Saturn is ancient, but its rings are only as old as the dinosaurs
How the gas giant got its rings.
Saturn’s rings are one of the most gorgeous sights to behold in the solar system, but we’ve never quite understood their origin story. That’s changing fast thanks to the last downpour of data collected by NASA’s Cassini probe before it met its demise in September 2017. According to findings published Thursday in the journal Science, Saturn’s rings are much, much younger than the planet itself—only about 10 to 100 million-years-old, compared to the gas giant’s age of roughly 4.5 billion years. For most of its existence, Saturn has been a big naked baby, and the solar system only put some rings on it relatively recently.
“There were already clues from Voyager and Cassini measurements that the rings had not formed with Saturn,” says Luciano Iess, a researcher at Sapienza University of Rome and the lead author of the new study. “But now we have a much more concrete evidence, which was only possible to obtain during the final phase of the mission—the Grand Finale.”
He’s referring to the last days of Cassini—in which the spacecraft ended its 13-year orbit of the Saturnian system in a wild last hurrah, making six close dives toward the planet between the atmosphere and the rings, and finally diving headfirst into the atmosphere to collect as much data as possible before disintegrating.
In that last phase, Cassini was close enough to measure the gravitational field around Saturn and its rings. In preparation for the measurements, Iess and his team created detailed models that predicted what the gravity field might look like using what was already known and predicted by about Saturn’s size and composition.
“It turned out, all those models were proven wrong,” says Burkhard Militzer, a planetary researcher at the University of California, Berkeley and a coauthor of the new study. “We were totally blown away by these results. The gravity field is weird, and we just did not understand what it would actually look like.” The gravity pull from Saturn turns out to be 40 million times larger than the pull the rings exhibit.
Two big findings from the gravity field measurements emerged. First, there must be massive flows around the equator to explain the unusual gravity field. And second, the ring mass estimates (derived from the gravity field measurements) show that the rings are less massive and lighter in color than previously thought, which strongly suggests a young age, and a possibility that the rings used to be more massive than they are today.
“This is the first time ring mass has been measured with gravity,” says Militzer. “There have been other attempts to do that using density waves, but the measurements from gravity are much more reliable and robust. And this is the first time we’ve ever really had them for Saturn.”
Deriving ring mass from gravitational field data is relatively easy and intuitive, but going from ring mass to ring age is, as Iess puts it, much more subtle. We already know meteorites gradually make rings darker over time. If we assume the rings were pure ice to begin with, we can use the darkness of the rings to make estimates as to how many meteorites have impacted the rings. Combine that data with ring mass numbers, and we can determine how long it took for the rings to reach their present-day darkness, which effectively gives us the age of the rings and an approximate understanding of when they formed.
The new findings don’t tell us exactly how the rings came to be, but the team thinks it’s likely some sort of dramatic, catastrophic collision within the Saturnian system played a role. “Something blew apart and made these gazillion ring particles that we see today,” says Militzer. Given that the rings are so young, “it did not happen when the solar system formed.” Coincidentally, the timeframe of formation of the rings—10 to 100 million-years-old—is the same timeframe of when the dinosaurs were wiped out (65 million years ago), also by an impact. It’s far from clear whether those events are related in any sort of way, but these new findings suggest that perhaps the solar system was in the midst of some sort of unusual upheaval of objects and collisions at the time. Iess adds that some colleagues think the findings help support the idea that even the inner moons of Saturn are young bodies.
“I am delighted and surprised by how well the team managed to estimate the ring age with such precision,” says James O’Donoghue, an astronomer at NASA’s Goddard Space Flight Center who was not involved with the study. “It is unclear what cataclysmic event formed the rings, but the idea that they formed around the time dinosaurs became extinct is a profound result to end the Cassini mission on. That particular time period seems to have been a busy one for the solar system!”
O’Donoghue adds that the new mass numbers suggest you’d need about 5,000 Saturn ring systems just to make something the size of the moon. And yet, the rocky bodies within the ring are so spread out that they could cover up the surface of the Earth 80 times over.
“The more we learn about Saturn’s rings, the more fragile and transient they seem to be,” says O’Donoghue. Just last year, scientists released predictions based on Cassini data that the planet’s rings would be gone in just 300 million years. It’s likely Jupiter, Uranus, and Neptune boasted much larger ring systems in the past, and Saturn’s system will thin out in the same way.
There are surely more mysteries to unravel when it comes to Saturn’s rings, and Cassini’s legacy will continue to grow for a while. The team hopes to use more of the mission’s data to characterize precisely how fast the interior of the planet rotates, and how this might affect ring evolution and behavior. They have also measured a tiny component of Saturn’s gravity whose origin is unknown. “We call it the ‘dark side’ of Saturn’s gravity,” says Iess. “I’m really puzzled by this finding,” and he hopes to hone in on what’s causing it. The rings of Saturn might not be the only treasure the gas giant is flaunting in its orbit.