You’re as cold as ice… but less dense

By Phil Plait | July 4, 2007 9:50 pm

Yes, you want to click that picture.

Hyperion is a moon of Saturn, and it’s freaky. It’s one of the largest irregular moons in the solar system at 300 km across, and the surface is simply weird. I speculated about it before, and it looks like some of my thoughts have panned out.

When the Cassini spacecraft passed by Hyperion, the gravity from the tiny moon deflected the probe just a hair, and from that scientists have been able to find that the density of Hyperion is an astonishing 0.5 times that of water! For comparison, rock is about 2 – 3 times as dense as water, and even ice is 0.9 times water’s density. I think this makes Hyperion the lowest density object yet found in the solar system.

So why is it such a puffball? Probably it’s suffered multiple low speed impacts with other bodies. This ruptured the moon, creating cracks and fissures all through it. If it got whacked by something of just the right size and speed, it could have actually broken apart and recoalesced; forming what astronomers call a "rubble pile". It would have so many holes in it that this would account for the extremely low density. Note– I’m still speculating, but it’s hard to imagine what else could have caused this moon to be so lightweight.

It’s covered in craters, too. The surface is so porous that when an impact occurs, it actually compresses the surface rather than blowing out material. The moon can absorb the impact better without disturbing the neighboring terrain (and any material ejected tends to escape the feeble gravity of the moon, so that it won’t blanket nearby craters either). On normal moons, an impact is likely to erase several craters as the material is disturbed, but on Hyperion the impactor goes crunch, like punching a piece of Styrofoam. I wonder… when you walk on certain kinds of snow, you can feel it crunch as it compresses underfoot. Would an astronaut on Hyperion feel the same thing?

I will note that I speculated that this was the case way back in that first post in 2005 (I missed the idea of ejecta leaving the moon, but I was dead on about the crunchiness of it). Maybe the Cassini crew should hire me. I’ll ask Carolyn Porco, the imaging team leader (and fellow Boulderite) when I see her in August at Spacefest! OK, not really. But I may very well ask her about that weird black stuff at the bottoms of the craters; that is, if the paper which comes out tomorrow in the journal Nature doesn’t mention it. When I learn more, I’ll post about it.

Update (July 5, 2007): A second press release has come out, saying the black substance at the bottoms of the craters is made up of hydrocarbons.

“Of special interest is the presence on Hyperion of hydrocarbons–combinations of carbon and hydrogen atoms that are found in comets, meteorites, and the dust in our galaxy,” said Dale Cruikshank, a planetary scientist at NASA’s Ames Research Center, Moffett Field, Calif., and the paper’s lead author. “These molecules, when embedded in ice and exposed to ultraviolet light, form new molecules of biological significance. This doesn’t mean that we have found life, but it is a further indication that the basic chemistry needed for life is widespread in the universe.”

That’s not surprising; many outer moons contain hydrocarbons; these objects tend to be dark or reddish in color. This close-up look at how they behave will no doubt help astronomers and planetary geologists learn a lot more about the chemical nature of the outer solar system, too.

CATEGORIZED UNDER: Astronomy, NASA, Science

Comments (50)

Links to this Post

  1. Cool picture of the day - Noticias externas | July 5, 2007
  2. A Ler…-- Rastos de Luz | July 6, 2007
  1. Eric TF Bat

    Good guess, but wrong. The reason it’s so light is because it’s hollow. I knew I left my TARDIS somewhere, and now I know where. Now, how to get up there to retrieve it…

  2. Ian Regan

    Actually Proteus is the largest irregular moon in the solar system, with a rough diameter of 400 km.

  3. Oh, nuts. I deleted the email alert from Carolyn, and now I can’t remember if she said it was the largest irregular moon, or the largest one that tumbles. Oh well, I’ll fudge the entry to make it correct.

  4. I click on the image and just see something about who hosed the image. What incentive do I have to click on it again? There doesn’t seem to be a larger version…

  5. Damnit, I meant to say “hosted,” not “hosed.”

    Freud was right!

  6. Spaaaaaaaaaace Loofa!

    (that is all)

  7. Here’s a link to the full res version:
    http://photojournal.jpl.nasa.gov/jpeg/PIA07740.jpg

    Interestingly enough, the lead story on the Cassini-Huygens site (posted today) is the fact that they found hydrocarbons on Hyperion!

  8. Brad

    From the Armchair Astronomy Department: maybe Hyperion is literally a big snow ball, wherein orbital snowflakes gather and collect. Of course, the ice doesn’t necessarily be of water, maybe it came from old high-strata atmosphere from Saturn. Alternatively, maybe one of Saturn’s icier rings began coalescing into Hyperion. The light structure may be slowly collapsing on itself, which would be the huge moon-sized crater taking up its face, not an impact crater.

    It might, of course, have a measure of the “dust” that seems to make up more traditional moons and give Hyperion its color. That could also be what the black stuff is in the smaller craters, where a collision melts away the structural ice to a certain depth, leaving heavier dust to pool in the bottom.

  9. It looks like pumice. But pumice is volcanic, so not icy. Is there any correlation between solid pumice and frozen foam?

  10. tacitus

    This is a link to yesterday’s press release regarding Hyperion and hydrocarbons:

    http://saturn.jpl.nasa.gov/news/press-release-details.cfm?newsID=758

    As a Brit, I’ve always thought Hyperion looks like a bit of crumpet (i.e. *not* the female variety, but the pancake variety!) or “pikelet” as we called it up in Yorkshire:

    http://www.lorry.org/Misc/20021223-stuff/pikelets2.jpg

  11. Darth Robo

    Tres cool. Kinda looks like a sponge! Itz got lotsa bubbles in it! :)

  12. Lay person

    So the black stuff is something other than just shadow?

  13. I’m with Eric. Except that it’s obviously inhabited by Purple Octopus Aliens who are studying earthlings covertly and occasionally visiting to kidnap people and do butt experiments.

    Not that they really care about the experiments. They just think that’s funny.

    Obviously.

  14. complex_field

    I see a ridge encompassing most of the moon. Is that one of the afore-mentioned impact craters? If so, it’s a biggun.

  15. Clicking the picture gives me a tiny ‘hosted by…’ picture.

    There’s a double-slash in the URL, remove it and enjoy: http://ciclops.org/media/ir/2005/1507_3730_1.jpg

  16. So what you’re saying is, we could drop it in the Pacific Ocean and it’d float?

  17. Except that it’s obviously inhabited by Purple Octopus Aliens who are studying earthlings covertly and occasionally visiting to kidnap people and do butt experiments.

    hey, what are you trying to say. :)

  18. I still get the “hosted by” thing, even with Bjorn’s link.

  19. Jarno

    It looks like a giant sponge, it’s got the density of a sponge, so any chance we’ve just discovered that the sponge-kind are a spacefaring species? ;)

  20. Thad Hatchett

    My guess is that the black stuff is the actual impactor itself. Maybe Hyperion is the solar systems own version of Aerogel. The impactor just gets absorbed, and since there is no dust plume it retains it’s dark color?

    Thad

  21. GK

    But, as David Letterman asks, “Will it float?”

  22. aiabx

    I suspect that if it were dumped into the Pacific, it would melt like ice cream in a root beer float.

  23. I fixed the bad link (which I cut and pasted from the Cassini site, so I’m not sure how that happened!).

    When I wrote the entry last night, the paper and release about hydrocarbons were not yet up, but I saw the followup and added an update to the entry.

  24. Evolving Squid,

    I have an ongoing affair with Purple Octopus Aliens. Sometimes I find it easier to deal with heavy duty science issues if I use silly hypothetical examples and situations. Snuffleupagi and Transgendered Purple Octopus Aliens make fairly regular appearances on my blog.

    They first made their appearances last year when I was posting at Uncommonly Dense, tweaking the creobots’ noses undercover. Great fun, that.

    Kisses for each of your limbs,
    JanieBelle

  25. Ian Regan
  26. Mister Earl

    That’s nice, but will it blend?
    ;)

  27. Ian Regan

    The version of the picture that Phil used in his blog entry is actually a false-colour image. A true-colour portrait of Hyperion was constructed by Gordan Ugarkovic, and can be found here: http://img472.imageshack.us/my.php?image=hyperionmosaicli2.jpg

  28. Gary Ansorge

    So many moons, asteroids and comets to play with. I can’t wait for Dr. Bussards fusion rocket to be built,,,

    Gary 7

  29. Ian Regan

    Gordan also produced a colour movie of the little moon rotating as Cassini flew past:

  30. Ian Regan

    I’d love to post the link to the Hyperion rotation movie, but this STUPID software won’t let me; I’ve tried over 30 times already!

  31. Ian Regan
  32. Ephraim

    Wouldn’t an astronaut on Hyperion need a good deal more gravity to feel the crunchiness? What’s little-g on Hyperion?

  33. I have an ongoing affair with Purple Octopus Aliens. Sometimes I find it easier to deal with heavy duty science issues if I use silly hypothetical examples and situations. Snuffleupagi and Transgendered Purple Octopus Aliens make fairly regular appearances on my blog.

    They first made their appearances last year when I was posting at Uncommonly Dense, tweaking the creobots’ noses undercover. Great fun, that.

    But you’re letting them in our our secrets!

    nevertheless, we tentacled ones appreciate your homage :)

  34. Another armchair speculation:

    That relatively large ring does appear to be an impact crater to my non-astronomically educated eyes. But if it is, it seems like the force to create it would have smashed Hyperion to smithereens (admittedly that is an assumtion that could easily be wrong).

    I wonder if the crater was created before Hyperion was its own entity. Maybe Hyperion was once a part of a larger body, received an impact forming that large ring, and then was ejected from the larger body, forming Hyperion, following some other major impact on the larger body.

  35. Donnie B.

    Here we go with the wild speculation.

    Those “craters” don’t look like impact craters. In fact, they look more like vents.

    I propose that Hyperion was once a comet, one that made several visits to the inner solar system. During its close encounters with Sol, it did what comets do: outgassed like mad, leaving the vents that we see. This also explains its low density, as it has lost much mass as gas volatilized and was blown away by the solar wind.

    Then on one pass, it came close to Saturn (either inbound or outbound) and was captured. Now it’s a “museum” that preserves a cometary nucleus for us to study.

    (CT mode) Now that I have developed this theory I shall stand by it despite any and all evidence to the contrary. (/CT mode)

  36. That’s one freaky-looking moon. That really odd ridge or escarpment running around the center of the image looks as though the material on the side of the moon facing the camera has subsided somehow — perhaps Hyperion had a big piece knocked away and it’s self-gravitating into a more spherical shape?

    Cambias

  37. slang

    Or the large crater was the result of a relatively small impact into a very loosely held together pile of rocks and ice. The type of impact that would create a much smaller crater on a ‘normal’ moon. Or perhaps a slow impact of a huge (by comparison) impacter that later moved away. Or sunk in. *leans back in armchair*

  38. slang

    Donnie B. Says: “I propose that Hyperion was once a comet”

    Interesting.. that was my first thought too when I saw a close Hyperion image the first time. And even more interestingly, it seems this guy thought so too, but as an afterthought :)

    http://www.badastronomy.com/bablog/?p=161

    (BA, could you include a little help link or text to explain how to embed URL’s properly for those of us not familiar with wordpress?)

  39. slang

    Aw, shame on me, it’s the same page as the post links to but a different URL. Oh well, nice try, slang.

  40. Slang, it’s pretty easy. You just go like so:

    [a href=”http://pagethatslangwantstolinkto.html”]Whatever slang wants the text to display[/a]

    Just replace the [ and ] with (and of course replace the url and the text with whatever you want).

  41. Crap. WP read the angle brackets like code. My total bad.

    Instead of square brackets [ and ] use the angled ones (greater than and less than)

  42. WM

    If you made that thing up, no one would believe you.

  43. Oooh! I bet it’s got a gooey marshmallow center!

  44. icemith

    Well, one thing’s for sure, they won’t be sending any Mars type Rover to cruise around the surface! And it is hard to get past the Bath Sponge concept.

    To me, it seems the impact has been a very low angle event. The lower smoother surface is either “recent” or there are considerable dust covered irregularities. It also appears to be not as steep as the opposite escarpment. Squinting at the whole image, (the higher res version), it seems as though the central core is partly visible, though it has suffered a large impact crater. The question is, where is the evidence for any remnant of the impact? Or is it actually the only reasonably solid mass in Hyperion’s make-up?

    Of course the deep “shafts” could be the burrowings of Dune like Worms, or the biggest termites ever!

    Ivan.

  45. slang

    Alright, html style then, not bbcode. Thanks JanieBelle.

  46. Hyperion is also notable for rotating chaotically! iirc we only know of one other celestial body which does so.

  47. Skepterist

    Donnie B,

    I like your idea that Hyperion is a captured comet. That could explain many of the recent findings. According to the saturn.jpl.nasa.gov website, they’ve found evidence of hydrocarbons on the surface.

    “Of special interest is the presence on Hyperion of hydrocarbons–combinations of carbon and hydrogen atoms that are found in comets, meteorites, and the dust in our galaxy,” said Dale Cruikshank, a planetary scientist at NASA’s Ames Research Center, Moffett Field, Calif., and the paper’s lead author.

    The latest hi-res images do certainly give it a sponge-like appearance. However, in many of the videos, the moon has a more solid-looking surface. Probably due to lower-resolution images?

    Interesting, if true.

  48. @Ian Regan: Actually, the natural color mosaic you linked to was done before I realized there’s a problem with Cassini PDS calibration data. Namely, the red channel consistently turns out too bright. I’ve since fixed this and the “correct” approximately natural color mosaic can be found here. It fits well with the small true color image CICLOPS released some time after.

NEW ON DISCOVER
OPEN
CITIZEN SCIENCE
ADVERTISEMENT

Discover's Newsletter

Sign up to get the latest science news delivered weekly right to your inbox!

ADVERTISEMENT

See More

ADVERTISEMENT
Collapse bottom bar
+

Login to your Account

X
E-mail address:
Password:
Remember me
Forgot your password?
No problem. Click here to have it e-mailed to you.

Not Registered Yet?

Register now for FREE. Registration only takes a few minutes to complete. Register now »