Saturn Hasn’t Always Had Rings

Saturn Hasn’t Always Had Rings

One of the last acts of NASA’s Cassini spacecraft before its death plunge into Saturn’s hydrogen and helium atmosphere was to coast between the planet and its rings and let them tug it around, essentially acting as a gravity probe.

Precise measurements of Cassini’s final trajectory have now allowed scientists to make the first accurate estimate of the amount of material in the planet’s rings, weighing them based on the strength of their gravitational pull.

That estimate — about 40 percent of the mass of Saturn’s moon Mimas, which itself is 2,000 times smaller than Earth’s moon — tells them that the rings are relatively recent, having originated less than 100 million years ago and perhaps as recently as 10 million years ago.

Their young age puts to rest a long-running argument among planetary scientists. Some thought that the rings formed along with the planet 4.5 billion years ago 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.

The new mass estimate is based on a measurement of how much the flight path of Cassini was deflected by the gravity of the rings when the spacecraft flew between the planet and the rings on its final set of orbits in September 2017. Initially, however, the deflection did not match predictions based on models of the planet and rings. Only when the team accounted for powerful flowing winds deep in Saturn’s atmosphere — something impossible to observe from space — did the measurements make sense, allowing them to calculate the mass of the rings.

“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 Burkhard Militzer, a professor of earth and planetary science at the University of California, Berkeley, who models planetary interiors. “That turned out to be massive flows in the atmosphere at least 9,000 kilometers 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.”

They also calculated that the surface clouds at Saturn’s equator rotate 4 percent faster than the layer 9,000 kilometers (about 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, 33 minutes.

Saturn hasn’t always had rings

Saturn’s interior is composed of three primary layers: a deep, inner rocky core made mostly of heavy elements, enveloped by liquid metallic hydrogen and surrounded by a thick layer of gaseous molecular hydrogen (H 2) . (Background image courtesy of NASA/JPL-Caltech)

Did rings come from icy comet?

Earlier estimates of the mass of Saturn’s rings — between one-half and one-third the mass of Mimas — came from studying the density waves that travel around the rocky, icy rings. These waves are caused by the planet’s 62 satellites, including Mimas, which creates the so-called Cassini division between the two largest rings, A and B. Mimas is smooth and round, 246 kilometers in diameter. It has a big impact crater that makes it resemble the Death Star from the Star Wars movies.

Saturn hasn’t always had rings

During the Grand Finale, Cassini 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. (Background image courtesy of NASA/JPL-Caltech)

“People didn’t trust the wave measurements because there might be particles in the rings that are massive but are not participating in the waves,” Militzer said. “We always suspected there was some hidden mass that we could not see in the waves.”

Luckily, as Cassini approached the end of its life, NASA programmed it to perform 22 dives between the planet and the rings to probe Saturn’s gravity field. Earth-based radio telescopes measured the spacecraft’s velocity to within a fraction of a millimeter per