A series of flybys by NASA’s Cassini spacecraft in the months before its final plunge into Saturn have revealed five of Saturn’s innermost moons to be made of the same material as the rings themselves.
It has also found the moons to be so light and porous that, if the smallest of them were to be dropped into the Pacific Ocean, it would float like a giant beach ball, with only a small fraction beneath the waves.
These moons — Pan, Daphnis, Atlas, Pandora, and Epimetheus — range in diameter from eight to 120 kilometres, and are so close to Saturn that they lie within its main ring system.
During the flybys, which came as close as 3625 kilometres to Epimetheus and within 8300 to 22,500 kilometres of the others, Cassini was able to turn a suite of six instruments toward each of them, verifying that their composition matches that of the rings within which they are embedded.
“They are the same stuff,” says Bonnie Buratti, a planetary astronomer at NASA’s Jet Propulsion Laboratory in Pasadena, California, and lead author of a study in the journal Science.
Most likely, she says, the ring moons are shards from whatever larger object (or objects) once ventured too close to Saturn and broke up to form the rings.
“They are probably big chunks left over from the formation process,” she adds.
They are also continuing to interact with the rings, sweeping up dust that gets in their way in a process Buratti says might provide insights into how planets grow in solar systems.
Several of them even look as if they have skirts of fine particles around their middles.
“Atlas looks like a ravioli,” Buratti says. “It has this disk of particles we believe are accreting from the rings.”
Intriguingly, she notes, the asteroid Bennu, currently being visited by NASA’s OSIRIS-REx sample-return mission, also shows a skirt, though “it’s not as spectacular as that on Atlas”.
In the case of Bennu, one hypothesis is that the skirt formed when an impact created debris that then accreted back onto Bennu’s surface in a manner similar to that by which Saturn’s ring moons are collecting material from the rings.{%recommended 1545%}
“So, this process that we’re looking at in an extreme form on Saturn, we see elsewhere,” she says.
The data also shows that all of the ring moons are far less dense than water, or even an iceberg. The densest, Epimetheus, weighs only 0.62 grams per cubic centimetre. The lightest (and smallest), Daphnis, weighs in at a meager 0.27 grams per cubic centimetre — barely a quarter the density of liquid water. “It’s a rubble pile,” Buratti says.
The new results are particularly exciting because none of the flybys were planned in advance. “They were serendipitous,” Buratti says.
Cassini’s grand finale was intended to study the rings, Saturn itself, and the planet’s magnetic field, before the spacecraft was allowed to burn up in its atmosphere, thereby preventing it from ever hitting one of the moons and contaminating it with Earth bacteria.
“It just happened that the orbit brought us close [to these moons],” Buratti says. “It’s a whole new mission we weren’t supposed to have … a little bow wrapped around the present that was Cassini.”
Meanwhile, Matthew Hedman, a planetary scientist at the University of Idaho, US, has found four “clumps” that mysteriously appeared in a thin ribbon of Saturn’s innermost D-Ring, called D68, which is only 67,630 kilometres away from the giant planet’s centre.
That makes it only 8000 kilometers above Saturn’s surface, vastly closer than any of the moons examined by Buratti’s team.
It is so close to Saturn, in fact, that if anything akin to one of the ring moons had existed there, gravity would have ripped it apart.
D68 had been studied for years, but in 2014, Hedman wrote in a paper recently published in the journal Icarus, it suddenly produced four bright spots that appear to be clumps of ring particles.
That doesn’t mean that new ring moons are somehow trying to form, despite Saturn’s gravity. Rather, Hedman says, the clumps appear to be clouds of dust, scattered by some type of collision.
“The easiest explanation,” he says, “is that something shattered.”
That released dust and larger fragments, the latter of which then hit other ring particles to produce more dust and another round of fragments, in a collisional cascade.
These collisons needn’t have been huge. To produce the amounts of dust seen, Hedman says, each one only needed to have pulverised the amount of material contained in an ice block a few metres in radius.
The fact that four clumps appeared at the same time is intriguing, but, Hedman says, might simply mean that a meteor shower slammed through, happening to hit four ice blocks in a single onslaught.
Meanwhile, he notes, similar clumps of dust have periodically appeared farther out from Saturn in its F-Ring, which hosts the moons Prometheus and Pandora.
The bright spots, he says, might also be dust clouds created by collisions. If so, they might reveal the type of process that formed the dust now collecting on the ring moons’ skirts. If so, he adds, it’s an interesting “potential connection” between the two studies.
