Cassini eyes Saturn flashes, discovers radiation belt


The U.S.-European Cassini spacecraft discovered a new radiation belt around Saturn and has found that lightning in the ringed planet’s atmosphere is occurring much differently now than when NASA’s Voyagers flew by in the early 1980s, scientists said.

The discoveries announced Thursday are part of an early wave of information about the Saturn system from Cassini, which arrived June 30 on a US$3.3 billion exploration expected to last four years.

Managed by Jet Propulsion Laboratory, the Cassini mission is a joint project of NASA, the European Space Agency and the Italian Space Agency.

“This is exactly the point of doing a mission like this,” said Bill Kurth of the University of Iowa, the deputy principal investigator for Cassini’s radio and plasma wave instrument, which detects “cracks” and “pops” from lightning like those that can be heard on an AM radio during a thunderstorm on Earth.

“Cassini now has evidence for changes in the thunderstorms that occur on Saturn over more than 20 years since we first started making measurements of these,” Kurth said Thursday.

One difference is in the patterns in which lightning is occurring.

“Currently Cassini is seeing lighting on a very sporadic basis. It comes and goes,” Kurth said. “Some days we see no lightning at all; other days we see evidence perhaps of more than one storm. Back in the early 1980s on Voyager such storms were detected extremely regularly.”

The other major difference is that the thunderstorms observed by Cassini are taking a longer time to rotate around the giant gas planet, about 10 hours and 45 minutes, compared to the 10 hours and five minutes in the 1980s.

Voyager scientists associated those storms with “super rotational clouds” near the equator, where a high-velocity windstream can move them around Saturn faster than the planet itself rotates.

“The fact that Cassini is seeing a longer period suggests that the storms are coming from a higher latitude” where the winds don’t move as fast, Kurth said.

Scientists suspect that cause of the difference in the lightning patterns may be due to the difference in the way shadows from Saturn’s rings are falling on the planet now compared to when the observations were made in early ‘80s.

Then, the sun was nearly on the same plane as the rings, and their shadow fell along a narrow band close to the equator. Now, the sun is illuminating the southern hemisphere and the rings’ shadows are falling broadly across the northern hemisphere.

“They’re no longer concentrated in one place and they’re spread out over a large part of the planet,” Kurth said.

Scientists’ hunch is that during the Voyager era the very deep ring shadow being very close to the region where the sun shined most directly on the upper atmosphere caused extraordinary convection to drive the storms.

“Currently we don’t have that situation,” Kurth said. “The ring shadows are in the north and the direct sunlight is in the southern hemisphere so you don’t have the potential for that temperature contrast that you might have had at Voyager.” The new radiation belt was discovered just above Saturn’s cloud tops by a Cassini instrument, showing that the belts extend much closer to the planet than had been known.

Radiation belts are invisible, symmetrical, doughnut-shaped regions in space where high energy ions and electrons are confined by a planet’s magnetic field.

Over the long term, radiation belts have an important role in the evolution of a planet’s atmosphere and any moons and orbiting gas and dust, said Donald G. Mitchell, a scientist for the magnetospheric imaging instrument, from Johns Hopkins University, in Laurel, Maryland.

Temperature and chemistry of a planet’s upper atmosphere are also affected by radiation belts, he said.

Kevin Baines, a scientist on Cassini’s visual and infrared mapping spectrometer, also released new observations of Saturn’s planet-sized moon Titan, documenting the extreme thickness of its atmosphere, which is greater than Earth’s.

Cassini will launch its Huygens probe toward Titan in December. The probe will plunge into the moon’s atmosphere in January, transmitting back images and science data.