# Saturn’s biggest kids play peek-a-boo

I spent all day yesterday writing a 2000-word article for a print venue to be named later, and the weather outside is sunny and delightful and begging to be biked in, so I am disinclined to write something deep and philosophical today. So instead here is just a simply way-cool picture from Cassini taken in 2009, showing the Saturnian moon Rhea peeking out from behind the much larger Titan:

[Click to eneldergodenate.]

[UPDATE: I messed up here. In the original post I misread Titan’s radius when I looked it up, and was comparing it to Rhea’s diameter. This changes my numbers enough that I have simply corrected everything below; otherwise it would be too confusing to read. Thanks to the commenters for pointing this out!]

Rhea is a little over 1500 km (900 miles) across, and Titan 5150 km (3100 miles). However, in this shot, Rhea was almost two and a half times farther away than her big sister, so it looks smaller than it really is. Titan has a thick atmosphere, which is pretty obvious in the picture, while Rhea is basically a ginormous iceball.

Still, hmmm. Titan and Rhea are the two largest moons of Saturn, but to be honest Titan really is a lot bigger than Rhea, more than 3 times wider. Why such a big gap in sizes? Jupiter’s two largest moons, Ganymede and Callisto, are much closer together in size (5260 and 4820 km, respectively), and Ganymede is only 1.7 times bigger than Jupiter’s fourth largest moon, Europa. After that, though the rest are far smaller.

It seems to me that Saturn and Jupiter are telling us something about the physics of the way their moons formed. But what could it be? Titan orbits well over twice as far from Saturn as Rhea, while Ganymede is actually closer to Jupiter than Callisto. Is that important? Did those moons form at other distances and get their orbits jostled through gravitational interactions over billions of years, maybe even switching positions?

These are pretty basic questions, but it’s questions like these that lead to basic insights on how our solar system formed and changed as time went on.

And dangit! I guess I did get a little deep and philosophical here. Ah well, what can I say? Images like this are so pretty and so interesting to look at, they spark all kinds of thought processes in my head. And the more I do that, the more I want to do that. Science is like that: addictive, but in a good way.

Damn! Did it again.

Image Credit: NASA/JPL/Space Science Institute

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