Cassini visits a foamy moon

Bad Astronomy
The entire universe in blog form
Aug. 26 2011 11:40 AM

Cassini visits a foamy moon

I've said it before: Saturn's moon Hyperion is seriously freaky. New images from the Cassini spacecraft flyby of the tiny moon don't change my mind one bit:

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 enchronosenate.]

What a weird place! Hyperion is a lumpy chunk of ice only about 270 km (170 miles) across on average, but yesterday (August 25, 2011) Cassini passed about 25,000 km away from it, so it got a lot of high-resolution shots.

As you can see, it's saturated with craters. But they look funny! The overwhelming impression I get is that Hyperion is made of resilient foam, like a packing peanut. I'm also fascinated by the ginormous crater that dominates this face of the moon. If Hyperion were made of stiff rock, an impact that size would've shattered it like a bullet hitting a pebble. But if the composition of the moon is able to compress and compact -- like foam, or something with lots of pockets of empty space inside it -- the impact would do pretty much what we see here. Lots of asteroids appear to be "rubble piles" -- chunks of material held together by their own gravity, possibly due to impacts that produced myriads of cracks inside the body, or events that just barely shattered the rock, letting it re-accumulate under its own gravity. I'm not saying Hyperion is like that (it's still not well-understood what it's like), but that's one way to bet.

Note also all the dark spots in the bottoms of the craters. Those are from hydrocarbons, complex organic molecules formed when ultraviolet light from the Sun hits simple molecules like methane, rearranging the atoms into bigger molecules. Even under the weak gravity of the moon they flow to the bottoms of the craters, giving Hyperion that decidedly odd black-eyed pea look.

The image here, and the others released today, are raw: that is, they haven't been processed yet to remove camera artifacts and the like. I imagine they'll be cleaned up pretty soon, and no doubt my pal Emily Lakdawalla is working on an animation of the flyby right now even as I write this. One was made for a 2010 flyby and it's awesome. This new flyby is pretty dramatic, showing Hyperion full as well as shadowed, so I can't wait to see what a Cassini's eye-view animation will look like!

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