One of the last acts of NASA's Cassini spacecraft before it died in the Saturn's hydrogen and helium atmosphere, was to be shrouded between the planet and its rings and let them drag it around, essentially acting as a gravity probe.
Accurate measurements of Cassini's last trajectory have now given researchers the opportunity to make the first accurate estimate of the amount of material in the planet's rings, weighing them out of the strength of their gravity.
It estimates – about 40 percent of Saturn's mass Moon Mimas, which is itself 2,000 times smaller than Earth's Month – tells them that the rings are relatively recent, dating from less than 100 million years ago and perhaps recently like 1
Their young age settles down on a long-standing argument among planetary scientists. Some believed that the rings formed with the planet 4.5 billion years ago from icy wastes that remained in orbit after the formation of the solar system. Others believed that the rings were very young, and that at one point Saturn had caught an object from the Kuiper Belt or a comet and gradually reduced it to encircling rubble.
The new mass ladder is based on a measurement of how much Cassini's flight path was deflected by the gravity of the rings, as the spacecraft flew between the planet and the rings on its last set of circuits in September 2017. Initially, the deflection did not match predictions based on models of the planet and rings. At first, when the team faced too much deep wind in the atmosphere of Saturn – something impossible to observe from space – the measurements made sense so they could calculate the mass of the rings.
"The first time I looked at the data I didn't believe in because I trusted our models and it took a while to sink in that there was some effect that changed the gravity field we hadn't considered ", said Burkhard Militzer, a professor of earth and planetary science at the University of California, Berkeley, modeled on planetary interiors. "It turned out to be massive currents in the atmosphere at least 9,000 kilometers deep around the equatorial area. We provisionally believed that these clouds were like clouds on Earth that are limited to a thin layer and contain almost no mass. But on Saturn is really massive. "
They also estimated that surface clouds on Saturn's equator rotate 4 percent faster than lay 9,000 kilometers deep. The deeper layer takes 9 minutes longer to rotate than the top shoots on the equator that goes around the planet once every 10 hours, 33 minutes.
"The discovery of deeply rotating layers is a surprising revelation of the inner structure of the planet," said Cassini project researcher Linda Spilker of NASA's Jet Propulsion Laboratory in Pasadena, California. "The question is, what makes the faster rotating part of the atmosphere go so deeply, and what does it tell us about Saturn's interior."
Militzer could also expect that the world's stone core should be between 15 and 18 times the Earth's mass, which corresponds to previous estimates.
The team, led by Luciano Iess at Sapienza University in Rome, Italy, reported their results today in the journal Science .
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Earlier estimates of Saturn's mass – between a half and a third of Mima's mass – came from studying the density waves traveling 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 are smooth and round, 246 kilometers in diameter. It has a large battle crater that resembles Death Star from the Star Wars films.
"People didn't trust the wave measurements, because there could be particles in the rings that are massive but not participating in the waves," said militzer. "We've always suspected that there was a hidden mass that we couldn't see in the waves."
Fortunately, as Cassini approached the end of his life, NASA programmed it to perform 22 dives between the planet and the rings to probe Saturn's gravity Mark. Earth-based radio telescopes measured the spacecraft's speed within a fraction of a millimeter per square meter. Second.
The new ring mass value is in previous estimates and allows researchers to determine their age.
These age calculations, led by Philip Nicholson of Cornell University and Iess, built on a connection that scientists had previously made between the mass of the rings and their age. Lower mass points at a younger age, because the rings are first made of ice and are bright, but over time become contaminated and darkened by interplanetary waste.
"These measurements were only possible because Cassini flew so close to the surface in his final hours," said militzer. "It was a classic, spectacular way to finish the job."