Cassini Finds Radiation Between Saturn and Innermost Ring

NASA’s Cassini spacecraft has found a new radiation belt around Saturn, inside the orbit of the rings where scientists thought such a belt couldn’t exist.

Data from the craft released Thursday also showed that the prevalence of storms on the planet’s surface was lower than it was when Saturn was visited by Voyager 2 two decades ago, possibly because of changes in the shadow cast by the planet’s rings.

The discovery of the radiation belt was “a huge surprise,” said Donald G. Mitchell of Johns Hopkins University, who is in charge of the on-board magnetospheric imaging device that identified the belt.

Previous Saturn missions had shown the presence of strong radiation belts -- essentially ring-shaped concentrations of ionized particles -- outside the orbit of the rings. Researchers had thought, however, that the rings would absorb any particles that tried to get closer to the planet.

As Cassini swooped between the rings and Saturn’s cloud tops when it entered orbit around Saturn on June 30, the instrument revealed the presence of a belt stretching from near the surface almost to the inner edge of the ring closest to the planet.

“This new radiation belt had eluded detection by any of the spacecraft that had previously visited Saturn,” Mitchell said. “With its discovery, we have seen something that we did not expect, that radiation belt particles can ‘hop’ over obstructions like Saturn’s rings without being absorbed by them in the process.”

Mitchell speculated that the radiation belt was produced by electrically neutral particles that originated beyond the orbit of the rings and were dispersed in all directions. Some make their way through the rings and emerge close to Saturn, where they are stripped of an electron by the planet’s magnetic field, forming the radiation belt.

This type of isolated radiation belt could only be produced in the presence of rings, which keep the individual belts separated, Mitchell said.

The storms and lightning bolts on Saturn are detected by Cassini’s radio and plasma wave science instrument, producing “the same crackle and pop one hears when listening to an AM radio broadcast during a thunderstorm,” said the University of Iowa’s Bill Kurth, who is in charge of the instrument.

“These storms are dramatically different than those observed 20 years ago” by Voyager, he said.

The earlier missions saw storms in a pattern that was highly regular. Those measurements showed that storms would sweep around the planet in 10 hours and 5 minutes, indicating that they were in the high-speed winds circulating at or near Saturn’s equator.

But Cassini has been observing the storms “on a sporadic basis,” he said. “Some days there are none, and some days there are more than one.” Those that are observed for longer periods appear to take 10 hours and 45 minutes, indicating that they are at a higher latitude.

Kurth attributes the difference in storm patterns to the angle at which the planet’s rings are inclined to the sun. In the early 1980s, sunlight was striking the rings edge-on, producing a narrow, sharply defined shadow. Kurth thinks the temperature difference between the areas in shadow and sunlight could have produced severe thunderstorms.

Now, however, the rings are tilted more in relation to the sun and the shadow is much broader and blocks less sunlight at any location. That means a smaller temperature differential and a lower chance of producing storms.