In the final phase of NASA’s Cassini mission, the spacecraft dived between Saturn and its innermost ring, at altitudes 1,616-2,423 miles (2,600-3,900 km) above the cloud tops. During six of these crossings, a radio link with Earth was monitored to make the first accurate estimate of the amount of material in the planet’s rings. The estimate — about 40% of the mass of Saturn’s moon Mimas, which itself is 2,000 times smaller than Earth’s Moon — tells the Cassini team that the gas giant’s rings are relatively recent, having originated less than 100 million years ago and perhaps as recently as 10 million years ago. The results appear in the journal Science.
NASA’s Cassini spacecraft in orbit around Saturn. Image credit: NASA / JPL-Caltech.
Saturn formed 4.5 billion years ago, in the early years of our Solar System.
Some planetary researchers thought that its ring system formed along with the planet from icy debris remaining in orbit after the formation of the Solar System. Others thought the rings were very young and that Saturn had, at some point, captured an object from the Kuiper Belt or a comet and gradually reduced it to orbiting rubble.
To figure out the age of the rings, scientists needed to measure something else: the mass of the rings, or how much material they hold.
They had the remote-sensing measurements from NASA’s Cassini and both of Voyager spacecraft.
Then came Cassini’s unprecedented, up-close data from its ‘Grand Finale’ phase. As the spacecraft was running out of fuel, it performed 22 dives between the planet and the rings.
The dives allowed the orbiter to act as a probe, falling into Saturn’s gravity field, where it could feel the tug of the planet and the rings.
Radio signals sent to Cassini from the antennas of NASA’s Deep Space Network and the European Space Agency relayed the spacecraft’s velocity and acceleration.
Once the researchers knew how much gravity was pulling on the spacecraft causing it to accelerate — down to a fraction of a millimeter per second — they could determine how massive the planet is and how massive the rings are.
“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 Dr. Luciano Iess, a scientist at Sapienza University of Rome.
“And with this work, Cassini fulfills 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.”
During the Grand Finale, NASA’s Cassini spacecraft passed between the inner edge of Saturn’s D-ring and the cloud top. This orbital configuration allowed the disentanglement of the tiny acceleration of the rings from the large acceleration due to Saturn. The two forces pull the spacecraft in opposite directions. Image credit: NASA / JPL-Caltech / Iess et al.
From Cassini’s super-close vantage point, immersed in Saturn’s gravity field, the spacecraft relayed measurements that led the team to another surprising discovery.
“The first time I looked at the data I didn’t believe it, because I trusted our models and it took a while to sink in that there was some effect that changed the gravity field that we had not considered,” said University of California, Berkeley’s Professor Burkhard Militzer.
“That turned out to be massive flows in the atmosphere at least 6,000 miles (9,000 km) deep around the equatorial region.”
“We thought preliminarily that these clouds were like clouds on Earth, which are confined to a thin layer and contain almost no mass. But on Saturn they are really massive.”
The team calculated that the surface clouds at Saturn’s equator rotate 4% faster than the layer 6,000 miles deep.
That deeper layer takes 9 minutes longer to rotate than do the cloud tops at the equator, which go around the planet once every 10 hours and 33 minutes.
“The discovery of deeply rotating layers is a surprising revelation about the internal structure of the planet,” said Cassini project scientist Dr. Linda Spilker, a researcher at NASA’s Jet Propulsion Laboratory.
“The question is 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 scientists also were able to calculate that the rocky core of the planet must be between 15 and 18 times the mass of Earth, which is similar to earlier estimates.
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L. Iess et al. Measurement and implications of Saturn’s gravity field and ring mass. Science, published online January 17, 2019; doi: 10.1126/science.aat2965
