Saturn Moons Have Class

Late last week scientists at NASA’s Jet Propulsion Laboratory in Pasadena announced the discovery of a new class of moons orbiting Saturn: the giant propeller moons.

Tracked over a four-year period by the Saturn-orbiting Cassini spacecraft, the moons create distinctive propeller shapes as they travel through the planet’s A ring, the outermost of the large, bright rings.


An unseen moon making a bright, white propeller feature in Saturn’s A ring.

—Image courtesy NASA/JPL/Space Science Institute

The moons had long been hidden, since they are embedded in the icy particles of the dense A ring.

“Scientists have never tracked disk-embedded objects anywhere in the universe before now,” Matthew Tiscareno, a member of the Cassini imaging team, said in a statement. “All the moons and planets we knew about before orbit in empty space.”

What Cassini’s new pictures revealed is that, as the moons orbit, they each kick up bright “propellers” of material several thousand miles long.

Such features aren’t entirely new: Small propellers had been previously spotted elsewhere in the A ring and were linked to tiny objects dubbed moonlets orbiting in the ring plane. These moonlets are thought to number in the millions.

But the new propellers are hundreds of time bigger, appear in a section of the ring farther out from Saturn, and are likely linked to full-size moons.

Cassini saw only 11 of the giant propeller features, but the science team thinks there could be dozens of moons hidden from view that are capable of creating propellers.

The find has implications for how larger bodies orbit inside disks of debris—an area of interest, for example, for scientists studying how planets take shape in the dusty disks around young stars.

But the find also highlights the enormous complexity of Saturn’s rings and moons. The ringed planet boasts more than 60 moons that vary dramatically in shape, size, complexion, and origin.

To get a handle on things, the moons have been grouped into ten classes based on where and how they orbit:

“Moonlets” is the general term applied to the not-quite-moon-size bodies that orbit inside the rings.

Ring shepherds are relatively small moons that “herd” the ring particles around them, sculpting the outside edges of rings or carving distinct gaps in their middles. The moons Pan and Daphnis, for example, created and maintain the Encke gap and Keeler gap, respectively. The newfound giant propeller moons aren’t exactly shepherds, since they don’t carve out defined paths in the rings—hence the suggestion that they make up a new Saturnian moon class.


Daphnis making waves in the Keeler gap.

—Image courtesy NASA/JPL/Space Science Institute

Co-orbital moons are pairs of moons that both orbit at almost the same distance from a parent body. In Saturn’s case, the moons Janus and Epimetheus orbit so close to each other that every four years it looks like a collision is unavoidable. Instead, thanks to some fun physics, the moons swap places, with one taking a slightly lower, faster path and the other taking a higher but slower route.

Saturn also hosts the only known trojan moons, pairs of moons that travel in stable positions either ahead of or behind a much more massive moon. The large moon Tethys, for example, is accompanied on its journey by the trojan moons Calypso (picture) and Telesto.

Tethys is one of the four inner large moons, which orbit in the diffuse, microscopic particles of the E ring, Saturn’s outermost ring. Its neighbors are Dione, Mimas, and one of Saturn’s most famous moons, the “ice-geyser world” Enceladus.

Between Mimas and Enceledus lie three smaller moons that make up their own class, the Alkyonides. Each of these tiny moons displays its own ring arc, a faint, partial ring of material that’s likely being knocked off the moon by minute impacts.

A ring arc around the moon Anthe.

—Image courtesy NASA/JPL


The outer large moons are the much more traditional satellites, orbiting outside the planet’s rings. Here we have Rhea, Hyperion, Iapetus, and Saturn’s largest moon, Titan.

Finally, Saturn has a few irregular moons orbiting beyond the big outer moons. These small satellites—some of which even orbit backward with respect to Saturn’s rotation—are likely passing objects that were captured by Saturn’s gravity. The irregulars are separated into (and named according to) three main groups: Inuit, Gallic, and Norse.

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