One of the last feats of NASA’s Cassini spacecraft before its death in the hydrogen and helium atmosphere of the planet Saturn was to continue collecting data until its last breath.
Now, Cassini’s final trajectory measurements have allowed scientists to make the first accurate estimate of the amount of materials present in the planet’s rings, weighing them based on the strength of their gravitational pull.
That estimate, about 40% of the mass of the moon Mimas, which itself is 2,000 times smaller than Earth’s moon, states that the rings are relatively recent, originating less than 100 million years ago and perhaps only 10 million years ago, which is to say that Saturn did not always have those magnificent rings.
That’s how it is. This young age ends a long discussion among planetary scientists. Some thought that the rings had formed along with the planet 4.5 billion years ago from ice debris that remains in orbit after the formation of the solar system. Others believed that the rings were very young and that Saturn, at some point, captured a Kuiper belt object or comet and gradually reduced it to orbiting debris.
The new mass estimate is based on a measure of how much Cassini’s flight path was deviated by the gravity of the rings when the spacecraft flew between the planet and the rings on its final flight in September 2017. Initially, however , the deflection did not match model-based predictions of the planet and rings. Only when the team took into account the very deep winds 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 understand that there was an effect that changed the field of gravity that we had not considered,” said Burkhard Militzer of the University of California, Berkeley ( USA), which models the planetary interiors. “They turned out to be massive flows in the atmosphere at least 9,000 kilometers deep around the equatorial region. We preliminarily thought that these clouds were like clouds on Earth, limited to a thin layer and almost massless. But on Saturn they are really massive . “
They also calculated that surface clouds at Saturn’s equator rotate 4% faster than the 9,000-kilometer-deep layer. That deeper layer takes 9 minutes longer to rotate than the clouds at the equator, which travel 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 Linda Spilker of NASA’s Jet Propulsion Laboratory in Pasadena, California. “The question is what causes the fastest spinning part of the atmosphere to sink so low, and what does that tell us about the interior of Saturn?”
Militzer was also able to calculate that the planet’s rocky core must be between 15 and 18 times the mass of Earth, which is similar to previous estimates.
Origin of the rings?
Previous estimates of the mass of Saturn’s rings – between half and a third of the mass of Mimas – came from studying the density waves that travel around the icy, rocky rings. These waves are caused by the planet’s 62 satellites, including Mimas, which creates the so-called Cassini split between the two largest rings, A and B. Mimas is smooth and round, 246 kilometers in diameter. It has a high-impact crater that makes it look like the Death Star from the Star Wars movies.
“We always suspected there was a hidden mass that we couldn’t see in the waves,” says Militzer.
Fortunately, as Cassini approached the end of its useful life, NASA scheduled it to make 22 dives between the planet and the rings to probe Saturn’s field of gravity. Earth-based radio telescopes measured the speed of the spacecraft at a fraction of a millimeter per second.
The new value for the mass of the ring is in the range of previous estimates and allows researchers to determine its age.
“These measurements were only possible because Cassini flew very close to the surface in its final hours,” Militzer said. “It was a classic and spectacular way to finish the mission.”
Thus, the rings are only between 10 and 100 million years old, while the age of Saturn is about 4.5 billion years.
Reference: L. Iess el al., “Measurement and implications of Saturn’s gravity field and ring mass,” Science (2018). science.sciencemag.org/cgi/doi… 1126 / science.aat2965
