“Ring Rain” Pelted Cassini Spacecraft During Its Final Orbits

Artist’s view of Cassini flying between Saturn’s innermost rings and atmosphere during last year’s “grand finale.” NASA/JPL-Caltech

NASA’s Cassini spacecraft flamed out in Saturn’s atmosphere a year ago, but the data it collected during its final orbits led to an amazing discovery — ring rain! As the probe dove between the rings and upper atmosphere, it was bathed in a downpour of dust. Hsiang-Wen Hsu (Colorado University-Boulder) and colleagues report that they successfully collected more than 2,700 pieces of charged bits of dust. Based on that amount, they estimate that about 2,205 pounds (one metric ton) leave the rings and enter Saturn’s atmosphere every second.

“Our measurements show what exactly these materials are, how they are distributed and how much dust is coming into Saturn,” said Hsu, lead author of the paper. The findings were made using data from Cassini’s Cosmic Dust Analyzer and Radio Plasma Wave Science instruments. Never before has dust from Saturn’s rings been directly examined, a real coup for the mission.

Cassini (orbit shown in blue) was dusted by “ring rain” as it passed through the narrow gap between the planet and the innermost D-ring 22 more times before ending its mission. The dust was encountered during eight of the final orbits. NASA

Hsu gathered the data with Cassini’s Cosmic Dust Analyzer and Radio and Plasma Wave Science instruments, which beamed the information back to Earth shortly before the probe was intentionally burned up in Saturn’s atmosphere. You might recall that NASA went for broke in the final months before the mission’s end, commanding the spacecraft to zip between the planet’s rings and upper atmosphere at speeds of 75,000 mph.

Snagging those 2,700 bits wasn’t easy. The probe had to get close enough to the innermost ring but not so close as to be sandblasted and shredded from impacts by larger particles. In a classic eye-of-the-needle scenario, Cassini kept to a 1,200-mile gap between the ring and upper atmosphere … or else.

Five images of Saturn’s rings taken by Cassini show clouds of material ejected from impacts of small objects into the rings. Clockwise from top left are two views of one cloud in the A ring, taken 24.5 hours apart; a cloud in the C ring, one in the B ring, and another in the C ring. The images show how some of the dust making its way from the rings to the planet’s atmosphere originates from space rock (meteoroid) impacts. NASA/JPL-Caltech/Space Science Institute/Cornell

Rather than crashing into the atmosphere directly after pulled in by Saturn powerful gravity, the particles most likely got caught up in the planet’s magnetic field, where they bounced up and down before slamming into the air. Their composition was most interesting. Since Saturn’s rings are primarily ice, most of the particles contained but water ice, but there were also lots tiny minerals made of silicates. Silicates are compounds of iron, magnesium and oxygen common in meteorites as well as in interplanetary dust, material from asteroid collisions or left by passing comes. That’s not all. The Cosmic Dust Analyzer also sniffed out organic compounds, too.

The material appears to originate when high speed interplanetary dust particles slam into the icy chunks that compose the rings, ejecting release fine clouds of dust and water ice. According to the research paper, the biggest contribution comes from Saturn’s B and C rings (both visible in a modest-sized telescope) with smaller amounts coming from the A and D rings. These tiny bits find their way, with help from Saturn’s magnetic field, to the atmosphere. Once there, they serve as “nuclei” for the formation of clouds.

Pure ice is tough to maintain in a dirty solar system, where meteorite and comet dust are common. Hsu says that if we can figure out the exact type of silicates that coat the rings, we may be able tell whether they’ve been around for billions of years or are a more recent feature.

More ring discoveries are in the offing as there’s still plenty of data remaining to analyze from the Cosmic Dust Analyzer.