The solar system's weirdest moon

Bad Astronomy
The entire universe in blog form
Nov. 2 2011 11:00 AM

The solar system's weirdest moon

Our solar system is a fantastically bizarre place. There are worlds as varied as our imagination can grasp -- in fact, they exhibit features we never imagined before we saw them up close. Storms larger than planets, moons with undersurface oceans, lakes of methane, worldlets that occasionally swap places...

... and that's just at Saturn. But of all these, if I had to pick, I'd say the strangest place in the entire solar system would be the ringed planet's distant moon Hyperion. Why? Well, maybe this will help: in September, when the Cassini spacecraft was within just 88,000 km (54,000 miles) of the weird little moon, it snapped this picture:

Phil Plait Phil Plait

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!  


[Click to entitanate.]

Just looking at it, you get a sense of strangeness, don't you? It's little, only about 270 km (170 miles), but packed into that tiny moon is a Universe of weird. It looks like a sponge! Or more like a piece of packing foam that's been pinged by a BB gun. It has a very low density -- about half that of liquid water, even less dense than water ice, indicating it must not be entirely solid. It's porous, like a sponge, or a pile of rubble.

And those craters... they just look funny. They have sharp rims, shallow slopes, and flat bottoms, and it's thought that this is because of how crunchy Hyperion is. Instead of blasting out material like on rocky moons, impacts compress the surface, like punching a block of Styrofoam. The bottoms of many of the craters are dark, filled with hydrocarbons that form when sunlight changes the structure of simpler molecules.

That giant flat region on the right is actually a huge impact crater -- you can see the central peak in the middle, typical for big impacts -- and it reminds me strongly of the huge south polar impact basin on the asteroid Vesta, which itself is a weird place. But it can't hold a candle to Hyperion.

I've talked about Hyperion before (see the Related Posts links below) but I can't get enough of just how freaky this moon is. While it bears some resemblance to other objects in the solar system, it has a bizarre nature all its own. Perhaps I'm showing my American fondness for underdogs, but Hyperion really is one my favorite worlds in all the solar system. We may never understand everything about it, but with every pass by Cassini, we learn a little bit more, and that's cool enough for me.

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