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Big Trouble on Little Mimas

(I wrote and discarded about 20 titles for this post, and that was trying hard to avoid the usual “Mimas looks like the Death Star!!!” jokes.)

While we’re on the subject of Saturn, Cassini recently got some really good images of Saturn’s “Death Star moon”, Mimas. The two images show why we sometimes call it that, and a more oblique view in which the giant crater (known as Herschel) is seen in a kind of profile. Scientists believe the crater is the relic of an ancient impact that nearly shattered the moon. A crater of this size relative to Earth would be wider than the lower United States!

As others (notably Phil Plait) have pointed out, the crater in profile is rather flat-looking. But another thing that struck me in this image is how nonspherical Mimas is: you can see that very clearly if you overlay a circle on top of the image. To really see this, I rotated the image by 75º and zoomed in; my altered image can be found below the fold.

Objects like planets, stars, and moons are governed by a delicate balance between the attractive force of gravity and the repulsive force atoms exert on each other, which manifests itself in the form of pressure. If you have enough mass in one place, this balance of forces tends to make objects spherical; rotation makes objects flatten at the poles and bulge at the equator, but the shape is still mostly spherical. (For those who like fancy geometrical language, the term for this is oblate spheroid.) Mimas isn’t flattened in this way: it’s actually distorted in the opposite direction (prolate for the math buffs following at home).

I don’t know what the consensus among planetary scientists is about whether the impact that created the Herschel crater is responsible (at least in part) for the elongated shape. In this orientation, it looks like axis of elongation is perfectly aligned with the impact—but that could very well be an illusion, caused by the fact that we’re seeing a three-dimensional object in a two-dimensional image. If any planetary scientists reading this care to enlighten me, I’d be thrilled to hear what you have to say.

Whether or not the non-spherical character of Mimas is due to the impact, this is still a stunning image of a very interesting object, and I’m as always excited to see what new discoveries the Cassini mission makes in its orbit around Saturn.

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7 Responses to “Big Trouble on Little Mimas”


  1. 1 Matthew R. Francis April 15, 2011 at 18:18

    With regard to the image overlay, I didn’t do anything too fancy: just estimated a circle to fit over the top using the Gimp.

  2. 2 Nancy April 15, 2011 at 19:15

    Good pictures, great article! I’m noticing a weird phenomenon with the pink sphere overlay: When I’m reading text nearby, the pink sphere in my peripheral vision appears to PULSE? I’m going to think on/research an explanation for the illusion, but wondered if you’d a) noticed it, and b) had a ready explanation?

    • 3 Matthew R. Francis April 16, 2011 at 08:35

      Hm, no idea where that illusion comes from! I don’t get it on this monitor, so perhaps it’s a product of the display? The overlay is a 70% transparent red circle in a separate layer (for those who have used image manipulation software), so it’s possible the monitor is dithering between the underlying layer (with Mimas) and the overlay (with the red circle). That’s my best guess, at least!

  3. 4 Richard Francis April 15, 2011 at 21:43

    I think the second photo looks more like one of my old golf balls, after I’d hacked it a few too many times.
    Good title, BTW.


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