Cassini data shows Saturn’s rings likely a recent phenomenon

An artist’s impression of the Cassini spacecraft during its final orbits, repeatedly passing between Saturn’s rings and its cloud tops. Data collected during those orbits show the rings are a relatively recent phenomenon. Image: NASA/JPL-Caltech

More than a year after NASA’s Cassini spacecraft ended its hugely successful mission with a suicidal plunge into Saturn’s atmosphere, researchers have analysed data collected during its final orbits to determine the mass of the planet’s spectacular ring system and in so doing, its age. They also gained new insights into the planet’s interior structure and, in unrelated research, came up with a more definitive value for Saturn’s rotation period.

Some researchers theorised the rings formed along with the planet some 4.5 billion years ago while others thought they must be a relatively recent phenomenon, the result of a Kuiper Belt body that wandered too close and was ripped apart by Saturn’s gravity.

The Cassini data indicate the latter case is the most probable. The material making up the rings represents about 40 percent the mass of Saturn’s moon Mimas, a diminutive body 2,000 times smaller than Earth’s Moon, and that, in turn, indicates the rings likely formed between 10 million and 100 million years ago.

“Only by getting so close to Saturn in Cassini’s final orbits were we able to gather the measurements to make the new discoveries,” said Cassini radio science team member and lead author Luciano Iess, of Sapienza University of Rome. “And with this work, Cassini fulfils a fundamental goal of its mission: not only to determine the mass of the rings, but to use the information to refine models and determine the age of the rings.”

Running out of propellant near the end of its life, Cassini was directed to repeatedly fly between Saturn’s rings and the giant planet’s cloud tops before its final plunge into the atmosphere. Analysis of subtle changes in the spacecraft’s velocity due to the gravitational tugs of the ring system and the planet, accurate to fractions of a millimetre per second, allowed the researchers to come up with the mass of the rings.

Iess’ paper in the journal Science builds on earlier research indicating a younger age for the rings based on the brightness of the particles. Older material likely would have been contaminated and darkened over astronomically long periods.

In this Cassini photograph, the moons Mimas and much smaller Janus are seen above the plane of Saturn’s edge-on rings with Tethys visible below. Cassini data shows the rings contain about 40 percent the mass of Mimas. Image: NASA/JPL-Caltech/Space Science Institute

Cassini made another surprising discovery about the internal motions of Saturn’s atmosphere during the spacecraft’s final close-in orbits.

Researchers have long known Saturn’s equatorial atmosphere rotates faster than its inner layers and its core, with layers eventually rotating in lockstep the deeper one goes. The Cassini data show the layers start rotating in synchrony three times deeper than they do at Jupiter.

“The discovery of deeply rotating layers is a surprising revelation about the internal structure of the planet,” said Cassini Project Scientist Linda Spilker at NASA’s Jet Propulsion Laboratory. “The questions are what causes the more rapidly rotating part of the atmosphere to go so deep, and what does that tell us about Saturn’s interior?”

The measurements allowed researchers to determine the mass of Saturn’s core: 15 to 18 Earth masses.

In unrelated research, Christopher Mankovich, a graduate student at the University of California at Santa Cruz, used Cassini observations to come up with a more accurate value for the time it takes Saturn to complete one rotation: 10 hours 33 minutes and 38 seconds.  The result was based on detailed observations of oscillations in the rings that are tied to vibrations within the planet. The research shows Saturn rotates several minutes faster than estimates based on data from the Voyager spacecraft during flybys in the early 1980s.

“The researchers used waves in the rings to peer into Saturn’s interior, and out popped this long-sought, fundamental characteristic of the planet. And it’s a really solid result,” said Spilker. “The rings held the answer.”

Mankovich’s research was published in the Astrophysical Journal.