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Perhaps no planet in our solar system resembles a miniature solar system more than Saturn.
“Saturn plus its moons are like the sun plus the planets,” said Linda Spilker, Cassini’s project scientist. Saturn’s moons also orbit Saturn in generally the same plane and in the same direction as one another* like planets in a solar system. “And Saturn’s ring disk is like going back in time to before the planets formed,” Spilker said.
Solar systems billions of years younger than ours have an accretion disk — a flat-ish cloud of gas and dust that orbits a young star and from which planets form or “accrete.”
Compared to Saturn, other ringed planets (Jupiter, Uranus and Neptune) in our solar system have only faint ring systems. And Saturn’s 60 or so known moons offer more than enough complexity to serve as models of our solar system’s planets.
But the Saturnian system doesn’t merely look like a mini solar system. It can also be studied to better understand how our solar system works, both in the present and how it evolved in the past.
Our solar system’s accretion disk accumulated into planets a few billion years ago, so scientists can’t study it directly. But Saturn’s ring system is still there, ripe for study. “It gives us clues about how planets form in an accretion disk,” Spilker said.
Cassini scientists study how Saturn’s rings change over time and how Saturn’s moons gravitationally affect the rings, creating disorder in some regions and tidying up in others. The planets of the early solar system interacted similarly with the disk of dust and gas around the young sun.
The moons also interact gravitationally with each other just as planets do, but in some cases more extremely than seen between planets.
For example, Saturn’s moon Enceladus has a warm subsurface liquid water ocean, and it blasts some of that water into space through fractures in the ice around its south polar region. Those jets of ice are the primary source for material in Saturn’s E ring. But the Enceladean ocean, the water jets, and the E ring probably would not exist if weren’t for another moon, Dione.
The orbits of Dione and Enceladus line up at regular intervals, and Dione gravitationally keeps Enceladus in an eccentric orbit that puts it farther from Saturn, then nearer, then farther, and so on. Its ever-changing distance from Saturn causes stress within Enceladus, like bending a paper clip back and forth until it heats up, but on a larger scale, keeping Enceladus warm enough that its ocean doesn’t freeze.
The Saturnian system isn’t just a convenient proxy for the entire solar system because it has all the components. The Saturnian system is also faster. The gravitational interactions, and exchange of matter and energy between Saturn, its moons and rings are on a timescale that’s easier to observe than the timescale of our solar system as a whole.
Most of the planets in our solar system have longer orbits than Earth does. Neptune, for example, needs 165 Earth years to orbit the sun. Since its discovery in 1846, Neptune has completed only one orbit, and just barely. With a few exceptions, much of the Saturnian system moves on a scale of days and hours rather than years or several human lifetimes.
The advantages of timescale and proximity have allowed Cassini to collect valuable data about how Saturn, its moons and its rings interact and change over time. But they have also made it possible for Cassini and previous spacecraft to collect images that are strung together into videos, showing the Saturn system in motion. The videos make it possible to marvel at the motions of an entire solar system on a stage small enough for humans to comprehend, but immense enough that we still marvel.
*Some of Saturn’s moons, such as Phoebe, orbit in the opposite direction (retrograde) from Saturn’s other moons. Scientists suspect they orbit that way because they didn’t form from Saturn’s accretion disk like its other moons. Rather, they wandered by (probably from the Kuiper Belt in Phoebe’s case) at just the right time and place to get locked into Saturn orbit, albeit a “backward” orbit.