The solar system's weirdest moon

By Phil Plait | November 2, 2011 11:00 am

Our solar system is a fantastically bizarre place. There are worlds as varied as our imagination can grasp — in fact, they exhibit features we never imagined before we saw them up close. Storms larger than planets, moons with undersurface oceans, lakes of methane, worldlets that occasionally swap places…

… and that’s just at Saturn. But of all these, if I had to pick, I’d say the strangest place in the entire solar system would be the ringed planet’s distant moon Hyperion. Why? Well, maybe this will help: in September, when the Cassini spacecraft was within just 88,000 km (54,000 miles) of the weird little moon, it snapped this picture:

[Click to entitanate.]

Just looking at it, you get a sense of strangeness, don’t you? It’s little, only about 270 km (170 miles), but packed into that tiny moon is a Universe of weird. It looks like a sponge! Or more like a piece of packing foam that’s been pinged by a BB gun. It has a very low density — about half that of liquid water, even less dense than water ice, indicating it must not be entirely solid. It’s porous, like a sponge, or a pile of rubble.

And those craters… they just look funny. They have sharp rims, shallow slopes, and flat bottoms, and it’s thought that this is because of how crunchy Hyperion is. Instead of blasting out material like on rocky moons, impacts compress the surface, like punching a block of Styrofoam. The bottoms of many of the craters are dark, filled with hydrocarbons that form when sunlight changes the structure of simpler molecules.

That giant flat region on the right is actually a huge impact crater — you can see the central peak in the middle, typical for big impacts — and it reminds me strongly of the huge south polar impact basin on the asteroid Vesta, which itself is a weird place. But it can’t hold a candle to Hyperion.

I’ve talked about Hyperion before (see the Related Posts links below) but I can’t get enough of just how freaky this moon is. While it bears some resemblance to other objects in the solar system, it has a bizarre nature all its own. Perhaps I’m showing my American fondness for underdogs, but Hyperion really is one my favorite worlds in all the solar system. We may never understand everything about it, but with every pass by Cassini, we learn a little bit more, and that’s cool enough for me.


Related posts:

Cassini visits a foamy moon
Video of Cassini’s Hyperion flyby
Raw hypermoon
You’re as cold as ice… but less dense
Hyperion!

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

Comments (43)

  1. Carey

    about half that of liquid water, which is less dense even than water ice

    Liquid water is denser than water ice, no? Since water takes up less volume as a liquid? Doesn’t change your point about Hyperion being extremely light, of course.

  2. I must have misread. Isn’t liquid water *more* dense than water ice? Otherwise, why would icebergs float? I realize most liquids are less dense than solids, but I thought water/ice was an exception.

  3. ScienceMC

    Hyperion! I can almost see the Shrike from here… (If you’ve read Hyperion by Dan Simmons, you’ll know what I mean.)

  4. Blargh

    Carey: it’s an unfortunate sentence construct by Phil. That my brain corrected (together with the typo in the same sentence!) until you pointed it out.

    It has a very low density — about half that of liquid water, which is less dense even than water ice, indicating it most not be entirely solid.

    What he means is that Hyperion – whose density is about half that of liquid water – is even less dense than water ice. Not that liquid water is less dense than ice. I.e.

    It has a very low density — about half that of liquid water, which makes it less dense even than water ice, indicating it must not be entirely solid.

  5. Carey

    Ah, an unfortunate confusion of antecedents then. Half the density of liquid water is indeed less than the density of water ice.

  6. J Marton

    I want a Hyperion pedicure stone.

  7. What causes that central peak in big impact craters?

  8. My favorite fact about Hyperion is that it doesn’t have a fixed rotation axis: it tumbles chaotically, under influence of the tides from Saturn and Titan. Its orientation isn’t even predictable over longish time scales.

  9. LabCad

    “Crunchy Hyperion” – an awesome name for a breakfast cereal, or candy bar.

  10. Chris N

    It kind of looks like a really big pumice rock.
    …it has a low density and a soft, foamy surface.

    A moon that’s really a giant cinder ejected from a volcano the size of Neptune, maybe? :-)

  11. Bubba

    @ #5: exactly. It doesn’t look like sponge to me. It looks, to me, like a piece of volcanic rock. As #5 implies, it makes me think of pumice.

  12. Joseph Marcus

    Does anyone else see the skull outline?

  13. Dutch Railroader

    Sorry, Phil, but I’m going to cast my vote for Titan. There’s a hell of a lot going on there, and it’s completely unique among planets and moons alike. It’s also presently active and dynamic.

  14. Regner Trampedach

    Chris N @ 10: Well, I am just glad you didn’t make the obvious 4th grader joke involving another ice giant in the outer reaches of the solar system. And I, of course, would never have made that joke either – that would have been imature…
    Carl @ 7: A large enough impact will liquify both the target and projectile and you get a rebound effect like when a drop of water drops into water (search the inter-tubes for high-speed camera pictures of water drops, or something, and you’ll invariably find pictures of the rebound and a snipped-off drop just above it. Molten rock can probably not get liquid enough for that latter effect…).
    Cheers, Regner

  15. Ross

    It does look like a huge lava bomb. I remember seeing boulders in Hawaii that looked like rocky sponges. According to the tour brochure, these formed when large blobs of lava were blasted out of the volcano, and the sudden release of pressure caused the gases in the magma to come out of solution, “foaming” the lava as it cooled. Could Hyperion be a moon-size version of a lava bomb?

  16. James

    I’ve always thought it was a good candidate for a captured comet.

  17. Chief

    It’s amazing that a body this small can have an impact crater that large and still be existing.

  18. Thameron

    “Our solar system is a fantastically bizarre place.” When compared to what exactly? Some other stellar planetary system? It just is what it is. How well we understand how it came to be what it is is of course a different issue.

  19. Torbjorn Larsson, OM

    Titan. Enceladus. Then perhaps Hyperion.

    @ Chief:

    Remember though that a hypervelocity impactor (i.e. ~ orbit velocity difference in speed) will leave a crater ~ 20 times its diameter in size purely for energy reasons.

    Presumably then it is when the crater diameter overtakes the diameter of the body we will have to ponder whether the original body or the resulting aggregate is most significant object.

  20. Joseph G

    I agree, Hyperion is one of the coolest moons out there. And the name is pretty badass as well :)

    I do wonder, though – with moons like this, and large “rubble pile” asteroids – what keeps them so “fluffy”? I would think that with no gases in the voidy parts to support the rocks, and with thermal flexing and numerous small impacts over billions of years, wouldn’t objects like this eventually collapse and become fairly dense?

  21. Regner Trampedach

    Joseph G @ 20: “collapse” from what? The 0.017 N/kg gravity at the surface?
    That’s about 600 times weaker than at the Earth’s surface. On Hyperion you would weigh about a 100g (for US types: about the weight of a stick of butter…). Of course surface gravity on Hyperion depends very much on where on it’s very non-round surface you stand.
    Cheers, Regner

  22. Joseph G

    @21 Regner: I guess I was just assuming that the low gravity would still tend toward a collapse, just much, much more slowly. Again, assuming that repeated impacts manage to “shake things loose.” But I guess that doesn’t necessarily follow. The coefficient of friction doesn’t change, and impact energy probably doesn’t travel very far through the body precisely because it’s so “holey”. There’s probably some ice in there acting to glue everything together as well? :-P

  23. tardis_blue

    Phil, would you please explain how the central peak forms, and is it the same mechanism in liquids? I tried to explain it to my kid, and showed him a slo-mo video of a drop of liquid being dropped into milk, but then I couldn’t explain why that bit in the middle shoots up. I found an explanation, but I’m not very sure I understand.
    Thanks!

  24. Joel

    @3 ScienceMC: You beat me to it. I’m just worried about that pointy tree of his. It sounds sharp.

  25. Chris A.

    @Thameron (#18):
    “‘Our solar system is a fantastically bizarre place.’ When compared to what exactly?”

    Given that, for most folks, the definition of bizarre is something akin to “outside ordinary experience,” I think what Phil’s saying is that there are a lot of environs in our solar system that are VERY different from what we are used to (i.e. the surface of Earth). Whether or not that’s typical of other planetary systems doesn’t change the fact that there is a large variety of temperatures, pressures, chemistry, morphology, etc. to be found around old Sol.

  26. Au contraire, to quote Eddie Izzard: “Just rock and ice! We’ve got the most boring solar system I’ve ever heard of! I haven’t heard of any!”

    Still, cool moon.

  27. flippertie

    Under the picture is a caption : [Click to entitanate.]

    Entitanate ? Is that a word that means anything? Like if you Entitanate Hyperion it will be pushed into Titan or something?

  28. MKS

    Phil,

    this might be an idea for a future post of yours:

    What top [pick some number] places in universe would you absolutely love seeing with your naked eye? IE, you could actually be there

  29. Jerffersonian
  30. Joseph G

    @27 MKS: That is a cool question.

    Me, I’ve always wanted to see an active black hole (with accretion disk) up close. Or even an inactive black hole, with the gravitational lensing around the event horizon.
    I’d also love to see a type 1a supernova progenitor (a white dwarf slurping matter from an orbiting red giant).
    There’s something about accretion :-P

  31. Dr_cy_coe

    I’ve had that picture as my desktop for over 3 days and only now do I see what BA points out is a giant impact crater!
    I just didn’t zoom out of the details enough to notice the huge pattern.

  32. jennyxyzzy

    La Esponja Grande!

  33. flip

    #27, MKS

    I’d start with Saturn, steal Joseph G’s idea of going to a black hole… and then see everything else too! What drew me initially to the topic of astronomy is all the wonderful images. I wish every day I could be ‘out there’ (even though I know we kind of are).

  34. Bob_In_Wales

    If it is a sponge I vote re rename it Bob.

  35. Gary Ansorge

    Hyperion, a fine example of a snow cone, randomly mixed with soot,,,

    Gary 7

  36. Nigel Depledge

    Flip (32) said:

    I’d start with Saturn, steal Joseph G’s idea of going to a black hole… and then see everything else too! What drew me initially to the topic of astronomy is all the wonderful images. I wish every day I could be ‘out there’ (even though I know we kind of are).

    Don’t get your hopes up too much, though.

    Many of the most dramatic photos are long exposures of nebulae, which consist mostly of rarefied gas.

    Getting closer would make them bigger and brighter, but not nearly bright enough to look like the amazing long-exp pics we get from instruments such as Hubble.

  37. flip

    #37, Nigel

    My hopes are just high enough. I know I’ll never get to see any of it, let alone the way they (falsely) look in many of the images sent back. However, when you’re having a bad day – and my whole year has been one long bad day – it does make you happy looking at the pretty pics and imagining what it would be like out there. LEO would be good enough for me :)

  38. Messier Tidy Upper

    @27. flippertie asked :

    Under the picture is a caption : [Click to entitanate.]
    Entitanate ? Is that a word that means anything? Like if you Entitanate Hyperion it will be pushed into Titan or something?

    Not been here long? Its a fun practice Phil Plait has of turning the word ‘enlarge” into something different usually with the ~ate suffix – always connected with the image in question – because, well, he wants to. ;-)

    If you click on the image to “entitanate” it turns out to mean going to a larger version of that image on another original website. Which you can, natch, experimentally verify for yourself.

    Personally, I’d say the most bizarre place in our solar system is Erath – but of course we’re accustomed to all the weirdness (Liquid water surface? H2O present in *all* its phases on the surface? A flammable oxygen-rich atmosphere? What the .. blazes!?) Although Titan, Enceladus and Hyperion certainly seem weirder to us.

    Bizarreness / weirdness of course is a subjective value lying in the eyes of the beholder.

  39. Messier Tidy Upper

    @28. MKS asked :

    What top [pick some number] places in universe would you absolutely love seeing with your naked eye? IE, you could actually be there.

    If I could survive near enough there somehow then the Pulsar planets would rate pretty highly for me as wella sbeing able to look down or up on our Milky Way Galaxy as seen from the Galactic Halo or Magellanic Clouds. Preferably with a nice nearby Globular cluster – but not so nearby as to spoil the view.

    Seeing Eta Carinae up close would be high on my list too although at about five million times as brightas our Sun it would need strong protective starglasses.

    Seeing a Cepeheid or Mira star up close and watching it change through a full cycle or Epsilon Bootis from a nearer perspective would be incredible too. So many possibilities!

    @21. Regner Trampedach :

    Joseph G @ 20: “collapse” from what? The 0.017 N/kg gravity at the surface? That’s about 600 times weaker than at the Earth’s surface. On Hyperion you would weigh about a 100g (for US types: about the weight of a stick of butter…). Of course surface gravity on Hyperion depends very much on where on it’s very non-round surface you stand.
    Cheers, Regner

    Thanks for that info & comparison Regner. Appreciated. :-)

    @26. Arik Rice :

    “Au contraire, to quote Eddie Izzard: “Just rock and ice! We’ve got the most boring solar system I’ve ever heard of! I haven’t heard of any!”

    Well, our solar system does have one planet unlike any other we’ve so far found – and we’ve found a fair number of different exoplanetary systems so far – Earth. (Otherwise known as “Erath” because I can’t type too well, d’oh!)

    We may know of a few other planets that share Earth’s mass and radius but we know of no life – let alone macroscopic sentient life anywhere else nor (now Gliese 581g seems to have been an error) – are we aware of any other potentially habitable planets where H20 is at its triple point able to exist simultaneously as liquid water, vapourous steam and the type of sedimentary rock commonly known as ice.

    I think our solar system generally is pretty impressisve, fascinating and wonderfully strange myself. ;-)

    *****

    “There were no fires in the Martian desert. In fact, of all the worlds in the solar system only Earth with its oxygen-rich atmosphere knew fire.”
    – Page 43, ‘Voyage’, Stephen Baxter, Harper-Collins, 1996.

  40. Nigel Depledge

    MTU (39) said:

    Personally, I’d say the most bizarre place in our solar system is Erath

    Gotta agree with you there.

    It’s even more bizarre than that place – what’s it called again . . . Dirt or Soil or something . . . Earth! That’s it : Earth. A planet whose ape-descended life forms are so amazingly primitive, they still think digital watches are a pretty neat idea.

  41. It looks like Hyperion has taken a serious beating over the years. Still pretty cool but not my favorite.

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