That’s no moon– wait, yes it is

By Phil Plait | November 30, 2005 10:00 pm

The Cassini Saturn probe never seems to disappoint. This latest image is another stunner:

That image shows Saturn’s weird moon Mimas, which is uncannily like the Death Star, floating in front of Saturn’s magnificent rings. Cassini was pretty far out when it took this image; nearly twice as far from Mimas as the Earth is from the Moon.

I’ve written about Mimas before, but while poking around the web to find out more info just now, I found out that Mimas is not even close to being a sphere! Check this out:

That’s amazing! I had no clue it was so ovoid. This image exaggerates the effect somewhat, the top part of the moon as seen here is in shadow, so it makes Mimas look more egg-shaped than it really is. But the equatorial diameter is 10% larger than the polar diameter, which is a lot, given its 400 kilometer average diameter. Compare that to the Earth, where the equatorial diameter is only about 0.6% bigger than the polar diameter. The Earth is far more round than Mimas. Earth’s oblateness is caused by its rotation: basically, it’s just the centrifugal force* making the Earth bulge. Mimas doesn’t spin nearly fast enough for that to be the reason. So I’m not sure why Mimas is so goofy looking (tides from Saturn? Maybe; it’s closer to Saturn than the Moon is to Earth). I’ll ask Carolyn Porco, leader of the Cassini imaging team, when I meet her at the Amaz!ng Meeting in January. Gloat gloat.


* Yes, the centrifugal force is real. Please don’t leave angry comments or emails here. The centripetal and centrifugal forces are the same thing, just seen in different frames of reference. People get all snarky about this, and I don’t know why. If you sit in a car that is making a turn, you darn well feel a force pushing you. If there were no force, you wouldn’t feel it. It’s really that simple.

CATEGORIZED UNDER: Cool stuff

Comments (54)

  1. Andy

    is it’s rotation synced with it’s orbit (i forget the term) like luna? that could make the near side bulge (just like luna), but i don’t see how that could possibly make the equator bulge out like that. you want my theory? (bear in mind i’m a HS freshman, so go easy on me :p)

    way back, perhaps when mimas was a little gooier, perhaps not, it was spinning fast enough to cause the “flattenting” observed. over time, it’s rotation got slowed by tidal forces to match it’s orbit (again, like luna), and perhaps it solidified, if it was even warm enough to be “gooey” (certainly not “molten” though).

    as for herschel, he came along long after mimas got flattened out, and the crater doesn’t seem to be related to the ovoid shape.

    /my two cents. have a few whacks at it

  2. Kyle_carm

    Sounds like a pretty decent hypothesis to me Andy, good use of technical terms also, “gooey”. It is mainly made of water ice so a certain amount of gooeyness as it was forming is very possible and with a large impact like Herschel it might have caused enough heating along with tidal forces to cause a pronounced amount of oblateness. Also just my 2 cents. (Hmmmm, on second thought it was probably the FSM that cause the crater, he bumped with a meatball.)

  3. Andy

    while this is interesting, it’s nothing compared to Iapetus. afaik, we still don’t have any better theory than clarke’s “celestial beacon from the aliens/star-people/FSM/etc. perhaps in the near future we can get a series of general-pupose lander probes on the cheap to go take a look at some of the outer planet’s moons.

  4. Andy, I believe all the moons in our solar system are tidally locked (the term) with their respective planets (I could be mistaken though, anyone know for sure?).

  5. Oh, and that’s an awesome picture of Mimas. :)

  6. Chris CII

    [Quote]
    The Cassini Saturn probe never fails to disappoint.
    [/Quote]

    So the Cassini probe always succeeds in disappointing you ? You must be a very demanding person. ;-)

    Stunning images, and a very interesting blog.

  7. Tom

    Oh man, I saw the photo and thought it WAS the death star, so slightly dissapointed, but still it’s pretty cool and my desktop wallpaper until I find another good astronomy picture.

  8. Dan Gerhards

    Eric: Many of the outer moons of the gas giants are not tidally locked. They are too far away to experience strong tides, so there hasn’t been enough time for it to have happened yet.

  9. Centrifugal… centripetal… for every force there’s an equal and opposite reaction force. Duh.

  10. Cindy

    Runolfr,

    I think Phil put that footnote in for me because I emailed a “snarky” comment to him when I read it in his book. ;-)

    It’s not really a Newton’s third law action/reaction pair. Centripetal force is what you use if you’re in a non-rotating coordinate system. Centrifugal is what you use if you’re in a rotating coordinate system. When you’re in a car and go around a corner, you feel the centrifugal force pushing out because your perspective is in a rotating coordinate frame. Somebody standing by the side of the road would say you have a centripetal force pointing in.

    Can’t you tell I’m a high school physics teacher? ;-)

    Cool picture! Phil, I hope you will pass along the reasons why it’s not spherical to us in a future blog.

  11. Ashley Zinyk

    “The Cassini Saturn probe never fails to disappoint.”

    I think you mean “Cassini never fails to amaze” or “Cassini never disappoints”. Now, the shuttle program, that regularly disappoints.

  12. Phil, that’s a great image, thanks for posting it. Also, thanks in general for the blog, I read it every day and it’s wonderful to see someone so passionate about their job. Especially when the job in question is as cool as astronomy! Keep up the good work.

  13. Leon

    About the whole cetripetal/centrifugal force thing…

    My understanding is that centrifugal force isn’t technically a force. It’s the inertia of motion: a body wants to move in a straight path, and when it’s pushed/pulled to the side, the inertia feels like a force acting on it.

    So centrifugal force may not technically be a force, strictly speaking, it acts just like one–so for day-to-day purposes there’s no reason not to call it one.

  14. Oops! I fixed the opening line.

    And there is an explanation for Iapetus’s ridge. I will blog about it at some point.

  15. BA, I’m sorry to say that I don’t share your disappointment. :-D

    And, oh yeah that old nitpicking discussion about centrifugal force Vs. inertia of motion.

  16. aiabx

    There’s just something about moons and rings together that looks unreal. I think we’re just not used to seeing multpile objects together. Seeing images like that is like taking a step into the science fiction future I dreamed of as a child.

    As for centripetal and centrifugal, I’d love to argue about it, but I’m all unfortunately sober.
    -Andy B

  17. Darn! I took to long to hit “Submit Comment”.
    Better do the Time Warp, put your hands on your hips……

  18. Irishman

    The BA Said:
    >I’ve written about Mimas before, but while poking around the web to find out more info just now, I found out that Mimas is not even close to being a sphere! Check this out: [snip]

    >That’s amazing! I had no clue it was so ovoid. This image exaggerates the effect somewhat, the top part of the moon as seen here is in shadow, so it makes Mimas look more egg-shaped than it really is.

    Okay, let me get this straight: you wish to show that Mimas is ovoid and not spherical, then you use a picture that has an optical illusion to make it even more ovoid than it is? That seems like a poor choice of demonstration. That picture is hindered by the shadow hiding part of the object. Might as well show us a crescent image of Mimas and use it to prove your point that Mimas isn’t spherical. Your main point about the shape of Mimas is that the equatorial diameter is larger than the polar diameter – 10% larger – but I can’t tell that from the picture you’ve shown. All I can tell is that the upper left is “missing”. Don’t get me wrong, the picture is cool, I just don’t think it has anything to do with your point, and using it there is misleading. IMO.

    Cindy is correct, it’s not an action-reaction pair. When you are riding in your car and make a turn, there are two effects on your body. One is the force of the car seat against you. That is the Centripetal (center-seeking) Force. The second effect is your inertia, i.e. momentum to continue in the direction you were going. We call that the Centrifugal (center-fleeing) Force, because from your vantage point, you don’t feel a pull from the car seat, you feel your body lunge away from the car seat. Ergo, the effect you detect is what you call the “force” you feel, regardless of what is actually moving you (the “real” force).

    We could all comfortably define the “real” force as the force that makes you change motion versus inertia which is, by definition, maintaining your previous motion. Except General Relativity tells us that the Force of Gravity is not a “real” force, either, it is an inertial effect due to the curvature of space-time. As long as we want to call gravity a force, it’s hypocritical to be pedantic that Centrifugal Force isn’t really a force.

  19. Sudro II

    I can’t speak for anyone else, but I know the reason folks from my high school would be snarky about the whole centripetal/centrifugal force thing is that our physics teachers drilled into us the idea that centrifugal force was an illusion and that centripetal force was the actual actor. Cindy’s explanation above actually makes more sense and I hope that’s what kids are learning in high-school classrooms now.

  20. Tim G

    Accelerating and rotating frames of reference are not legitimate, you freak. And keep General Relativity out of this.

    ;-)

  21. Tim G

    BA,

    Why are you gloating about meeting Carolyn Porco? She’s no celebrity. When was the last time she was on the cover of People with Tom Cruise?

  22. Sudro II: I can only speak for myself, of course, but I’m only a few years out of high school physics and my teacher was adamant about the centripetal/centrifugal thing. For that reason, I’m really glad for Phil’s footnote: I suspected that my physics teacher was lying/nitpicking about the whole thing since I’ve seen actual physicists use centrifugal unapologetically, but this is the first time I’ve seen anyone take the issue head-on. I never liked my physics teacher, anyway, so I’m kinda pleased that he was being a nitpicky dick.

  23. Leon

    Irishman says:

    General Relativity tells us that the Force of Gravity is not a “real� force, either, it is an inertial effect due to the curvature of space-time.

    Ah, now you’ve got me. Well, darn my poor old Newtonian brain! ;)

  24. Irishman

    To be fair to high school (and college) physics teachers, let’s look at definitions a bit. A Force is something that causes a change in motion. Inertia is resistance to change of motion. If you’re dealing with rotating systems, you can either treat your point of view as rotating and the environment as a stationary frame of reference, or you can treat your POV as the stationary frame of reference and the environment as rotating around the object. The first is called an inertial frame of reference, the second is non-inertial. In a non-inertial frame of reference, intertia gets treated like a force (like centrifugal). But by definition inertia isn’t a force, it’s resistance to force. How can resistance to changing your motion cause a change in motion? That’s a bit of an oxy-moron.

    Also consider that high school physics teachers are trying to get their students to focus on inertial frames of reference. In those frames, the cause of motion is pulling you toward the center of the curve, not pulling you away from the center of the curve.

    The only downside to that distinction is that inertia feels like a force. Treating it like a force (in the proper reference frame) makes the math work out. So the distinction is more philosophical than physical.

  25. Karnalis

    An egg-shaped moon… So, does that mean you could balance Mimas on its end (equinox notwithstanding)? :P

  26. You know, I only JUST “got” the title of this post! It is, of course, a Star Wars reference. I’m such a bad, bad nerd!

  27. NelC

    Irishman, I suspect the reason BA used this image of Mimas is that there aren’t any showing a fully illuminated hemisphere. At least, a quick google couldn’t find any for me.

  28. Leon

    So, as long as we’re noting the Star Wars reference, I have to ask: is The Force centripetal or centrifugal? If it does truly bind the universe together, doesn’t that mean it must be centripetal?

  29. Irishman

    NelC Said:
    >Irishman, I suspect the reason BA used this image of Mimas is that there aren’t any showing a fully illuminated hemisphere.

    So what? My point is that this image doesn’t show the effect he is trying to demonstrate, and is misleading. He shouldn’t use it to prove his point. If he can’t find an image that does work, then don’t post an image. Or post an image to show Mimas, but don’t claim it demonstrates the point.

  30. Irishman, the image does indeed show that Mimas is ovoid. It also happens to exagerrate the effect. But if the image showed Mimas fully lit, you’d still see it as ovoid. It’s the only image I’ve seen showing the effect, and I stumbled on it right before posting the entry, so I added it.

  31. Leon: click on my name for an explanation of the Force.

  32. Irishman

    Sorry, Phil, from the picture shown, I can’t identify that the ovoid nature of the image is due to a real difference in shape, only that the shadow is blocking part of the moon making it appear ovoid. Perhaps it’s just me, but that image does not demonstrate that Mimas is ovoid, only that it looks ovoid with that lighting.

    I can’t even identify the poles or the equator in order to judge. Am I supposed to assume the poles are oriented North up, like standard Earth map alignment?

  33. Nigel Depledge

    Tom said:
    “Oh man, I saw the photo and thought it WAS the death star, so slightly dissapointed, but still it’s pretty cool and my desktop wallpaper until I find another good astronomy picture. ”

    Er, so does that mean that you are disappointed that the Death Star is NOT in our solar system and heading our way …?! Me, I’d be relieved!

    The BA said:
    “And there is an explanation for Iapetus’s ridge. I will blog about it at some point.”

    Yes, if you go to Richard Hoagland’s website, he explains it all !

    Just kidding!

  34. Tracey

    Irishman, stop whining already. It’s just a picture.

  35. Shaggy

    It’s really, really simple how Mimas got that shape. It’s really close to Saturn, and it probably was heated enough by some whacking it got early in life to make it semi-molten, probably an even bigger impact than Herschel. The Saturn system is one of the messiest in the Solar system, with massve rings of debris and huge impact craters everywhere, so a large impact like this well after planetary formation is plausible.

    Imagine the tides that the Moon causes on Earth, and what would happen if they were instantly frozen. Earth would have two tidal bulges and assume a more ovoid shape. If Mimas was warm enough to be flexible, it would have had similar tides. Since Saturn is so massive, and Mimas has a much smaller mass, I’d imagine that the tides on Mimas would be on the order of kilometers. Someone with a background in physics could probably do the calculations easily. If it were tidally locked so that the tides didn’t move over the surface, it would have cooled into the ovoid shape we see today. I’m not sure if that would happen the same way if it weren’t locked.

    Now, I’d really like to hear a good explanation for the bulge on Iapetus.

  36. NelC

    I think Irishman’s got a point, though. Unless you have Leonardo da Vinci’s eye for a perfect circle, and your monitor isn’t misproportioned, and you know the extent of the dark side of Mimas in this picture, it doesn’t illustrate Mimas’ mis-shape at all well.

    Now, if it had a guideline showing the dark edge of Mimas, and another one showing a true circle, then one could appreciate the true ovoidity of the thing.

  37. NelC

    Tried drawing circles and ellipses in Photoshop, but not having much luck with that picture; I’m just too uncertain about where the terminator lies and I can make either a circle or a 10:9 ellipse fit.

    Had a bit better luck with the first picture, since that actually shows more of Mimas’ edge, thanks to Saturn’s ring:

    http://tinyurl.com/bs563

    You can just about see that the polar diameter is a smidgen less than the equatorial in this picture, though not as much as 10%. I’m guessing that Mimas’ equator is actually elliptical, and that the bulge is aligned with Saturn (somewhere above the top of the picture), so this picture isn’t as useful as I thought. Still, you get a hint of the mis-shapeness of it.

  38. Tom

    Hell yeah I’m disappointed. A giant orbital battle station hiding in our solar system, how cool would that be? How much gravity would the death star exert though? I’ve checked it out and it’s diameter is 900km, making it bigger than Mimas, although it would have a lot less mass.

  39. Kyle_carm

    Hey Tom if you read David Weber remember that “the moon” aka Luna, is a battle station. If I remember right its name is Dahak, from the series Heirs of Empire I think.

  40. Irishman

    Thanks, NelC, that picture does a much better job of showing oblateness.

    Tracey Said:
    >Irishman, stop whining already. It’s just a picture.

    [whine]I’ll whine if I want to![/whine]

    Seriously, I just don’t think the picture shows what is claimed it shows. It certainly doesn’t to me. I could just as easily and accurately say this picture proves the Moon isn’t a sphere:
    http://www.nineplanets.org/luna.html

    There’s also this comment from The Bad Astronomer:
    >Irishman, the image does indeed show that Mimas is ovoid. It also happens to exagerrate the effect. But if the image showed Mimas fully lit, you’d still see it as ovoid.

    That’s a serious if. Yeah, if it showed Mimas fully lit, it probably would show the oblateness. But it does not. My point still stands, the oblateness is no shown in that picture as it is.

    If you don’t see my point, I’m sorry. If you see the oblateness that isn’t just an optical illusion, you have better eyes than me. If the BA wants to post pretty pictures, I have no objection. But if he makes a claim about what that picture shows, I have every right to scrutinize that claim, and to voice my disagreement.

    Look, I’ve said my piece about it. Phil can do whatever he wants on the issue – remove the pic, clarify the remark, let is stand. People can read my comments, too, and either agree or disagree. At this point, I’ve said what I need to say unless someone else brings it up.

  41. Anthony Wright

    Actually, I’d have to congratulate the Irishman for whining. After all, this site drills into us to “think” and not just take what we see for granted (There was actually a blog post about this recently too, with the film in the mine!). While I’m sure that most of us here trust the BA to give us reliable information, I think the Irishman’s request for a better image was not only worth a bravo, but exactly the kind of mentality that BA tries to get us into. AS the BA said, “Don’t just ask for proof: demand it!”.

  42. Nigel Depledge

    Tom said:

    “Hell yeah I’m disappointed. A giant orbital battle station hiding in our solar system, how cool would that be? How much gravity would the death star exert though? I’ve checked it out and it’s diameter is 900km, making it bigger than Mimas, although it would have a lot less mass.”

    Well, what I was getting at, Tom, was that if the Death Star were in our solar system, its next target was unlikely to be Rhea or Io (for instance), but most likely to be the only inhabited body in our system. I think the gravity it exerts would be the least of our worries. Or maybe it got overtaken by a Vogon Constructor Fleet, I dunno …

  43. Norm

    Centrifugal force is only real to people who don’t understand F=mA. Both the F and the A are vector quantities.

    Velocity is also a vector with speed & direction. Changing either over time is an acceleration. Moving the velocity in a circle at constant speed creates an acceleration vector that points toward the center of the circle. Or the “instant center” of a varying curve like an elipse. That’s what gravity does.

    You wouldn’t create an imaginary force holding your head back when the car accelerates forward, would you? You know it’s inertia, a property of mass. Lateral acceleration creates the same feeling, only “sideways. It’s still inertia. The problem is that you’re inside the car, itself a non-inertial reference frame.

    Norm

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