Nadia Drake

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.

“These moons kind of sweep out cavities within the rings,” said Bonnie Buratti of NASA’s Jet Propulsion Laboratory, who described the mini-moons in a study published Thursday in Science.

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 UFOs, 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, 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,” 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,” Howett said. “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.”

This article originally appeared in <a href="https://www.nytimes.com/2019/03/28/science/saturn-moons-rings.html" id="e133169b-e037-3c02-a3a1-f4d9e2e88442">The New York Times</a>.

(Trilobites)

A Crew of Mini-Moons That Sculpts Saturn's Rings

Saturn’s icy rings are among the most iconic features in the solar system. But they are raining so much water onto the planet that in 300 million years they could rain themselves nearly out of existence, leaving Saturn startlingly ringless.

“What we’re seeing is something on the order of about a ton and a half per second,” said James O’Donoghue of NASA’s Goddard Space Flight Center, in Greenbelt, Maryland, who reported the conclusions Monday in the journal Icarus.

“The rings of Saturn haven’t been around forever,” he said. “And they’re going to disappear someday.”

It is difficult to imagine a solar system without Saturn’s rings, but it turns out they are an ephemeral, if majestic, phenomenon. Scientists have long debated the age and expected life span of the rings, and last year, some published findings suggesting that they are not ancient relics from the birth of the solar system, 4.5 billion years ago. Instead, they were fastened onto Saturn within the last few hundred million years — during the time of the dinosaurs. And now, multiple lines of evidence are converging to suggest that the rings come with an expiration date.

“Saturn’s rings appear to be young,” said Linda Spilker of NASA’s Jet Propulsion Laboratory in Pasadena, California, project scientist for the Cassini mission that studied Saturn, who was not involved in the study. “Maybe we’re just in that interesting, lucky period where we get to see Saturn’s rings to the level that we see them.”

Stretching some 175,000 miles across, Saturn’s bangles easily outshine the dark, fragmented rings feebly encircling Jupiter, Uranus and Neptune. In fact, without its icy accouterments, Saturn would look boring and beige compared to some of the other gas giants.

O’Donoghue and his colleagues suggest the rings could wither away in the near future, on the relative scale of solar system time.

Earlier this year, the team took a close look at measurements of a charged molecule found in the upper reaches of Saturn’s yellowish atmosphere, using the Keck II Telescope in Hawaii in 2011. The molecule, H3+, is made up of three hydrogen atoms (minus an electron). It is created in abundance when minuscule, charged particles escape Saturn’s C-ring, spiral into the planet along magnetic field lines and collide with atmospheric atoms in what is called ring rain.

After measuring the amount of H3+ in Saturn’s upper atmosphere, O’Donoghue and his team concluded that as much as 4,400 pounds of water are falling onto Saturn each second.

Assuming a constant rate of ring rain — which Spilker said is a substantial unknown — the team calculated that Saturn’s rings could mostly shed themselves into oblivion within 300 million years.

“It’s not out of the question,I would say, that the rings might degrade on this kind of time scale,” said Jeff Cuzzi of NASA’s Ames Research Center in Moffett Field, California, who was not involved in the research.

But, he added, “it doesn’t mean that if you come back, there would just be nothing there.” The rate at which the rings might waste away depends not only on how much material is still in the rings, but on other physical forces, Saturn’s shifting seasons and the way in which ring material is replenished.

Recent research using data from the Cassini spacecraft, which winged its way among Saturn and its moons for more than a decade, support the group’s ring rain estimate. Before Cassini took a fiery, fatal plunge into Saturn in September 2017, it completed a daring series of loops between the planet and its rings.

The spacecraft took a census of the particles it encountered that were falling toward the planet; the amount of ring rain Cassini caught is “completely consistent” with O’Donoghue’s measurements, Spilker said.

But Cassini also found that a colossal amount of organic molecules and water ice (on the order of 22,000 pounds per second) are falling onto the planet’s equator through a different, potentially transient process that could help hasten the rings’ demise. That could mean the rings might disappear even sooner, in less than 100 million years, O’Donoghue said.

Although Cuzzi is skeptical of the time frame in the new study, he agreed it is likely that the rings will gracefully degrade as the solar system matures.

“I don’t think it’s unreasonable, with all these numbers being so high, that we have to seriously consider that the rings won’t be around forever,” he said.

This article originally appeared in <a href="https://www.nytimes.com/2018/12/17/science/saturn-rings-vanish.html" id="1167e883-b91a-351c-aee5-a8fa579c5e71">The New York Times</a>.

Saturn With No Rings? It Could Happen, and Sooner Than Astronomers Expected

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Nadia Drake

Articles written by the author

A Crew of Mini-Moons That Sculpts Saturn's Rings

Saturn With No Rings? It Could Happen, and Sooner Than Astronomers Expected