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.
 |
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:
 |
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.
April 16th, 2009 at 7:14 am
Wouldn’t it be possible for a big asteroid impact to knock a planetal body out of its spherical shape?
April 16th, 2009 at 7:14 am
Imo Mimas is a death star : http://inquizition.files.wordpress.com/2007/10/mimas.jpg
April 16th, 2009 at 7:17 am
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?
April 16th, 2009 at 7:28 am
“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…..
April 16th, 2009 at 7:29 am
(Obligatory John Varley reference)
Are we sure it’s not one of Gaia’s Eggs?
April 16th, 2009 at 7:33 am
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.
April 16th, 2009 at 7:36 am
“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.
April 16th, 2009 at 8:00 am
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!
April 16th, 2009 at 8:03 am
Mimas has one big crater on it!
April 16th, 2009 at 8:16 am
The BA said:
Whoa!
That’s one seriously not-spherical object!
Unless you meant diameter along its orbit . . . ?
April 16th, 2009 at 8:17 am
Someday, we’ll get there…
April 16th, 2009 at 8:21 am
Mimas is an egg-shaped UFO | Bad Astronomy | Discover Magazine
Astronomy | 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.
April 16th, 2009 at 8:22 am
@Nigel Depledge
No. Radius. It’s the classic cigar shape. After all, Phil said it was a UFO.
April 16th, 2009 at 8:25 am
That’s no moon… it’s an EGG!
April 16th, 2009 at 9:17 am
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.
April 16th, 2009 at 9:19 am
Are you sure you meant to say “egg-shaped” and not “ellipsoidal?” Eggs are bigger at one end.
April 16th, 2009 at 9:19 am
@ 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.
April 16th, 2009 at 9:20 am
Which brings to mind another question: is there a mathematical term for “egg-shaped?”
April 16th, 2009 at 9:28 am
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.
April 16th, 2009 at 9:31 am
And here I was thinking, “Why are stars visible in the moon? Is it a clear, translucent sphere…errr….egg?”
April 16th, 2009 at 9:31 am
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?)
April 16th, 2009 at 9:31 am
Oh, and for the humor impaired,
April 16th, 2009 at 9:37 am
All I can think of when I hear the name Mimas is the first Red Dwarf novel.
April 16th, 2009 at 9:56 am
Heres a good shot of Mimas looking very ovaloid…
April 16th, 2009 at 9:57 am
Well, that didn’t work… whats the image tag rule here?
anyway,
http://en.wikipedia.org/wiki/File:Mimas_shape.jpg
April 16th, 2009 at 9:58 am
Enceladus is also somewhat ellipsoid-shaped:
513.2 × 502.8 × 496.6 km
April 16th, 2009 at 10:01 am
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.
April 16th, 2009 at 10:01 am
@zandperl/Joe Meils:
“That’s no moon — it’s a dwarf moon” (or moonoid/Mimasoid/…)
April 16th, 2009 at 10:01 am
Phil,
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.
April 16th, 2009 at 10:06 am
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???
April 16th, 2009 at 10:08 am
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.
April 16th, 2009 at 10:34 am
Sorry, the word “radius” was a typo. I changed it to “diameter”.
April 16th, 2009 at 10:34 am
And chili, there is software that automatically holds up some comments in moderation. As you will note it is now posted.
April 16th, 2009 at 10:41 am
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.
April 16th, 2009 at 10:51 am
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.
April 16th, 2009 at 10:58 am
# 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.
April 16th, 2009 at 10:58 am
@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:
April 16th, 2009 at 10:59 am
Nope, no special privileges for me; so, the tentative hypothesis stands.
April 16th, 2009 at 11:10 am
Sure it looks like the death star… If Darth Vader was drunk.
April 16th, 2009 at 11:43 am
…although, I gotta admit that Herschel looks more impressive than any of the larger craters mentioned.
April 16th, 2009 at 11:52 am
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.)
April 16th, 2009 at 12:04 pm
I’m with Gary on this one.
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?
April 16th, 2009 at 1:00 pm
So my completely off-the-cuff guess was right? Huh.
April 16th, 2009 at 1:29 pm
Stupid people. goddidit.
April 16th, 2009 at 1:31 pm
Phil, I daresay even you would have trouble balancing an egg that big.
April 16th, 2009 at 2:02 pm
@ 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…
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.
April 16th, 2009 at 2:43 pm
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.
April 16th, 2009 at 2:56 pm
@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!
April 16th, 2009 at 2:59 pm
@IVAN3MAN,
D’oh, I should have noticed that. Thanks.
(consider my tentative hypothesis thrown out)
April 16th, 2009 at 3:03 pm
[...] the original post here: Mimas is an egg-shaped UFO | Bad Astronomy | Discover Magazine Related to UFO’s reading: Visitors: A New Look at UFOs Interdimensional Universe: The New Science [...]
April 16th, 2009 at 6:17 pm
Did I hear somebody say, “Commence primary ignition”?
April 17th, 2009 at 12:09 am
@ !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!
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?
April 17th, 2009 at 12:22 am
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. 
April 17th, 2009 at 12:36 am
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 …
April 17th, 2009 at 12:42 am
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.
April 17th, 2009 at 1:02 am
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 ..)
April 17th, 2009 at 11:18 am
Phil has a special fondness for eggs so Mimas is egg shaped.
April 17th, 2009 at 11:31 am
Egg shaped? Does it stand on its end at equinox?
J/P=?
April 17th, 2009 at 3:45 pm
Balls, now it is planet envy?!
April 18th, 2009 at 2:24 am
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?
October 17th, 2009 at 8:22 am
cooooooooooooooooooooooooooooooooool awsome!!!!!!:) :p