Saturn’s orbit in fine detail
After its launch in 1997 and a seven-year journey, Cassini spent 13 years orbiting Saturn. The rings had previously been observed close up by the Voyager 1 and Voyager 2 missions, but for a few days only. Cassini was therefore able to study the fine structure and evolution of the rings over a long period, making many discoveries in the process.
“The surprising thing about the rings is that they’re not lifeless, but in fact extremely dynamic,” explains Sébastien Charnoz. “They have wave structures and vibrate like a membrane in the gravity field of the planet and its moons. But these gravity variations also occur because Saturn isn’t uniform, and the motions in the rings reveal insights into the planet’s inner structure.”
Cassini’s sojourn has enabled it to observe the rings in unprecedented detail. Saturn has seven main rings, each separated by gaps. Each ring is in turn formed of ringlets and gaplets, seen in the high-definition images from Cassini.
“The rings consist of water ice, whose porosity we have been able to measure to obtain clues about how they formed. The very-high-resolution imagery has even revealed new moonlets residing within the rings. Some of these are very elongated like rugby balls or peanuts, which is the sign that they were formed by accretion. Others like Pan and Daphnis have bulges at their equator, which suggests they must have formed at the same time as the rings. Others still are smaller and more irregularly shaped, and are not orbiting in the equatorial plane of the rings, like Phoebe, which Cassini flew past early on in the mission and which must have formed at a distance of around 40 astronomical units (40 times the Earth-Sun distance) in the Kuiper Belt before migrating inward and being captured by Saturn’s gravitational pull.”
How moons formed from the rings
The formation of planets’ moons is still poorly understood. The most important thing when validating any theoretical model is to be able to constrain it from observations, but this isn’t made easy by the fact that the solar system is more than four and a half billion years old, so today the planets and their moons have finished forming… except for Saturn’s moons.
“Three years ago, a new model was validated by observations of the outer edge of the B ring, where Cassini saw a new moon that wasn’t there 10 years previously. So maybe moons are still forming in the solar system, even though its planets finished forming four billion years ago. These processes of accretion outside the rings are the discovery I find the most striking. Seeing a phenomenon postulated by the theory of planet formation that is now no longer observable in the solar system was an unexpected bonus. What’s more, it’s enabled us to establish a relationship between Saturn’s rings and the formation of its moons.”
Establishing this reliable theoretical moon-formation model was no easy task. So how exactly does it explain the formation of this new moon?
“We devised numerical simulations from Cassini’s observations,” Sébastien Charnoz continues. “Normally, the rings tend to flatten and spread out like pancake mixture, and when they reach the theoretical Roche limit at which a body is torn apart by tidal forces, clumps and moonlets are formed by aggregation. These simulations work very well and we think that most of the moons were indeed formed by material from the rings accreting and migrating away from the planet. These discoveries are already finding their way into textbooks, even though the mission is only now set to end.”
Into the gap between Saturn and its rings
For the last six months, Cassini has been on its final trajectory in a highly elliptical orbit diving repeatedly between the planet and its rings, above the atmosphere that it will be analysing one last time before plunging to its demise. On this occasion, the rings are set to give up some more of their secrets.
“First of all, we’ll get the opportunity to gauge the rings’ mass. That’s important, because no direct measurement of their mass has ever been made and that information will enable us to estimate their age. If they formed at the same time as Saturn four and a half billion years ago, they should be more massive. Conversely, if they’re lighter, they could be as young as 100 million years. That’s a big question for planetary science, because we still don’t know how rings form or why gas giants have ring systems, unlike rocky planets. We still have much to learn about Saturn and the rest of the solar system.”
Artist’s impression of Cassini between Saturn and its rings en route to the mission’s Grand Finale. Credits: NASA/JPL-Caltech
- Francis Rocard, Head of CNES’s solar system programme francis.rocard at cnes.fr
- Sébastien Charnoz, astrophysicist at Paris Diderot University and research scientist at the IPGP global physics institute in Paris charnoz at ipgp.fr