Just in case you need reminding how nice a place to live Earth is.

By Phil Plait | September 4, 2010 12:00 pm

When all is said and done, the outer solar system must be a pretty scary place to live. How’d you like to live in a location that has a battle-scarred history like this?


That’s Saturn’s moon Dione as seen in by the space probe Cassini in a recent pass of the small world. Earlier today (September 4, 2010), Cassini dipped to a height of less than 40,000 km (25,000 miles) above Dione’s surface — that’s about the same height as weather satellites above the surface of the Earth!

The landscape of Dione is shockingly battered. Craters are everywhere, indicating a fierce history of meteoric bombardment. It is also heavily littered with cliffs and chasms.

I learned something incredibly interesting looking up information about the moon: like all moons in the solar system, it’s tidally locked with its parent planet, meaning it spins at the same rate it orbits, always showing the same face to Saturn. This is a natural consequence of gravity and isn’t terribly surprising. What did surprise me is that the trailing hemisphere of Dione is more heavily cratered than the leading hemisphere!

Because of its spin-orbit coupling, Dione always has the same half of its surface pointing in the direction it orbits, and the other half pointing away. Think of it like a car: the front of your car always faces in the direction you travel, and the rear in the other direction (ignore going in reverse). On your car, the front windshield takes the brunt of any debris in your way; insects, gravel, and what-have-you tend to hit the front windshield, not the rear one.

But Dione has that backwards! The rear-facing half is more heavily cratered. This has prompted scientists to wonder if Dione actually got spun around by an impact some time in the past. Think about that! Over a long period of time, Dione got hammered by debris. The half facing into its orbit got more battered. Then there was some pretty big impact that hit it at an angle, and that gave the moon a bit of spin. Eventually, tides from Saturn’s gravity wore that spin down, and it settled into its current configuration, with the formerly leading hemisphere now facing the other way.

That to me is incredible. Even a small moon is a pretty big chunk of material, so imagine an impact — or even a series of impacts — so catastrophic that it actually spun the moon up and made it face the other way!

And then I learned that an impact big enough to do that to Dione would leave a crater only a few dozen kilometers across. Much, much bigger craters than that can be seen in images.

As I write this, the temperature outside is quite lovely, the sky is blue, and I’m yearning to hop on my bike and ride around the hills a bit. Earth is an amazingly beautiful and hospitable planet. Certainly it wasn’t always that way, but water and air eroded away the history of our violent past, and have left us with a pretty nice place to live. If you ever doubt that, take a good look at Dione’s surface. Things could be a lot, lot worse.

Image credit: NASA/JPL/Space Science Institute. Tip o’ the meteor deflection shield to Carolyn Porco.

Related posts:

Dione’s Atlas shrugged
Incredible quadruple transit on Saturn!
An otherwordly eclipse
Helene of Saturnian Troy

CATEGORIZED UNDER: Astronomy, Pretty pictures
MORE ABOUT: Cassini, Dione, Saturn

Comments (42)

  1. Dave L

    Okay, but what are all the white lines, which seem to be casting shadows?

  2. Randy A.

    I’m curious: is Dione turned exactly 180°? Is the moon a little lopsided, so that the current position and the former position are preferred? Because it seems to me that if Dione was a uniform sphere, the side that becomes locked facing Saturn would be random, and an exact 180° flip would be very unlikely.

  3. Orihara

    Tidally locked satellites tend to be slightly prolate sphereoids, rather than uniform spheres, IIRC. Thus, the most likely situation if it was tidally locked both before the hit and now would be to end flipped exactly 180 degrees.

  4. andy


    When did the IAU revise the definition of “moon” to exclude Hyperion?

  5. Greg in Austin

    “like all moons in the solar system, it’s tidally locked with its parent planet, “

    ALL moons? I thought only some were tidally locked. I’ll have to look that up!


  6. XPT

    What strikes me about some of Saturn’s moons is that at close-up they actually look like ice, or melted snow. Is it just me? They don’t have that crisp jagged look that out Moon has.

  7. “like all moons in the solar system, it’s tidally locked with its parent planet, “

    I think all major moons are tidally locked, but some of the smaller more distant orbiting moons are not.

    If you really sit and ponder what it would be like to leave Earth and colonize another planet or moon, it starts to lose its appeal quite rapidly. Especially when you look at the options located in our solar system. Even if we do find another Earth like planet rich with life outside our solar system, the time living in the void without our mother Earth to comfort us will be psychologically devastating. If any of you get the chance, check out “Packing for Mars”, a recently published book by Mary Roach. This has really opened my eyes to what living in space for an extended amount of time might be like. Suddenly I realize just how awesome living on Earth is, and my dreams of floating in space for 2+ years have lost their luster!

  8. @Dave L

    Looks like the light is coming from the right hand side of the photo. Look at the craters. Left side is illuminated and the right side is in shadow.

    Those are probably (for lack of a better word) valleys.

  9. I seem to recall a short story, possibly by Isaac Asimov, in which space travelers visit an alien world and find it incredibly hostile to life: corrosive oxygen in the air, oceans of universal solvent sloshing about, violent weather, scorching radiation, active tectonics….you know where this is going.

    I guess it’s all in the marketing.

    Oh, and these days I would add infested with vermin of the not-so intelligent kind.

  10. Simon

    I always have great trouble getting the surface features in images like this to go the right way. The craters always want to look like hills and the “valleys” look like ridges. I had to rotate the image 90 degrees so that the light came from above before I could see it properly.

    I suspect most astronomers have spent enough time looking at craters and honed their skills in these perceptual gymnastics that light direction doesn’t matter to them anymore, but maybe you could help us common folk by rotating the images so that the light comes from the top?

  11. I submit that when Maestro Cassini-Huygens is ready to retire (and may that be a long time from now), we somehow return this mechanical marvel to earth and make it the centerpiece of a grand museum dedicated to its glorious achievements. It is truly one of the wonders of the age. Saluto questa macchina meravigliosa!

  12. Markle



    Uh oh moons of the solar system! Pluto’s fate is upon you. If you don’t get yourself a tidal lock the IAU will demote you to ‘minor moon’ status. 😉

  13. Dave L: The white lines are canyons. The leading hemisphere of Dione is covered with “wispy” markings, which turn out to consist of systems of these canyons. Rhea has them too.

    XPT: The surfaces are ice. Really, really cold, hard ice, but ice. Our Moon isn’t ice, it’s rock–though the surfaces there have still been softened somewhat by being pounded down by micrometeoroids for billions of years.

    Tribeca Mike: There’s no way to get it back, absent sending another spacecraft to pick it up. The current plan is to send Cassini plunging into Saturn, to avert the extremely slim chance that it might contaminate any of the moons with Earth germs (NASA tries hard not to do this just in case there is indigenous life somewhere). But it’s going to make some close orbits between the rings and the planet before that, which should be awesome.

  14. RAF

    That image reminds me of the far side of the Moon…craters everywhere.

  15. cmflyer

    I don’t think tidally-locked necessarily means orbit=rotation. It just means there’s a mathematical relationship between orbit and rotation caused by tidal forces.

  16. Nick

    Could the observation that the leading edge of Dione appears less-heavilly cratered than the trailing edge be due to the fact that the leading edge will collide with much more dust than the trailing edge, thus smoothing the surface and giving the appearance of a more heavily-cratered trailing edge?

  17. NAW

    @ Nick, did you plan to have “trailing edge” line up like that? And that is a good point, I guess they need to check on how deep the regolith is on the leading edge. (somehow)

  18. Laura

    Actually, our own moon’s composition is so strikingly (hah!) similar to Earth’s that one leading theory on its origin is that a giant comet or asteroid hit Earth and threw all of that material into orbit. Major solar-system-shaping impacts have happened here too! Kind of scary. Definitely get out and enjoy that bike ride while you still can… muahahahaha!

  19. sil-chan

    Re: The Wisps:

    No matter how I flip the image and rotate the image, I cannot see them as being Canyons. They are physically floating above the surface.

    The ones dead center at the bottom cast a shadow that is not connected to them in any way. If it were a canyon, and light were coming from the left, top, or bottom, there wouldn’t be a shadow, and if it was coming from the right, the shadow would connect to the ridge at the top of the canyon…

    So can someone explain how they are canyons given the nature of light (with detail) or explain what they are if otherwise?

  20. alfaniner

    This is one of the most difficult optical illusions with craters/mesas that I’ve seen so far. No matter which way I turn it, I can only get the crater effect for a few seconds, then it snaps back to the opposite. Neat.

  21. It would seem there’s at least one alternative to “the leading hemisphere should be more cratered”… if the primary source for impactors was from crossing orbits with larger semi-major axis than the moon. In that case, the potential impactor will always have a velocity greater than the moon, and so there will be a bias to strike the *trailing* side of the moon. The possible implication is that a significant fraction of the Dione impactors may have come from an event like a moon break-up or interaction outside Dione’s orbit, which seems unlikely… but when you look at the established resonances (and things that should have had resonances swept through them), and things like largange point satellites etc., it seems there’s a lot of potential chaos in the system.

    I’m not sure how you’d verify this hypothesis however – I suspect that if could date enough of the craters, you’d see a cratering peak associated with such an impactor-generating event. Which at least has the advantage of being a possible test for a hypothesis. As opposed to “is there a small impact event around that could have temporarily broken lock”, although I guess if it was of the right age, you might be able to find a crater around the right age and in the right location, and correlate that single crater’s age to the bias between the leading and trailing faces and… ugh.

    Fun to think about, for some of us astronomer-types however :).

  22. Buzz Parsec

    @Brian –

    Exactly! I was thinking about this not long ago in the context of the ISS, which is in a very low orbit. I couldn’t figure out how to express it; “semi-major axis” does the trick… Anyway, any object with a longer semi-major axis and an inclination within 45 degrees of the ISS (which is almost everything in orbit) would hit it from behind. Only objects with a smaller semi-major axis would hit it from the front, but these by necessity have a perigee lower than the station’s almost circular orbit, and would decay very rapidly due to atmospheric drag. Therefore the station should have most of its MMOD shielding on the aft end, which seems very counterintuitive.

  23. Tim

    I always think in terms of hemispheres facing the planet or facing away for tidal locking, so traling/leading edge was new and I had to sketch it…which leads to the obvious conclusion that Brian Davis describes: If there are other bodies at greater orbits that somehow drift in from instabilities then collisions are only possible on the trailing side. Why would such a radical proposal such as a spin flip collision be the preferred theory? I am transitioning into astronomy from physics, so I don’t have any clue what is known. I’m wondering if there are other observations of moons that display the same collision-littered asymmetry? Enough of those observations should invalidate the spin-flip theory…

  24. TheBlackCat

    It isn’t that surprising, didn’t something similar happen to Venus (flipped upside-down), Uranus (flipped on its side), and Earth (big chunk gouged out to form the moon)?

  25. Messier Tidy Upper

    @ ^ TheBlackCat :

    Well yes & no.

    Earth’s moon was almost certainly created in the “Big Splash” impact between Earth and a roughly Mars sized proto-planet which threw a huge amount of material into orbit which coalesced into our Moon.

    Pluto’s moons – all three of them Charon, Hydra and Nyx (& a possible Plutonian ring system) probably also arose the same way. Unless I’m very much mistaken, Pluto is tilted over at a fairly steep angle too.

    Venus might well have been knocked upside down by an impact – it probably was the case. (Although it may also have something to do with tidal factors notably the Sun.)

    Ouranos however, although we used to think that was the case, now seem s much more likely to have been tipped over by Saturn’s and Neptune’s gravitational influences. I’ll see if I can find a source or two for this and link it in a separate comment for you.

    As for Earth Vs Dione well given a good enough home there I’d love to live on any of the Saturnian moons. Just think of the view! 😉

    Not to knock Earth which is fantastic and definitely the easiest planet to live on but if we had people living on Dione and in the outer solar system generally – how cool (& interesting & an achievement) would that be! 😉

  26. Messier Tidy Upper

    Hmm … While looking for the Ournaos flipping cause I stumbled on this blog item which is pertinent here :


    Then this is the link for the flipside of the question of was Ouranos’ sideways-ness caused by an impact :


    Which seems to be another idea again involving a large moon being ejected.

    PS. If you’re wondering why I’m calling it Ouranos :

    All the planets except one are named after Roman gods, and Ouranos is the one exception. Unfortunately it has become the brunt of joke after joke due to a Latinization of the original Greek name, Ouranos. We do not call Poseidon Posidon, so why make Ouranos the exception? Ouranos is a magnificent planet that has been subject to a bromidic and stale joke for far too long.

    Source among svereal other places this “Ouranos not Uranus” Facebook group :


  27. Rae

    I know these are craters, I *know* they are, but I just cant stop my mind from reading them as lumps, like looking at bubble wrap! It is SO frustrating! I remember your post from a while back about the opticle illusion of craters, usually I can teach my brain to see right through illusions, but I guess looking at a distant moon is so alien I just can’t do it.

  28. Interestedbystander

    Funny, this is one time that I immediately saw them as craters and the lines as canyons. I had to work to get them to look like bumps. It is usually the other way. Go figure!
    The bright lines are from the sun striking the far rim of the canyon and ridge tops. The sun angle is very low and I’d guess that there is fairly clean ice exposed on the rims and or ridges. But I’m no astronomer or geologist.

  29. Messier Tidy Upper

    Oops that last link for the “Ouranos” facebook group just goes to my fb page rather than the one its meant to go to which is :


    Hopefully this will work this time. Sorry.

  30. Lupine

    But Ouranos is nowhere near as funny as Uranus.

  31. Matt McIrvin — awesome indeed!

  32. andy

    First off, Ouranos is no better or worse a transliteration than Uranus is. The values of the classical Greek letters do not precisely map to the letters in modern languages. Besides, if the Latin/Greek issue is such a bugbear to you, why not go the whole hog and call it Caelus?

    (And I suppose you spell the name of the Greek warriors as Akhilleus, Phoinix, etc., the notable Gorgon as Medousa, and the leader of the Argonautai as Iason?)

  33. Zucchi

    “Ouranos” would still be an exception: Greek instead of Roman. If you want to change the name, why not make it Caelus, the Roman god that more or less corresponds to Ouranos?

    Sil-chan: the light is coming from the right. The shadows are cast by the canyon walls on the right side. This isn’t mysterious. Go outside, take a screwdriver or something and carve a little trench in the dirt, running North and South. In early morning or late afternoon, look at the light and shadow. There you go.

  34. Edward James Olmos should play Dione in the movie…

  35. Deadly Pacifist

    I’m curious, is that thing supposed to still be geologically active in any way? I ask because in some cases the craters have wiped away the cliff features (as you’d expect if the fractures formed relatively early in the moon’s history, then got hit by meteors) but in other places the cliffs are quite defined and seem to interrupt the craters (as if they’d formed after the impact or impacts, like in the lower left there where the three impacts occurred inside the larger one and there’s still a clear cliff running through all the craters). I wouldn’t expect an icy crust to retain features like that through an impact, but then I didn’t go to a very good school, heh.

    Neat shots, that probe’s worth its weight in gold.

  36. tony

    Why would we expect the leading side to be more heavily cratered? The windshield analogy doesn’t apply here. The insects and gravel have no directional bias with respect to the ground, while the car does, hence the windshield takes the brunt of the collisions. But the debris that strikes the Dione also orbits Saturn. Some will orbit Saturn a little faster than Dione, and some a little slower. Dione should get rear-ended by the debris as often as it rear-ends the debris.

  37. And you think living in an oxygen reducing atmosphere is a PLEASANT place to live?

    Beg to differ.


  38. Tim

    Is that a Meerkat on Dione? Take a look far left and just below the middle – the ‘Face on Mars’ is dead, long live the Meerkat on Dione! It’s only a matter of time before the marketing men at a certain comparison site jump on this (Popular UK thing, in case you were wondering). I can’t see anything else in the picture now ha ha


  39. Gary Ansorge

    7. Eli Misel


    You’re exhibiting what we in the space colonization camp like to call “planetary chauvinism”, the idea that only a planet is an adequate place for humans to live.

    I’ll bet that the average Saudi Arab would gladly surrender living in their wonderful desert for the chance to inhabit an O’Nielle colony, with it’s perfect weather, gardens and forests and no damn mosquitos.

    The average oil field worker in Arabia adapted quite readily to living in air conditioned quarters, with their 7 foot ceilings. Outings to a mall were considered a treat and the sky was usually a queasy looking orange, so being outdoors wasn’t as much fun as one might think.

    Most people think we’ll be living in constrained tin cans in space. In the early days of exploration, that is pretty much a given but, successful exploitation of space resources must eventually lead to mega scale engineering projects, like 20 km long space colonies, with ceilings high enough to allow random cloud formations.

    Personally, with all the people I’ve seen spending their waking hours in malls, I expect humans would find similar space constructs quite adequate as a living environment.
    ,,,and the “sky” is always visible through windows(really THICK windows).

    The only real advantage to a planetary environment is that it is self maintaining,ie, it doesn’t require constant intelligent maintenance. Great for growing non-sentint species but a lousy place for a technological civilization.

    Gary 7

  40. Mike
  41. @Gary Ansorge


    I never really considered people living in desolate areas. I suppose they would be more apt to volunteer for a long winded trip through space. However, I still feel they would suffer psychologically without the blue skies of Earth. Very interesting to think about.

    If we are talking O’Neill cylinders for space travel though, COUNT ME IN!!! :)

  42. gravityhomer

    So I’m skeptical on the theory that the moon was spun 180 degrees as some other commenters are for three reasons:

    1) The impact of the crater would set it spinning and then it would have to stop again the opposite way. It could happen, but certainly is not a high probability.

    2) If there are many craters on the surface of the moon that are large enough to cause this type of spinning, then this must have happened many times in the past. The moon must have been given the spinning kick over and over, and the last one must have stopped it in this configuration. This configuration should be completely random. If this the theory is possible, then we actually do not know which direction the high cratered side was facing originally. It could be that it was facing out, or facing in. Also, not sure what the spin down time would be compared to time between impacts, but it might even still be spinning.

    3) As others say, why would the leading edge get hit more in the first place? Things usually orbit in the same direction. To make a crater you need a certain difference in velocity between the moon and an impact object. Assuming the smaller object is more likely to have the higher velocity this would mean objects traveling fast enough in orbit of the planet would come from behind. Objects not orbiting the planet that make an impact would come from any direction.

    I think we should look at what would cause one side of a satellite to be more cratered across all configurations (cratered side in, out, trailing, leading). Compare the theories and see what is most likely, because given the information that the moon could be sent spinning by an impact, then this configuration is random.


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