A Buffer Zone Around Saturn May Have Kept It From Swallowing Its Biggest Moon

Among Saturn’s exceptional entourage of 82 moons, one stands out: Titan, a hulking giant that, at 3,200 miles across, besting even the planet Mercury in width. Researchers have long struggled to explain why only one of Saturn’s satellites—the second largest moon in our solar system—reached such gargantuan proportions.

Now, a team of planetary scientists has come up with a new theory on how this bulky behemoth became a solo act. Titan, they argue, may have been the only survivor of a feeding frenzy during Saturn’s early evolution, during which the ringed planet gobbled up similarly-sized moons orbiting closer to its surface.

The findings, published this week in Astronomy and Astrophysics, represent the first demonstration “that a system with only one large moon around a giant planet can form,” study author Yuri Fujii of Nagoya University explains in a statement.

On the whole, moon formation remains a mysterious business. Many likely formed alongside their parent bodies, sprouting out of the swirling disk of gas and dust that wreathes planets in their infancy. A few are thought to be the shrapnel from collisions between much bigger bodies, while others may be hostages, captured from elsewhere in space by a planet’s gravitational pull.

Simulations have shown that when a large moon like Titan, distinct from its much smaller lunar siblings, ends up in the vicinity of a planet like Saturn, it will simply get swallowed up—suggesting that researchers’ models of moon evolution have been overly simplistic, Fujii tells Nadia Drake at the New York Times.

To refine their understanding of Titan’s origins, Fujii and her colleagues created a model that more accurately captured the dynamics within Saturn’s gassy, dusty halo. Their data added information about temperature and density gradients, while also factoring in other variables such as the gravity of nearby moons, reports Jennifer Leman for Popular Mechanics.

After running their simulation over more than 100,000 years of moon evolution, the team discovered that planets like Saturn are, in fact, capable of gobbling up their moons—but only if they fall within a certain distance. Within that radius, moons will end up moving inward and getting swallowed early on. But just outside this zone of terror lies a thick buffer—a ring with a steep temperature gradient that drops off precipitously at its outer edge. Warm, high pressure gas within the band pushes any moons that exist outside it away from the planet, instead of pulling them toward it. Circling Saturn from afar, Titan may have been spared the grisly fate that claimed others of its kind.

Of course, this scenario doesn’t guarantee the formation of one giant moon, flanked by a gaggle of 81 runts, like Saturn has. The researchers also can’t prove that Titan actually did form this way. As such, the simulations are “interesting, but preliminary,” Luke Dones, a planetary scientist at the Southwest Research Institute tells the New York Times.

Other aspects of Titan’s history remain mysterious as well. Though the researcher’s findings could help explain how the moon survived, they leave open a variety of possibilities for how it formed and came to orbit Saturn in the first place.

As spacecrafts and telescopes continue to probe the cosmos, perhaps some answers will come from outside our solar system, study author Masahiro Ogihara of the National Astronomical Observatory of Japan says in the statement. Should Saturn-like exoplanets exist, “the formation mechanisms of such systems will become a red-hot issue.”

Katherine J. Wu | | READ MORE

Katherine J. Wu is a Boston-based science journalist and Story Collider senior producer whose work has appeared in National Geographic, Undark magazine, Popular Science and more. She holds a Ph.D. in Microbiology and Immunobiology from Harvard University, and was Smithsonian magazine's 2018 AAAS Mass Media Fellow.