Mimas is an egg-shaped UFO

By Phil Plait | April 16, 2009 7:00 am

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.

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.

CATEGORIZED UNDER: Astronomy, Pretty pictures, Science
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Comments (61)

Links to this Post

  1. Conspirama | April 16, 2009
  2. Mimas is an egg-shaped UFO | Bad Astronomy | Discover Magazine | eufo.biz | April 16, 2009
  1. Jonas E

    Wouldn’t it be possible for a big asteroid impact to knock a planetal body out of its spherical shape?

  2. Just how round is round enough? In other words, if Mimas were in its own orbit, would it be a planet or a dwarf planet?

  3. Glyphea

    “In science, if one object has two weird things about it, chances are they’re related”

    I’m tempted to use this teaching science majors today…..

  4. !AstralProjectile

    (Obligatory John Varley reference)
    Are we sure it’s not one of Gaia’s Eggs?

  5. MadScientist

    Wow – the Cassini orbiter is still functional? How long has that been? To think that all these Luddites keep blabbing that Radionuclide Thermal Generators are evil.

  6. “In science, if one object has two weird things about it, chances are they’re related.”

    That made me laugh out loud. I never thought of it that way before.

  7. Joe Meils

    Okay, so this means we should kick Mimias out of the “Moon” catagory, and reclassify it as something else entirely. “Space Egg” maybe. After all, there are probably hundreds of these space eggs floating around out there… they can’t ALL be listed as moons, can they? Call the European Astronomical Conference! Find out who discovered Mimias! If he’s an American, all the better! Stick it to as many American discoveries as possible! Because, as we all know, only old Europe owns THIS solar system, bitches!

  8. QUASAR

    Mimas has one big crater on it!

  9. Nigel Depledge

    The BA said:

    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 radius pointing along its orbit is 394.4 km.

    Whoa!

    That’s one seriously not-spherical object!

    Unless you meant diameter along its orbit . . . ?
    😉

  10. «bønez_brigade»

    Someday, we’ll get there…

  11. Todd W.

    @Nigel Depledge

    No. Radius. It’s the classic cigar shape. After all, Phil said it was a UFO.

  12. That’s no moon… it’s an EGG!

  13. hhEb09'1

    I’m not sure why we use the phrase “egg-shape” here. When talking about prolate ellipsoids (which Mimas is not, it’s a more a tri-axial ellipsoid), the canonical shape is a rugby ball, no? Or, sometimes, a football. Doesn’t “egg” connote that one end is heavier than the other?

    Probably has something to do with the spring equinox. :)

  14. wfr

    Are you sure you meant to say “egg-shaped” and not “ellipsoidal?” Eggs are bigger at one end.

  15. Nick G.

    @ Todd W.
    No. I think he meant diameter since Phil later says, “The diameter pointing to Saturn is the largest”. If he meant radius, then the diameter along its orbit would be close to 789.9 km, significantly larger than then diameter pointing to Saturn at 414.8 km.

  16. wfr

    Which brings to mind another question: is there a mathematical term for “egg-shaped?”

  17. Gary

    Phil, what can all the flaws and cosmic ray tracks on the image tell us? That’s almost as interesting as the Mimas’s non-sphericity.

  18. Todd W.

    It’s unprocessed, meaning the cosmic rays and other flaws have not been cleaned up yet.

    And here I was thinking, “Why are stars visible in the moon? Is it a clear, translucent sphere…errr….egg?”

  19. chili

    UFO??? Me thinks one is trying too hard to draw in more hits to their blog. What’s Unidentified about Mimas? I can’t see any other reason for using the phrase UFO in this context other than to get more hits and make this great blog look like it’s more popular than it is. Seems a little unworthy of Phil to be doing this, unless the Hive Overmind is pushing this subtle but ethically questionable practice. Even when one considers the reference to the Death Star, it’s a pretty well known object and not exactly a UFO (unless one is considering all spacecraft as UFO’s, in which case then the ISS, Voyager, etc are all UFO’s – NOT!! Or is it only extra-Terran spacecraft that qualify as UFO’s?)

  20. Todd W.

    Oh, and for the humor impaired, 😛

  21. All I can think of when I hear the name Mimas is the first Red Dwarf novel.

  22. Brian Schlosser

    Heres a good shot of Mimas looking very ovaloid…

  23. Brian Schlosser

    Well, that didn’t work… whats the image tag rule here?

    anyway,

    http://en.wikipedia.org/wiki/File:Mimas_shape.jpg

  24. Ian Regan

    Enceladus is also somewhat ellipsoid-shaped:

    513.2 × 502.8 × 496.6 km

  25. Brian Smith

    The diameter pointing to Saturn is the largest…. 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.

    Couldn’t centrifugal force account for the pole-to-pole diameter being the smallest? It has very low gravity, and a rotational/orbital period of less than a day.

  26. Matt

    @zandperl/Joe Meils:

    “That’s no moon — it’s a dwarf moon” (or moonoid/Mimasoid/…)

  27. The Other Ian

    Phil,

    And another thing: the crater Herschel sprawls across the Mimasian surface, the biggest crater with respect to its parent body known.

    What is the basis for this statement, if you don’t mind my asking? The way I figure it, using numbers from Wikipedia, Herschel’s diameter is 66% of the mean radius of Mimas. In comparison, Stickney’s diameter is 81% of the mean radius of Phobos. So Stickney would seem to be larger with respect to its parent body.

  28. chili

    Interesting… Something a little unflattering about possible actions on the blog and it gets censored. Maybe there was something to my thoughts after all. Conspiracy theory??? 😉

  29. The Other Ian

    Okay, I redid the calculation using surface area instead of diameter, and in that regard Herschel turns out to be more than twice as large as Stickney in proportion. Which is surprising to me, because Stickney just looks bigger.

  30. Sorry, the word “radius” was a typo. I changed it to “diameter”.

  31. And chili, there is software that automatically holds up some comments in moderation. As you will note it is now posted.

  32. The Other Ian, I think Matilde holds the record for biggest craters relative to size. Comes of being a giant bean bag. I imagine Phil was thinking of round(ish) bodies, though.

  33. Jim N.

    Even if you limit it to round moons, Tethys’ crater Odysseus is 37.5 the moon’s diameter (400 km vs. 1066 km), compared to Herschel being less than 1/3 Mimas’ mean diameter. Iapetus’ two largest craters are 39% and 34% its mean diameter. And depending on what you call a crater, the South Pole-Aitken basin is 72% the Moon’s diameter.

  34. # wfr Says:
    April 16th, 2009 at 9:20 am

    Which brings to mind another question: is there a mathematical term for “egg-shaped?”

    Pretty sure its “ovoid,” which is what we use in archaeology when we find something egg-shaped.

  35. «bønez_brigade»

    @Brian Schlosser,
    Your img-tag-foo looks good from the page source. My tentative hypothesis is that the Discover bloggregate’s SW has something against colons (:) being in a URL.

    To attempt to rule out myself as having any special img-posting privileges, I’ll re-post your exact line of code:

  36. «bønez_brigade»

    Nope, no special privileges for me; so, the tentative hypothesis stands.

  37. Michelle

    Sure it looks like the death star… If Darth Vader was drunk.

  38. Jim N.

    …although, I gotta admit that Herschel looks more impressive than any of the larger craters mentioned.

  39. The diameter pointing to Saturn is the largest…. 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.

    Phil, as someone noted, it’s the spin. Encleadus is similarly shaped (A, B, and C radii are 256.6 km, 251.4 km, and 248.3 km), for example. In fact, most of the medium-sized moons of Saturn follow this trend (from my quick check). Titan is spherical to within available measurements, it appears. (I’m checking the spice kernels used for spacecraft planning and navigation, incidentally.) More extreme cases can be found with the moonlets like Pan and Atlas. In that case, they fill their Roche lobes and are fairly lemon-shaped. (In those cases, the B and C axes are even closer relative to the A axis, but that’s to be expected.)

  40. Clair

    I’m with Gary on this one.

    Phil, what can all the flaws and cosmic ray tracks on the image tell us? That’s almost as interesting as the Mimas’s non-sphericity.

    Comparing this to seeing a video of putting a CCD in the way of a stream of protons… How do these cosmic rays affect the equipment? Does its CCD have a higher tolerance before becoming permanently damaged?

  41. Brian Smith

    So my completely off-the-cuff guess was right? Huh.

  42. Mark

    Stupid people. goddidit.

  43. In fact, Mimas is egg-shaped!

    The diameter pointing to Saturn is the largest, just as you’d suspect.

    Phil, I daresay even you would have trouble balancing an egg that big.

  44. IVAN3MAN

    @ Brian Schlosser, and @ «bønez_brigade»,

    The mistake that both of you have made, when attempting to post that image, was that the URL linked to the file page containing the image rather than the image itself. In order to access the image source at Wikipedia, it is necessary to click on the image and that will direct you to the image only source URL, which for this image is: upload.wikimedia.org/wikipedia/en/0/08/Mimas_shape.jpg

    Like this…

    Saturn's moon Mimas
    Mimas, imaged by Cassini, looking notably egg-shaped.
    (Click on the image for the Wikipedia article.)

    VOILÀ! 😎

    N.B. Images exceeding 600 pixels must be re-sized by specifying width="600" within the image source tags, because that is the image size limit for posting here.

  45. The Other Ian

    Even if you limit it to round moons, Tethys’ crater Odysseus is 37.5 the moon’s diameter (400 km vs. 1066 km), compared to Herschel being less than 1/3 Mimas’ mean diameter. Iapetus’ two largest craters are 39% and 34% its mean diameter. And depending on what you call a crater, the South Pole-Aitken basin is 72% the Moon’s diameter.

    Odysseus covers approximately 3.5% of the surface area of Tethys, whereas Herschel covers only 2.7% of the surface area of Mimas. Yep, I’d say that Odysseus wins.

  46. Brian Schlosser

    @IVAN3MAN and @«bønez_brigade»

    Thank you! I shall remember that next time!

    Related note: Wiki tells us that the adjectival form of “Mimas” is not “Mimasian” but rather “Mimantean”… Neat!

  47. «bønez_brigade»

    @IVAN3MAN,
    D’oh, I should have noticed that. Thanks.
    (consider my tentative hypothesis thrown out)

  48. JB of Brisbane

    Did I hear somebody say, “Commence primary ignition”?

  49. Asimov Fan

    @ !AstralProjectile : (April 16th, 2009 at 7:29 am)

    (Obligatory John Varley reference)
    Are we sure it’s not one of Gaia’s Eggs?

    Great reference there! 8)

    Wonder how many others will get it?

    I loved that ‘Titan’ novel of Varley’s & Cirocco “Rocky” Jones and her trusty companion Gaby are among my favourite & the most appealing of SF heros.If folks here haven’t read that novel – I’d certainly reccomend it – a great rollicking fun read! :-)

    Two very minor nits picked though : ‘Gaia’ was actually spelt ‘Gaea’ and was also named Themis. 😉

    Mimas as frozen death star comparison was also awesome thanks «bønez_brigade»

    … & I’ll second that Mimas is NOT an “Unidentified Flying Object” but a Quite Well Known & Properly Identified Moon instead. QWK&PIM anyone?

  50. Asimov Fan

    Oh & you know what caused the “Death Star” crater folks?

    That happened back when the Shooting Starr struck the moon and burrowed into its surface to escape the pursuing Sirians in Isaac Asimov’s young adult “Lucky Starr” series novel ‘The Rings of Saturn!‘ 😉

    Specifically Chapter 7 ‘On Mimas’ (Or “in Mimas” actually! 😉 ), first published in 1958 – long before any spacecraft had visited Saturn -under the pseudonym Paul French.(Doubleday, 1958 – my copy, New English Library, 1974.) One of the better Lucky Starr books, IMHO, naturally. :-)

  51. Spectroscope

    Thinking elliptically, when it comes to astronomical egg shapes, many stars esp. of types B & A are NOT round but have instead been warped into egg-shapes by extremely rapid rotation, basically spinning very fast, eg. Altair, Regulus and Achernar.

    Regulus (Alpha Leonis) has been imaged by the CHARA array and found to be very oblate – Ken Croswell describes it as “pumpkin shaped” -an image of this can be found on its Wikipedia page & a Ken Croswell article online.

    Altair has also been mapped showing “limb darkening” – like Regulus – where the poles are much hotter & brighter than the further out equatorial regions.

    Other stars such as very close binaries are distorted by tidal forces eg. Sheliak (Beta Lyrae) and possibly also Mira (Omicron Ceti) which is also affected by pulsating.

    Planets Saturn and Jupiter are also notably flattened as a result of their rapid rotation.

    However in our own solar system the best example of an ellipsoidal, egg-shaped fast spinner is probably the trans-neptunean ice dwarf planet Haumea. (ex-EL61 2003 -at leat I think 03?)

    So, Mimas is far from being alone in that trait anyway …

  52. Click on my name for a link to Ken Croswell’s “The Lion’s Pumpkin-shpaed heart” article on Regulus complete with an illustration comparing the increasing “egginess” of our Sun, Altair, Regulus & Achernar.

    The illustration was also used in Astronomy magazine some issues back last year.

    As noted, Wikipedia’s entry on Regulus has more on that stars flattened shape incl. an image via the CHARA array.

  53. Now ( Only if you’d like to, of course! 😉 ) click on my name for alink to the Wikipedia page for Haumea.

    This contains a photo plus an artists illustration and much more showing just how flattened this fast-spinning distant little world (with two moons of its own) really is.

    The “egginess” of Regulus, Haumea and esp. Achernar -theflatetst star so far known all make Mimas ellipsoidity (if that’s a word) seem a real yolk!

    (… Sorry.)

    —-
    Accompanying doggrel

    Achernar’s the flattest star
    It spins so very fast
    You wouldn’t want to live there though …
    It set you on your a**!

    Achernar is hottest too
    Of our skies brightest ten.
    You burn up if you got there
    So its probably good its far! 😉

    (NB. Achernar or Alpha Eridani is 144 light years away, spectral type B3, ninth brightets star in apparent magnitude after Sirius, Canopus, Alpha Centauri, Arcturus, Vega, Capella, Rigel, Procyon with Betelgeux following as the tenth if you’re curious ..)

  54. tjm220

    Phil has a special fondness for eggs so Mimas is egg shaped.

  55. Egg shaped? Does it stand on its end at equinox?
    😉

    J/P=?

  56. Torbjörn Larsson, OM

    Okay, so this means we should kick Mimias out of the “Moon” catagory, and reclassify it as something else entirely.

    Balls, now it is planet envy?!

  57. csrster

    I’m still puzzled. When I hear the word “egg-shaped” I think of something quite different from an ellipsoid. A triaxial ellipsoid has three planes of symmetry, whereas an egg has a single symmetry axis (see e.g. http://en.wikipedia.org/wiki/Ellipsoid#Egg_shape). So which is Mimas?

  58. c1234

    cooooooooooooooooooooooooooooooooool awsome!!!!!!:) :p 😀

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