Raw hypermoon

By Phil Plait | December 3, 2010 7:00 am

Stop me if you’ve heard this from me before, but Saturn and its moons are weird. Take Hyperion. 300 km across (180 miles), yet only half as dense as water! Even normal ice is denser than that, so something weird is going on there. It’s most likely that it’s full of holes, like a loosely agglomerated rubble pile. It may be nearly half empty space!

So it’s no surprise that such an odd, odd little world should make such a brooding target of the Cassini spacecraft’s cameras:

cassini_hyperion_crescent

Cassini passed about 75,000 km (45,000 miles) from Hyperion on November 28, 2010, and took dozens of images of the moon. It was still 140,000 km (84,000 miles) away and on approach when it snapped the dramatic shot above. The picture is raw and unprocessed, so it still has some "image artifacts" in it; defects in the camera that can be easily corrected using pipeline software. Still, you can get a sense that the moon, well, just ain’t quite right.

Then Cassini got closer, and took many pictures like this next one:

cassini_hyperion_half

Man, Hyperion is just so weird! The craters don’t look like what we’re used to. The rims are sharp, and the walls shallowly slope, making craters look like inverted cones. Some people say the moon looks like a sponge, but in my mind it looks like it’s got the consistency of loose, weak Styrofoam that some kid has poked at hundreds of times with his finger.

That may not be too far off. The Styrofoam part, I mean, not the bit about getting poked by a child’s finger.

Hyperion orbits Saturn at an average distance of 1.5 million km (900,000 miles), and has an elliptical orbit. Visits by Cassini are not common, so there’s still a lot to learn about the moon. Passes like this recent one will help figure out what the heck is going on here. Was Hyperion once a part of a bigger moon, an object shattered by a vast collision? Or was Hyperion itself once shattered by an impact, only to loosely reassemble itself as we think happens with some asteroids and comets? Or both, or neither?

We know a huge amount about Saturn and its moons, and a lot of that is due to Cassini. But I suspect we could keep it orbiting the ringed world for another hundred years and still have much to learn.


Related posts:

- You’re as cold as ice… but less dense
- Hyperion!


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

Comments (44)

  1. Pat Z

    is it true that Hyperion has a weird orbit which is very hard to predict ? (something about it’s shape) I read in a book by Ian Stewart ( Does God play dice ) ages ago.

    awesome picture by the way.

  2. Bigby

    Actually it looks like a hunk of pumice I had as a kid. Sounds like right level of density. Wonder if the structure is similar.

  3. Wow. This just sparked all kinds of creative ideas for a sci fi short story. I’m picturing a tunnel that winds its way all the way through to the other side. In fact, I think we put a colony up there and they live in the tunnels…
    Thanks for the spark Phil.

  4. BILL7718

    After having two top-of-the-line death stars blown up by the rebels, the empire switched to styrofoam to cut down on costs just in case…

  5. Andrew

    “That’s No Moon”.

    I mean really… You expect me to believe that a holllow structure orbiting a planet that has tons of rubble circling it is a MOON?

  6. Messier Tidy Upper

    Raw hypermoon? You saying its not cooked yet? ;-)

    Great image by Cassini as always. :-)

    Oddly reminiscent of the Lutetia asteroid fly-past by the Rosetta spacecraft earlier this year.

    @1. Bigby Says:

    Actually it looks like a hunk of pumice I had as a kid. Sounds like right level of density. Wonder if the structure is similar.

    Not sure about the structural similarity but certainly pumice is very low density and porous – it even floats on water. Although, Hyperion is more icy than rocky in nature.

    @3. BILL7718 : LOL. :-)

    But make that styrofoam *and* sponge to make it easier to clean up after the next time the Death Star gets trashed! Or alternatively to use in mopping up the resistence. ;-)

  7. Bigby said what I thought as I first looked the the pictures. Although, I am not sure of ALL the mechanisms to form pumice. How would a rock of that type end up out there all by itself? Very weird!

  8. Now that you mention it, it IS more like Styrofoam! Or a loofah. Or maybe one of those blocks of weird dark green stuff that they used to use to arrange flowers with. (I always used to get in trouble for poking holes in those.)

    Anyway, I’ve made up a couple color composites of the latest Hyperion shots…they’re on my Flickr album here. I had to fake the red and blue channels with IR and UV, but it gives a general idea. :)

  9. Messier Tidy Upper

    EDITED TO ADD : @ ^ J. Major : Wow. [admiring whistle.] Your album ones there are superb. Great work & thanks. :-)

    My previous personal fave image of Hyperion was this one :

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

    Via Cassini again & really showing off its “spongy” look.

    For more see :

    http://en.wikipedia.org/wiki/Hyperion_(moon)

    While this link :

    http://en.wikipedia.org/wiki/Pumice

    gives the wiki-basics on pumice.

    Look closely at that last picture on the bottom – & now imagine turning the lights out and just shining a torchlight on the rock there .. :-)

  10. James

    Could this be a captured comet, and the craters caused by outgassing?

  11. Alareth

    They look to me like a bunch of Antlion sand pits.

  12. Georg

    Would 300 km diameter of water snow be enough
    to compress the snow in the center and make the
    moon spherical?
    Georg

  13. Right off the bat I’d say James is right. Probably some mix of water & something more volatile… Methane ice, perhaps? The methane ice sublimates away, leaving ice with big holes in it. Then, eons later it gets captured by Saturn, resulting in its elliptical orbit.

    The craters remind me of ant lion holes, so maybe there are arsenic loving space ant lions there…

    Remember kids! Arsenic is 100% natural! :)

  14. DrivethruScientist

    @bigby

    That’s what i thought too. Looks like a pumice rock to me!

  15. Stubby

    The Captured Comet idea is an interesting one.

    Just out of interest, what is the planned lifespan of Cassini?

  16. Dwarf moon? Moonitoid? Sponge Bob Moonpants? Weird stuff out there. The images of Saturn’s moons where the rings are edge on are the ones that really wow me! What a great mission. :)

  17. drow

    its obviously hollow, the last outpost of a race of hyper-intelligent velociraptors.

  18. “We know a huge amount about Saturn and its moons, and a lot of that is due to Cassini. But I suspect we could keep it orbiting the ringed world for another hundred years and still have much to learn.”

    I certainly hope so: we have thousands of satellites studying the planet humans have lived on for 10′s of thousands of years and although we know a lot, we still don’t know jack.

    Thanks for the post. I am team sponge.

  19. Oli

    A 300 km comet?

  20. James

    Hmmmmmmm. Oli brings up a good point, didn’t think about that. Is there a size limit on comets? How big are they on average?

  21. Ajax Mike

    Just a chunk of leftover packing material from when the Creator put us together. There’s probably a giant Allen key floating around out there somewhere too. ;)

  22. VJBinCT
  23. Oli

    @James: Wikipedia says “Comet nuclei are known to range from about 100 meters to more than 40 kilometres across.”

  24. #3 Non-Believer: Look up the story “Into the Miranda Rift”. As a caver, it’s a favorite of mine. There’s also the book “Cold River” by Hal Clement.

    Again, as a caver… what looks like ant-lion pits, in an object that is so under-dense it almost certainly must have significant pore space. My first thought is “subsidence sinkholes”. Note that they can’t all by formed that way, as if all the “empty volumes” filled in with dust, you don’t have a drastically under-dense object again… so the collapses are only surface collapses. You get that on Earth because the surface is processed from above (erosion), but here… outgassing perhaps, but only if it occurs after Hyperion attains it’s current size and shape. Interesting.

  25. AliCali

    “Stop me if you’ve heard this from me before…”

    Am I the only one who now has The Smiths in my head?

  26. Ian Regan
  27. Pat@1, the orbit of Hyperion is very well defined. What is not is its rotation. Hyperion is unique among moons of our solar system in having a chaotic period of rotation. Not only that, but the axis of rotation constantly shifts around.

    In other words, you’ll always know where Hyperion is at any given point in time, but not which side of it you will see when you look, even if you thought you were approaching from one of its poles.

  28. Rodney

    @ Messier Tidy Upper: That first image almost looks like an impact point.

  29. Bill Nettles

    “Hyperion is unique among moons of our solar system in having a chaotic period of rotation. Not only that, but the axis of rotation constantly shifts around.”

    So, are you suggesting that the angular momentum is not conserved, or it is conserved but lopsided, i.e., non-uniform density? I’m highly skeptical that a stand-alone orbital object is chaotic in its rotational motion. Angular momentum must be conserved, and therefore has a predictable rotational behavior, unless there is some non-radial external force acting on it.

  30. ozprof

    Bill @ 26

    Hyperion does have a non-radial external force acting upon it. Titan.

    It can approach Titan relatively closely, (it is the next moon out from Titan) and so there is a constant tug of war between Titan and Saturn, each trying to cause the long axis of Hyperion to point towards them. As a result, the rotation period and the axis tilt vary chaoticly.

    Cheers,

    Ozprof

  31. @9 MTU: Thanks!

    Emily L. at TPS had some hi-res pics from the September ’05 flyby, here’s one showing some crater detail: http://vsnet.planetary.org/blog/article/00000008/
    And here’s a color view of the surface, showing variation between the inside of the craters vs. the surface: http://planetary.org/blog/article/00002794/ (also the first door of this year’s advent calendar!)

  32. chris j.

    Oli @23: that definition discounts chiron, which is significantly bigger.

    if you think about it, most of the large moons in the outer solar system would probably behave like comets if they somehow got close enough to the sun, since they are mostly ice. who’s to say there aren’t many sedna-type bodies with sufficiently elliptical orbits that bring them in close every few hundred thousand years, and that hyperion was “lucky” enough to get snagged by saturn on one of those approaches?

  33. Andrew W

    I’ll go for a KBO thats been much closer to the sun than it is now, and so lost many of its more volatile ices (methane) leaving it porous.

    It doesn’t look like the surface of the comets we’ve seen but they are of course still actively out gassing so craters don’t last long.

    The number of craters is amazing, has it crossed through the rings at some stage or are they in large part due to the surface collapsing into subsurface cavities?

    I wonder if Cassini can detect any out-gassing.

  34. Joseph G

    Just wondering, what’s this “pipeline software” Phil speaks of? Is it anything like GIMP or Photoshop, or something completely different?
    Also, for raw images, these are darned dramatic. I’m not sure what needs to be done other then tweaking the brightness curves so you can see a bit more of the surface. But then, I’m not a scientist or a photographer, so I’m probably missing something very important :P

  35. Joseph G

    @#21 Ajax: Maybe some day they’ll find a huge sheet of wrinkled paper several dozen kilometers across out in the Oort cloud, printed with step-by-step instructions in pictograph format, complete with cute cartoon stick-man (stick-diety?) ;)

  36. Matt B.

    Those craters look like impacts in sand, which are conical. But then since the moon as a whole isn’t spherical, there must be something non-granular underneath it all. Is there any chance the surface has been pulverized by ring material? Or maybe Hyperion gently collected ring material on top of its original surface. (I’m guessing that would keep the surface from being too much like the Moon’s regolith.) Then that sand surface was impacted by larger objects to make the conical craters.

  37. Pat Z

    @dcsohl thanks for the update ! :)

  38. Pete Jackson

    @29 Bill, the angular momentum of Hyperion’s rotation is constantly being exchanged with orbital angular momentum elsewhere in the Saturnian system, particularly with Titan.

    Look at the pdf attachment at:

    http://www.physics.udel.edu/~jim/PHYS460_660_10S/solarsystem/Chaotic rotation of Hyperion.pdf

  39. Joseph @ 34: The pipeline is an automated processing of all the pictures (pretty similar for all telescopes, although each telescope has its own idiosyncrasies) consisting of: flatfielding – each pixel in the camera has a linear response to incoming photons, but no by the same factor. To compensate the science picture is divided by a picture of uniform white (the flat field). Subtraction of dark current. Some pixels will have an off-set from the linear response, i.e., registering a signal despite no photons being around. Cosmic rays will litter any picture, especially taken from space, and for longer exposures. Locate them and interpolate over them. Over-exposed pixels will bleed into neighboring ones in the same row – sometimes that can be repaired. Also there will be tables of known dead pixels and rows to interpolate over. Also the coordinates of the picture/distance to source has to be established and the conversion factor from pixel response to flux (energy, per time, per surface area).
    No – it ain’t GIMP! At this stage you can actually start using the pictures for science, and you can also combine pictures from several filters and make false-color images to highlight various features, etc. I hope that helps.
    Cheers, Regner

  40. Joseph G

    @#39 Regner Trampedach:
    I was also wondering where these image artifacts are, exactly? Pixels activated by cosmic rays, for instance – I’m just not seeing anything that looks like that in these images, but then I have no idea what the resolution of the original images was.
    Anyway, that does help quite a lot, thanks!

  41. Messier Tidy Upper

    @32. chris j. Says:

    Oli @23: that definition discounts chiron, which is significantly bigger.
    if you think about it, most of the large moons in the outer solar system would probably behave like comets if they somehow got close enough to the sun, since they are mostly ice. who’s to say there aren’t many sedna-type bodies with sufficiently elliptical orbits that bring them in close every few hundred thousand years, and that hyperion was “lucky” enough to get snagged by saturn on one of those approaches?

    Yes indeed.

    If an ice dwarf – even a huge one like Eris or Pluto – fell into the inner solar system, it may well become a gargantuan comet.

    Even a hot Jupiter can have a tail of gas and other material being driven from its atmosphere as is the case with Osiris (HD 209458 b) so, how big can a comet get?

    Good question!

    Maybe we should start a new definition debate on what precisely makes a comet and whether an object can be both a planet and a comet! ;-)

  42. Oli

    @41. MTU: If the earth were to plunge into the sun, it would also temporarily be comet-like when the atmosphere and all the water from the oceans evaporated and were blown away. I think a comet should be defined as a small icy body on a very eccentric orbit which, when near perihelion, shows a tail.

  43. Keith Bowden

    I continue to become awestruck with every new photo or data from Cassini and Hubble and everything else that Phil finds.

    Comets lose mass with every transit, so how big were they when they started? Why can’t Hyperion be a former comet?

    I do think we need a new definition of what constitutes a “moon”, however. This thing ain’t even round! It’s definitely a satellite, but a moon? I dunno. A captured lumpy comet/asteroid/etc. just doesn’t seem to do it for me. (I say this only partially in jest over the “dwarf planet” definition.) ;)

  44. David S

    Ok one question a month after this bog post. Is there a possibility that this could be a lump of volcanic basalt? It certainly looks a bit like a worn piece of that and basalt can be very low density to boot.

    Thank you for reading a question from a curious citizen.

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