After spending a decade exploring Saturn, NASA’s Cassini mission is in the midst of a “Grand Finale,” a series of close passes between the planet and its rings that will culminate on September 15 with the spacecraft’s self-immolation via a dive into the planet’s upper atmosphere.
That fiery conclusion is for more than dramatic effect, serving as a way to ensure astrobiologically interesting Saturnian moons such as Titan and Enceladus remain unsullied by any microbes that may have hitchhiked on Cassini from NASA’s clean rooms all the way to the outer solar system. As a bonus, the extreme close-ups Cassini will gain of the planet and its rings during the Grand Finale could also allow scientists to solve some of the ringed planet’s long-standing mysteries. How long is the planet’s day? How massive are its rings? Although seemingly simple, these questions are actually quite complex, and have far-reaching implications for our understanding of Saturn. Only now, as its adventure ends, is Cassini in a position to answer them.
Just How Long Is a Day on Saturn?
You might think that, after studying the planet for centuries through telescopes and for decades with spacecraft, scientists would know exactly how long a day on Saturn lasts. You would be mistaken. Pinning down a rocky planet’s day is as simple as picking a convenient surface feature—a crater, say—and measuring how long it takes the sun to rise and set over it. This approach does not work for Saturn and other gas-giant planets, because they have no accessible surfaces, and their cloudscapes are continuously churned and pushed by winds.
To measure days for such worlds, scientists must look beneath the ever-changing clouds. By measuring the spin of magnetic fields produced by highly compressed metallic hydrogen at a gas giant’s core, they can measure the core’s spin, which is the closest approximation to a day one can hope for on a groundless world. “A magnetic field is generated in the interior of a [gas-giant] planet,” says William Kurth, leader of Cassini’s Radio and Plasma Wave Science team. “It’s very straightforward to measure the periodicities and intensities and determine the rotation rate deep inside the interior of the planet.”
Knowing how long it takes the planet to rotate is key to understanding the storms that rage in its atmosphere. Without knowing how fast the inner layers beneath it rotate, it is difficult if not impossible to gauge the speeds and origins of Saturnian winds. “The speed is relative to some frame, and unfortunately for Saturn, we don't have a well-defined frame,” Kurth says.
The magnetic fields of the sun’s other gas giants are significantly tilted relative to their rotational axes, creating a contrast that makes the magnetic fields’ spins easier to discern. Saturn is different; its field and rotation align almost perfectly, rendering its spin much harder to decipher. Despite this, careful measurements by NASA’s Voyager spacecraft during flybys of the planet pegged the length of Saturn’s day at about 10.7 hours—and before Cassini was launched, many scientists were confident the case was closed.
That changed when Cassini reached Saturn. As it orbited the planet, the spacecraft revealed the rotation of Saturn’s magnetic field varies, oscillating between 10.6 and 10.8 hours. What’s more, the magnetic field’s spin is surprisingly nonuniform, exhibiting rotation rates at Saturn’s north and south poles that seem to differ from each other. “Instead of clarifying what’s going on at Saturn, Cassini has opened up many puzzles,” Kurth says.
As Cassini swoops in and out of Saturn’s ring plane for the Finale, Kurth and other researchers are hoping it spots “lumps” in the magnetic field, regions of stronger or weaker magnetism similar to ones that exist in Earth’s magnetic field. These lumps cannot be spotted from a distance; the spacecraft must be “extremely close,” he says. If Cassini tracks one of these lumps as it rotates, it may solve the question of how long a day lasts. “In that sense, the lump is acting like that crater we can’t see,” Kurth says.
Bright Rings, Blurred Origins
Of the multiple sets of rings encircling Saturn, the A, B and C rings are the largest. The A and C rings are both thin and nebulous whereas the B ring between them is wider and thicker. The smaller D ring lies closest to the planet. No one really knows exactly how old the rings are, how much mass they contain or where they came from—and the answers to all three are related. “If you know the answer to one of those questions, really, in the end, you know the answer to all of them,” says Mark Showalter, co-investigator on the Cassini Infrared Spectrometer. “They’re so intimately tied together.”
Of the three mysteries, mass is the most fundamental, and the one Cassini will directly measure. Small ripples stirred up by moons reveal the densities of the A and C rings when light shines through them. When paired with the rings’ sizes, these density waves allow researchers to calculate their masses. But the B ring is thicker, and doesn't allow light to get through, so its mass remains a mystery.
Cassini hopes to change that during its final flybys. As it sails close to the ring, it will broadcast a radio signal to Earth. The gravitational pull of the B ring should cause extremely minute shifts in the signal’s wavelength, allowing scientists to directly measure the mass. By pinning down the B ring’s mass, scientists might then obtain better estimates of the ring system’s overall age. “There is an ongoing debate on whether the rings were created in the early years of the solar system 4.5 billion years ago or more like 100 million years ago,” says Cassini participating scientist Matthew Tiscareno.
Previous studies have shown Saturn’s rings are mostly composed of extremely pure and highly reflective water ice. Like freshly fallen snow, water-ice rings would be expected to be very bright if they are very young, gradually darkening through dust accumulated as they age. The rate of darkening, however, may depend not only on the influx of dust, but also the total mass and internal dynamics of the rings.
Earlier in its mission Cassini measured the rate of dust and other debris falling onto the rings, but the results were puzzling. Simple extrapolations suggested that if they are billions of years old, the nearly pristine rings should have been darkened by the observed amounts of dust, yet they still shine bright. One plausible solution is that the B ring’s total mass is far larger than anticipated, allowing it to effectively absorb and hide the dust. Cassini interdisciplinary scientist Jeff Cuzzi compares this with squirting an eyedropper of ink into a pint of white paint versus a paint-filled oil drum; paint in the small container would turn gray but in the drum the dark drops would disappear. According to Cuzzi, the mass and amount of material flowing in have been debated for years. Now, he says, “Cassini is on the verge of being able to determine both of those numbers.” In contrast to the “massive, relatively old rings” scenario, it could simply be that Saturn’s rings are young and would not have had enough time to become coated with dust.
“My prediction is that we’re going to find out that [the B ring] is not massive at all,” said ring researcher Paul Estrada. “That would basically make a very, very strong argument that the rings are pretty young.” Even if the rings prove to be massive, Estrada says, that could still point toward their youth—the B ring could simply have had less time to spread out.
Although experts may presently have no consensus on the mass and age of the rings, they do broadly agree that the rings must have been formed by the destruction of a large, moonlike object. But did that object come from outside of the Saturn system or from somewhere within?
In its early epochs, when the solar system was filled with the rocky flotsam and jetsam left over from planet formation, Saturn could have snatched up and torn apart a moon-size interloper, resulting in rings that are several billions of years old. That is not as likely for a younger ring, because most of the large objects disappeared billions of years ago, leaving behind precious few from which rings could form.
Instead, if the rings are young, they could have been born from one of Saturn’s own moons. In 2016 SETI Institute scientist Matija uk proposed that the gravitational pull of the sun could have created instabilities in the orbits of Saturn’s lunar retinue, resulting in the death of a moon only a few hundred million years ago.
Although the resilient Cassini spacecraft is doomed, its legacy will live on in the data it sends home in its final hours. “There’s a lot of controversy, and once Cassini measures the mass of the rings, then the debates are going to begin—again,” Estrada says.