Mimas is an egg-shaped UFO

The entire universe in blog form
April 16 2009 7:00 AM

Mimas is an egg-shaped UFO

Mimas is a moon of Saturn, most notable because it has a whopping ginormous crater on it, making it look like the Death Star. But a few years ago I stumbled on the fact that Mimas isn't spherical; it's actually quite noticeably ovoid. A few years back I wrote about it on this blog, and a few commenters took me to task because they weren't sure if the image posted really did make Mimas look elliptical.

Well, wonder no more.

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!  


Unprocessed Cassini image of Mimas

Whoa. This image from the Cassini spacecraft orbiting Saturn was taken in January 2009. It's unprocessed, meaning the cosmic rays and other flaws have not been cleaned up yet. They're distracting, but try to ignore them. Instead notice two things: the "dark" part of Mimas is being lit by light from Saturn (the crescent is from sunlight), and Mimas isn't even close to being a sphere.

You can see this again in this Cassini picture taken in October 2008:

Cassini image of a crescent Mimas

In fact, Mimas is egg-shaped! From pole to pole, the diameter is 381.4 kilometers. The diameter going through Mimas and pointing right at Saturn is 414.8 km, and the diameter pointing along its orbit is 394.4 km.

That's pretty far off of a sphere.

The diameter pointing to Saturn is the largest, just as you'd suspect. Saturn's immense gravity means Mimas feels huge tides, stretching it out in a line pointing right at Saturn's center. All big planets do that to their moons (including us). I'm not precisely sure why the other two diameters are so different (by about 3%) but it may have to do with the internal structure of the tiny moon. Interestingly, Mimas's shape is roughly like a flying saucer... and Pan and Atlas, two more of Saturn's moons, are far more obviously described that way. I wonder if the same forces are acting on all the moons, but it becomes obvious at some particular size?

Also, I think that Mimas may be the smallest body in the solar system that is nonetheless big enough to be almost spherical. Its gravity is just enough to form it into a ball instead of an irregular lump like smaller moons and asteroids.

And another thing: the crater Herschel sprawls across the Mimasian surface, the biggest crater with respect to its parent body known. In science, if one object has two weird things about it, chances are they're related. You can't be sure, but it does make me wonder what the heck is going on with this strange little world.


Frame Game

Hard Knocks

I was hit by a teacher in an East Texas public school. It taught me nothing.

Chief Justice John Roberts Says $1,000 Can’t Buy Influence in Congress. Looks Like He’s Wrong.

After This Merger, One Company Could Control One-Third of the Planet's Beer Sales

Hidden Messages in Corporate Logos

If You’re Outraged by the NFL, Follow This Satirical Blowhard on Twitter

Sports Nut

Giving Up on Goodell

How the NFL lost the trust of its most loyal reporters.

How Can We Investigate Potential Dangers of Fracking Without Being Alarmist?

My Year as an Abortion Doula       

  News & Politics
Sept. 15 2014 8:56 PM The Benghazi Whistleblower Who Might Have Revealed a Massive Scandal on his Poetry Blog
Sept. 15 2014 7:27 PM Could IUDs Be the Next Great Weapon in the Battle Against Poverty?
Sept. 15 2014 4:38 PM What Is Straight Ice Cream?
  Double X
The XX Factor
Sept. 15 2014 1:51 PM Why Not Just Turn Campus Rape Allegations Over to the Police? Because the Police Don't Investigate.
  Slate Plus
Tv Club
Sept. 15 2014 11:38 AM The Slate Doctor Who Podcast: Episode 4  A spoiler-filled discussion of "Listen."
Brow Beat
Sept. 15 2014 8:58 PM Lorde Does an Excellent Cover of Kanye West’s “Flashing Lights”
Future Tense
Sept. 15 2014 4:49 PM Cheetah Robot Is Now Wireless and Gallivanting on MIT’s Campus
  Health & Science
Bad Astronomy
Sept. 15 2014 11:00 AM The Comet and the Cosmic Beehive
Sports Nut
Sept. 15 2014 8:41 PM You’re Cut, Adrian Peterson Why fantasy football owners should release the Minnesota Vikings star.