Ring arcs show moonlets horde particles

Contributed by
Sep 8, 2008

How's that for a title? Well, how's this for a picture?

This image, taken by Cassini, shows an arc of particles orbiting Saturn. The bright spot is the tiny moon Anthe (pronounced ANN-thee), and it appears to be embedded in the arc. What's going on here?

Well, several things. The arc is probably caused by meteorite impacts on the surface of Anthe. Being just a couple of kilometers across, it doesn't have enough gravity to hold on to the ejected material, so the ejecta splash up and out, into space. Normally, that would wind up forming a complete ring around Saturn. So why do we see an arc and not a ring?

Anthe orbits Saturn in a resonance with Mimas, another, larger moon. A resonance is when two objects have orbital periods that are simple fractions of each other, like one takes exactly twice as long to orbit as another (a resonance of 2:1). When this happens, the smaller object gets tugged periodically by the larger object. In the case of Saturn's rings, this clears a gap in them.

In the case of Anthe, it shepherds the moon, making it perform a strange dance indeed; over the course of its orbit it slows down and speeds up due to the influence of Mimas, forming an arc-like pattern superimposed on its larger orbit. The particles feel this same tug, and perform the same arcing loop. This keeps the particles from completely circling Saturn, so we get an ringlet of material. The particles never get too far from their parent moon, as if its keeping them close to itself.

Another moon, Methone (meth-OH-nee), shows the same thing, and Cassini captured both arcs simultaneously:

The bright spots are the moons, and the arcs are pretty obvious. But don't be fooled! This is a long time exposure by Cassini; the arcs are intrinsically incredibly faint, and can only be seen by sending an amazing machine like Cassini to Saturn. We'd never see these from Earth.

Saturn is an astonishingly complex interacting system. And as we see over and over again in nature, whenever you get complexity like this, beauty is sure to follow. Saturn never disappoints.

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