One of the last acts of NASA's Cassini spacecraft, before its death plunged into the atmosphere of hydrogen and helium of Saturn, was to move between the planet and its rings and let them pull it, acting essentially as a gravitational probe .
Accurate measurements of Cassini's final trajectory have allowed scientists to make the first accurate estimate of the amount of material in the rings of the planet, weighing them based on the force of their gravitational pull.
That estimate – about 40 percent of Saturn's moon mass, Mimas, which is 2,000 times smaller than Earth's – says the rings are relatively recent, having originated less than 100 million years ago and perhaps 10 million. years ago.
His tender age puts to rest a long discussion among planetary scientists. Some thought the rings formed along with the planet 4.5 billion years ago from icy debris that remain in orbit after the formation of the solar system. Others thought the rings were very new and that Saturn had at some point captured an object from the Kuiper belt or a comet and gradually reduced it to a debris in orbit.
The new mass estimate is based on a measure of how far Cassini's flight path was diverted by the gravity of the rings when the spacecraft flew between the planet and the rings in its final set of orbits in September 2017. Initially, however , deflection does not coincide with predictions based on planet models and rings. Only when the team represented very deep winds in Saturn's atmosphere – something impossible to observe from space – the measurements made sense, allowing them to calculate the mass of the rings.
"The first time I looked at the data, I did not believe it because I relied on our models and it took a while to realize that there was some effect that changed the gravity field that we had not considered," said Burkhard Militzer. professor of earth and planet sciences at the University of California, Berkeley, who models planetary interiors. "This has resulted in massive flows into the atmosphere of at least 9,000 kilometers of depth around the equatorial region." We preliminarily thought that these clouds were like clouds on Earth, which are confined to a thin layer and almost contain no mass. really huge ".
They also calculated that the surface clouds at Saturn's equator spin 4% faster than the 6,000-mile (6,000-mile) deep layer. This deeper layer takes 9 more minutes to spin than the clouds at the equator, which circle the planet once every 10 hours, 33 minutes.
"The discovery of deeply rotating layers is a surprising revelation of the planet's internal structure," said Linda Spilker, Cassini project scientist at NASA's Jet Propulsion Laboratory in Pasadena, Calif. "The question is what makes the fastest part of the atmosphere so deep and what it tells us about the interior of Saturn."
Militzer was also able to calculate that the rocky core of the planet should be between 15 and 18 times the Earth's mass, which is similar to previous estimates.
The team, led by Luciano Iess of the Sapienza University in Rome, Italy, reported their findings today in the journal Science.
Did the rings come from the icy comet?
Earlier estimates of the mass of Saturn's rings – between one-half and one-third of the mass of Mimas – came from studying the density waves that travel around the rocky and 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 large impact crater that makes it look like the Death Star of the Star Wars movies.
"People do not rely on wave measurements because there may be particles in the rings that are huge, but that are not participating in the waves," Militzer said. "We always suspected there was some hidden mass we could not see in the waves."
Fortunately, as Cassini approached the end of its life, NASA programmed it to perform 22 dives between the planet and the rings to probe the field of gravity of Saturn. Earth-based radiosteps measured the spacecraft's speed by a fraction of a millimeter per second.
The new mass value of the ring is in the range of previous estimates and allows researchers to determine their age.
These age calculations, led by Philip Nicholson of Cornell University and Iess, built a connection scientists had made earlier between the mass of the rings and their age. Smaller mass points for a younger age, because the rings are initially made of ice and are shiny, but over time become contaminated and darkened by interplanetary debris.
"These measurements were only possible because Cassini flew so close to the surface in its final hours," Militzer said. "It was a classic and spectacular way to finish the mission."