Saturn has more than 60 moons, but a handful of them do more than spangle the planet’s skies.
Snuggled close to Saturn, these innermost moons are small — Epimetheus, one of the largest, stretches just 72 miles across. But they are hefty enough to help sculpt Saturn’s rings. Orbiting at the edges of some of the planet’s main rings, or within gaps between them, these shepherd moons wield enough gravity to herd icy ring particles into place. Some like Atlas tend the bangles by pruning and neatening their edges. Others, like Pan and Daphnis, mow lanes between the rings.
But the rings sculpt the moons, too, coating them with colorful mounds of ice and crafting some unusual shapes.
“They’re quite different than the rest of the moons,” said Carly Howett of the Southwest Research Institute, a co-author. “Some of them look absolutely crazy.”
After more than a decade in orbit around Saturn, NASA’s Cassini spacecraft plunged through the planet’s atmosphere in September 2017. Before it vaporized, the probe completed a series of ring-grazing loops that included five close fly-bys of Atlas, Daphnis, Epimetheus, Pan and Pandora. Now, using Cassini’s observations of shape, color and composition, scientists can infer the worlds’ origins — and are working on figuring out why some of them look so utterly different from the space potato-shape of most small worlds.
Atlas and Pan, for example, are shaped like raviolis or U.F.O.s, with a bulgy center and a flat, tutu-like disc around their middles. But the two moons are distinctly different.
Cracked and cratered, 17-mile-wide Pan is wearing a narrow, knife-edge fringe around its Equator, while 30-mile-wide Atlas is smooth and swaddled in a skirt that’s so voluminous it mostly hides the moon’s bulbous core. Based on Cassini’s observations, the team determined that these skirts are made from ring particles that collide with and stick to the moons, rather than from shifting terrain.
“Neither of these moons is actually orbiting in a cloud of ring material right now, so it wasn’t necessarily obvious that the equatorial ridges come from accreted ring material,” said Matthew Tiscareno of the SETI Institute, who was not involved in the study and says more work is needed to explain the differences between the two moons.
Like Saturn’s rings and some of its larger moons, these five moons are primarily made of water ice.
But it’s unlikely that scientists are seeing their primordial surfaces, Dr. Buratti says. The rings are shedding reddish material onto their surfaces — and Pan, the planet’s absolute innermost moon, is the most colorful.
At the same time, the little moons are coated in bright white material erupting from Enceladus, an ocean moon of Saturn that orbits just outside its main rings and could host alien life-forms in its sea.
“Enceladus is the cosmic graffiti artist of the solar system,” Dr. Tiscareno said. “The closer you are, the more your colors look like Enceladus.”
That’s why Epimetheus, which is the farthest-flung of the five from Saturn, is the whitest of the bunch.
The study helps confirm earlier research suggesting that the moons grew from the same violent collision that birthed the planet’s rings, and could contribute to finding the age of Saturn’s bangles.
It’s not exactly clear how the solar system put a ring on Saturn — or when that even happened — but the most widely accepted story is that a decent-size moon smashed into one of its buddies, flinging icy material into orbit around Saturn.
These innermost moons are likely the larger shards of that collision, and they have been gobbling up ring particles and growing ever since.
“They haven’t always been there and maybe they won’t always be there,” Dr. Howett. “It’s unlikely that the rings formed around them in their current state, so if you can date the ages of the surfaces, maybe you could at least provide a lower limit to the age of the rings.”