The Sciences

The tiny moon with the long reach

Bad AstronomyBy Phil PlaitAug 2, 2010 11:13 AM


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When I was a kid, Saturn had one big, flat ring system divided up into maybe three or four broad sections separated by gaps, and that was it. Turns out, we were wrong. Saturn has thousands of rings made up of billions upon billions of tiny ice particles. There aren't just a handful of gaps, there are thousands of them, too, and there are moonlets in those gaps. Those tiny moons tug and pull on the rings, distorting them into weird and fantastic shapes. And "flat"? Not quite. The Cassini mission apparently delights in showing us just how wrong we were:

This image from Cassini shows Saturn's broad A ring, the one you can see in small telescopes from Earth. On the right is the Encke Gap, a space carved out by the tiny moon Pan. On the left is the narrower Keeler Gap, where the even tinier moon Daphnis orbits Saturn. Daphnis is a lump, only about 9 kilometers (5.5 miles) across. But it has gravity, however feeble, and it's enough to affect the rings. The waves you see just inside and outside the Keeler Gap are from Daphnis poking and prodding at the ring material. Stuff closer to Saturn (to the right) orbits faster than Daphnis, and stuff farther out (to the left) moves slower. In the picture above, the ring particles move roughly from the bottom of the picture to the top. Remember, everything is in motion here. As the particles closer to Saturn pass the moon, they get tugged, and as the moon passes particles farther out, they get tugged too. This causes the ripples you can see in the rings. Here is the same image, rotated and zoomed a bit for clarity:

Man, that's bizarre. But it gets even weirder. The orbit of Daphnis is not exactly circular, nor is it exactly in the plane of the rings. It bobs up and down by a few kilometers (very roughly its own diameter) every orbit. This causes it to pull the ring particles out of the ring plane, and sometimes it pulls harder than other times. This motion and its effects are extremely complicated (as this technical paper outlines), but the cool thing is, Cassini shows us what happens.

This picture, taken when the Sun was shining straight along the edge of Saturn's rings in 2009, shows Daphnis slightly out-of-plane of the rings, casting a long shadow on them. You can also see that the ring particles are also being pulled out of the plane; the waves cast shadows too! What this and other pictures from Cassini are showing us is that the Universe isn't all that simple. When we first look at something, we may get low-resolution, fuzzy pictures, and that means our understanding may be equally fuzzy. The closer we get, the harder we scrutinize, the more we learn... and in turn we find out that the Universe is more complicated, more interesting, and more beautiful than we first thought.

Tip o' the Whipple Shield to Carolyn Porco (and for the link to the tech paper, too). Image credits: NASA/JPL/Space Science Institute

Related posts: - Saturn's rings do the wave - Like the fist of an angry god - More Saturn ring awesomeness - Ring shadowplay on a Saturn moon - Ringless

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