Moon Shadow

And Cat Stevens

Credit - Cassini / NASA

Credit - Cassini / NASA

Emily at Planetary.org has a wonderful post about some shadow-play that’s occurring right now on Saturn’s rings.  You see, the rings are quickly becoming “edge-on” to the Sun, as they do twice every Saturnian year (which is about 30 Earth years).  Right now, shadows caused by the moons of Saturn that happen to fall on the ring system are very long, as you can see by the spike shaped shadow cast by Mimas in the photograph.

But that’s not the cool part. That would be the jagged, “grassy” edge that you can see above (and mostly to the left of) the spike (click on the thumbnail to enlarge it).  What’s that caused by? Why, something much smaller, clearly.

Emily does some quick “back of the envelope” calculations right there in her blog (she’s a pro., so please don’t try this at home without a trained mathematician nearby!) to show that whatever is causing that jagged line effect is only about 3 km. high.

But Cassini has already spotted moons around the planet that are about that size, and whatever is causing this doesn’t look like a moon to Cassini.  So are they many small moons – hundreds of them?  Not exactly, probably.

I believe that what we’re actually seeing is clumpiness of particles at the outer edge of the densest B ring, where particles bunch together partially by self-gravity (which would make them more like moons) but also by the periodic gravitational shoves they get from Mimas. At least that’s what the imaging team has said about past images of the outer edge of the B ring, like this one. These clumps would be transient, torn apart by the same forces that bring them together. The B ring is so dense that particles rub up against each other as they orbit Saturn — an astronaut would be able to travel easily from one particle to the next, clambering around the rings, though it’d be a long trip to circle Saturn! And, evidently, the astronaut would have some climbing to do, traveling up and down the clumps of big particles that form the B ring’s outer edge.

Now that’s cool!

Here’s more from Nancy Atkinson at Universe today.

Because Saturn is approaching its equinox, in August the rings will “disappear” from our view from Earth, as the rings will be exactly edge-on. But as the rings ease into alignment with the sun, Saturn’s moons cast their shadows across the rings, growing longer as equinox approaches. See in the image above, a shadow is cast on the rings[.]

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2 Comments on “Moon Shadow”

  1. Becky Says:

    Unfortunately all the while I was reading this all I had going through my brain was Cat Stevens “I’m being followed by a moon shadow…” and it will probably stay lodged there for days.

  2. joe Says:

    Heh! My work here is done. ;>

    Emily Lakdawalla said exactly the same thing, Becky!


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