NASA's Cassini spacecraft has captured some stunning images of Saturn's rings as part of an effort to better understand how they've changed over time. Why is Saturn blessed with such spectacular rings while the Earth has none?
Size is a big reason. Saturn is 95 times more massive than Earth and thus boasts more moons and more gravitational pull, both of which are vital to ring formation. Moons, in particular, are the critical factor in the creation of Saturn's "dust rings," which are similar to the wispy circles that also surround Jupiter, Uranus, and Neptune. The icy "classic rings," which are the ones depicted in the Cassini images, are a bit more of a mystery, however.
The dust rings, composed of trillions of micron-sized specks, are almost certainly composed of debris from Saturn's 31 moons—although primarily from the innermost "major satellites," which include Dione and Tethys. When asteroids and other objects collide with these moons, tremendous amounts of dust are discharged and trapped in Saturn's orbit, forming rings. The Earth, of course, has only one moon, and it's relatively far away given our planet's small size (and subsequently small gravitational tug); the average distance between Earth and the moon is 384,467 kilometers, compared with just 133,570 kilometers for Pan, Saturn's closest satellite. Overall, Saturn has 13 moons that are, on average, closer to the planet than our moon is to the Earth. Debris from the moon is unlikely to be sucked in by the Earth's gravitational embrace because of the relatively great distance between the two bodies. In addition to our dearth of satellites, this is one of the biggest reasons why our planet is ringless.
There are a couple of theories as to how Saturn's classic rings—which are unique to the planet—were first formed. These rings, which consist mainly of house-sized ice chunks, might contain the vestiges of either moons or similar-sized objects that once orbited the nascent Saturn and were destroyed by space debris billions of years ago. But the rings may simply be a consequence of Saturn's formation, which occurred in a far different manner than that of Earth. The giant planet likely began as a core of rock and ice and slowly pulled in surrounding detritus and, more important, gases. (The planet is made up mostly of hydrogen and helium.) The rings, then, may be composed of materials that Saturn pulled close but never quite managed to add to the planet proper. The Earth, perhaps because of its comparatively small gravitational pull, simply didn't develop in this manner—good news for us humans, as gaseous Saturn could scarcely be more inhospitable to life.
Many astronomers believe, however, that Mars—the planet most like Earth—may have dust rings of its own. Mars, after all, has two moons smaller than our own, an important requisite for developing the wispy rings. If the rings are there, they're obviously so faint as to have escaped detection so far. But there are researchers who project that Martian rings, however modest, will be discovered within the next 10 to 15 years.
Explainer thanks Douglas Hamilton of the University of Maryland and Stephen P. Maran of NASA's Goddard Space Flight Center.