits the planet at an average distance of 215 times the radius of Saturn. When Phoebe is hit by wayward space rocks, the impacts could generate debris that fills the rings.

Moon to moon

The new ring is so far away from Saturn that it is not strongly affected by the gravity of the giant planet鈥檚 bulge, which pulls Saturn鈥檚 other, closer-in rings into a plane around the planet鈥檚 equator. As a result, the new ring shares the plane of Phoebe鈥檚 orbit, which is tilted by 27掳 with respect to the other rings.

Because the ring feels lower gravitational forces from Saturn, relatively weak pressure from sunlight can push its particles into new orbits. Computer simulations show that many of the ring particles should spread inwards to collide with Saturn鈥檚 two-toned moon Iapetus, Hamilton told meeting attendees.

Phoebe and its ring are orbiting in the opposite direction to Iapetus and Saturn鈥檚 other rings, so inward-moving material should hit Iapetus head-on. 鈥淚t鈥檚 going to strike Iapetus鈥檚 windshield like little bugs splattering on the windshield of a car,鈥� he said.

The team reported that over the course of the solar system鈥檚 history, enough material might have been created by collisions with Phoebe to coat Iapetus鈥檚 leading hemisphere with a metres-thick layer of ring material.

That may bolster previous suggestions that material from Phoebe is responsible for Iapetus鈥檚 dark side, although it to account for colour differences between the two objects. The dark material on Iapetus is reddish in visible light, while Phoebe鈥檚 surface appears gray.

Still, early reaction to the discovery is optimistic. 鈥淭he Phoebe ring here is the smoking gun for what has caused the colouration of Iapetus,鈥� planetary scientist of Cornell University, who was not part of the team, commented after the presentation.

Journal reference: (DOI: 10.1038/nature08515)