Ringing Cassini’s Epic Mission to a Grand Finale

Saturn’s rings have long maintained an aura of mystery. In 1610, when Galileo first observed them through his crude “optik tube,” he couldn’t resolve them clearly enough to describe anything more than “ears” or “handles” on either side of the planet’s disk. With gradual improvements in optical quality and design came much clearer views of the enigmatic feature, and even though similar rings have since been discovered around the other giant worlds orbiting the Sun, Saturn’s remain the only ones that can be easily seen from Earth. If you want to give it a try for yourself, Saturn is visible just after sunset right now, located in the south-southwest (don’t forget your telescope).

From long-term observations of the planet tilting up and down over a Saturnian year, astronomers determined that while the most visible portions, or the “main rings,” measure about 70,000 kilometers (44,000 miles) from inner to outer edges, the entire structure is comparatively thin—by some estimates only 10 meters (30 feet) thick, while others more generously suggest as much as a kilometer (0.6 mile). When seen edge-on, they practically vanish, and although Galileo also noticed this in 1612, he was baffled by the disappearance of Saturn’s “ears.” The next time the rings will be edge-on to Earth will be in 2025.

Theories explaining the rings have ranged from a flat, solid structure to a gaseous or liquid ring to a system of broad concentric rings. We now know that they’re a disk composed of countless particles of ice, in large part the remnants of a moon that approached too close and was ripped apart by the planet’s gravitational pull. To some extent, portions of the rings also consist of material ejected into space by certain moons, such as Enceladus, whose famous geysers feed the outermost, extremely diffuse E-ring. Transparent in places, yet substantial enough to cast shadows onto Saturn, the rings are shepherded by the gravity of Saturn’s moons into discrete zones.

NASA’s Pioneer 11 was the first spacecraft to encounter Saturn, passing within 21,000 kilometers (13,000 miles) of the planet in 1979. However, because Pioneer was constantly spinning, its imaging photopolarimeter produced grainy images whose details were smeared by the motion. A year later, the better-equipped Voyager 1 reached the Ringed Planet, sending back clearer, sharper images that revealed much more complex structure in the rings than had ever been seen from Earth, giving the appearance of concentric ringlets or the grooves of an old phonograph record (remember those?) with mysterious “spokes” that rotate with the rings. Voyager 2 made similar observations when it passed Saturn in 1981.

Cassini’s arrival at Saturn in 2004 provided an opportunity to observe the planetary system over a long period of time, thanks to two mission extensions that brought the entire mission duration to 13 years—nearly half a Saturnian orbit, including an equinox and a solstice. During the equinox phase in 2010, with the Sun lying along the plane of the rings, the spacecraft observed long shadows on the outer edge of the bright B-ring, indicating vertical structures towering above the ring-plane. Knowing the angle of sunlight and measuring the lengths of the shadows allowed scientists to estimate the height of the features (yay, math!), and the tallest comes out to roughly 2.5 kilometers (1.5 miles).

Although Cassini was never expected to resolve individual particles in the rings, the spacecraft’s orbital vantage point allowed it to image striking dynamics in the ring structure, along with density waves that compress adjacent ringlets closer together like the coils of a Slinky™, then ripple their way along the ring-plane. Other curious features seen up-close include kinks, disruptions, and wavelike ripples caused by gravitational disturbance by embedded moons or even the passage through the rings by streams of meteoroids.

Among the more intriguing ring features are so-called “propellers,” numerous persistent structures thought to be the signatures of interactions with moons too small to clear out permanent gaps in the rings like their larger cousins Pan or Daphnis. These tinier moons and their associated “propellers,” according to a 2008 paper, may measure between 40 and 500 meters (from 130 to more than 1,500 feet) and, according to chief author Matthew Tiscareno, appear to be limited to the A-ring, the outermost of the main rings. In light of the “propeller” classification created for them, the larger and longer-lasting of these features (one of which has been observed for a decade) have been nicknamed, whimsically, after famous historical aviators of the propeller-driven era such as Earhart, Santos-Dumont, and Bleriot.

As it turns out, rings aren’t unique to the giant planets. At least one asteroid has them—maybe even two. There’s speculation that Saturn’s second-largest moon, Rhea, may have a tenuous ring of material around it. Earth and Mars may have had or will have rings at some point in their respective histories, and even one extrasolar planet discovered in 2012 has been dubbed a “super-Saturn,” with a massive ring 200 times larger than Saturn’s.

So rings are a thing, and we have had the opportunity to study the most beautiful known example up-close to observe the interactions that occur within them and between the rings and moons, and to understand how rings form and what keeps them stable. Far more complex than our distant view from Earth reveals, Saturn’s rings have provided a remarkable phenomenon to study—one that has perplexed and puzzled astronomers while also gracing stargazers with one of the most amazing and awe-inspiring sights in the sky… And Cassini gave us a ringside seat!

Image(s): NASA/JPL