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Space Science

The Dark Side of Iapetus 73

Posted by kdawson
from the like-e-ink-writ-large dept.
Hugh Pickens writes "The difference in coloring between Iapetus' leading and trailing hemispheres is striking. NASA's Jet Propulsion Labs has just released a report on a bizarre 'runaway' process that may explain the strange and dramatically two-toned appearance recently revealed in images collected during a close flyby by the Cassini spacecraft. Scientists believe that initially dark material on one side of Iapetus may have come from other moons orbiting Saturn in the opposite direction. Since Iapetus is locked in synchronous rotation about Saturn, as dusty material from the outer moons spiraled in and hit Iapetus head-on, the forward-facing side began to darken. As it absorbed more sunlight, its surface water evaporated, and vapor was transported from the dark side to the white side of Iapetus. Thermal segregation then proceeded in a runaway process as the dark side lost its surface ice and got darker still. Now the leading hemisphere is as dark as a tarred street and the trailing hemisphere resembles freshly fallen snow."
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The Dark Side of Iapetus

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  • "... other moons orbiting Saturn in the opposite direction."

    From what I recall of planetary formation, moons all came from an accretion disk, and should be all orbiting the same direction. I suspect that more likely the materials coating the dark side came from same-direction objects that were in eccentric orbits.
    • by ajlitt (19055)
      Isn't there a theory that has the Earth's moon being formed by a collision with a large object? Wouldn't a moon formed this way not be dependent on the path of an accretion disk?
      • by rde (17364) on Tuesday October 09, 2007 @01:42AM (#20907819)
        It's a fairly substantial theory; you'd be hard-pressed to find any planetary scientist who thinks the moon formed any other way. The makeup of the moon (as it's currently understood) doesn't really accommodate alternate theories. As for its direction: when the Mars-sized planet whacked the nascent Earth, it most likely sent up an accretion disk of its own rather than a sending a huge chunk of proto-moon into orbit; this disk gradually formed the moon. Given that the disk's movement would be directed by the Earth, which would in turn be directed by the rest of the solar system, the moon's direction would, indirectly, be dictated by the solar system's original accretion disk. Pretty much the only reason (that I can think of, anyway) for a moon to have a retrograde orbit would be its capture as a more-or-less intact body.
        However, IANA[A-Z], so I'm willing to be contradicted on all this.
        • Re: (Score:2, Interesting)

          by E++99 (880734)
          If you look at the gravity simulations of such a planetary collision, it seems that the orbit of the resulting moon is a product of the angle of impact more than anything else. So I my impression is that a retrograde orbit could be completely consistent with a moon that is the product of a collision.
        • Re: (Score:2, Interesting)

          by Tablizer (95088)
          As for its direction: when the Mars-sized planet whacked the nascent Earth, it most likely sent up an accretion disk of its own rather than a sending a huge chunk of proto-moon into orbit; this disk gradually formed the moon. Given that the disk's movement would be directed by the Earth, which would in turn be directed by the rest of the solar system, the moon's direction would, indirectly, be dictated by the solar system's original accretion disk.

          But you couldn't change the orbital momentum of debris tha
          • by malilo (799198)
            There is an excellent book "What if the moon didn't exist" that actually details a ton of different scenarios for how the earth + rocky body collision could have been different (larger, smaller, different directions) resulting in all kinds of different outcomes (relies heavily on evolutionary theory - for instance, without tidal slowing of the earth's rotation, day's would be 4 hours long and animal life would certainly be adapted differently). Anyway, I recall the first chapter detailed the prevailing the
            • by MBGMorden (803437)
              There is also a television program (a bit older now - narrated by Patrick Stewart) called "What if we had no moon?" that deals with the same topic. It's available on DVD from the Discovery Channel's online store.

              One of the key points it touched on, aside from the length of the day, was just how much the moon serves the stablize our planet's "wobble factor". While planets like Mars for example, tend to wobble all over the place as they orbit, Earth tends to be more stable as the moon acts as a gravitationa
            • The book (out of print, published 1993) is currently available on Amazon [amazon.com] for as little as a penny. Looks very cool (as do some of the other books which Amazon mentions on the same page, like the one called The Big Splat, or How Our Moon Came to Be.

              Thanks for the recommendation.

            • by Tablizer (95088)
              without tidal slowing of the earth's rotation, day's would be 4 hours long

              I am curious how they know this because the collision itself may have added or subtracted rotation. Mars has no significant moon, and yet it has about the same period as the Earth.
                 
        • by Convector (897502)

          It is by far the best accepted theory, although I seem to recall that a few details that haven't been worked. The isotope ratios are a mystery; both the Earth's mantle and the Moon have identical oxygen and silicon ratios are identical. If the impactor had a different source than the Earth, it's hard to explain this unless the two bodies underwent complete mixing, and that's not predicted by the dynamical models. IAAPSBNAGCATIPMAA (I am a Planetary Scientist, but not a Geochemist, and this isn't precise

          • by CorSci81 (1007499)

            One of my office mates in grad school actually worked on this very problem. What I recall of his preliminary research was that one potential (but unlikely) explanation was the impactor maybe had formed along-side the Earth. However, isotope distributions among the terrestrial planets appeared to be completely random from dynamical models of accretion. While there may be some segregation in the protoplanetary disk there are a sufficient number of large accretion events from bodies from all over the inner

    • by Josef Meixner (1020161) on Tuesday October 09, 2007 @01:20AM (#20907695) Homepage

      There are also moons which are considered to have been once independent objects caught by Saturns gravity. E.g. in this list [wikipedia.org] the ones with a negative orbital period are retrograde. Saturn seems to have quite some of those.

    • by stevesliva (648202) on Tuesday October 09, 2007 @01:28AM (#20907733) Journal
      Wow, you're right to highlight the fact that they indicate retrograde satellites might be the cause. Iapetus itself is in an unusually inclined and distant prograde orbit... I hadn't heard any retrograde satellite theories for the dark region.
      • Phoebe was a leading contender as a source for the material for years (although it has recently been ruled out). And it is in a retrograde orbit.

        When a moon is far away from the central body, retrograde orbits are stable, and prograde orbits aren't. Pretty much every gas giant has retrograde moons far out.

        These moons likely escape and are captured over long period of times. They are probably the same population as the centaur asteroids near Saturn. To know for sure, we need to figure out the compositi
    • by techno-vampire (666512) on Tuesday October 09, 2007 @01:32AM (#20907753) Homepage
      Many of Saturn's moons are probably captured asteroids, and have highly eccentric orbits. For various reasons, it's a lot easier for a body to be captured into a retrograde orbit, going "the wrong way."
    • Perhaps these moons, which do orbit retrograde, are captured objects?
    • by aqk (844307)
      Check out Phobos and Deimos.

  • by ajlitt (19055) on Tuesday October 09, 2007 @01:04AM (#20907635)
    As a matter of fact it's all dark.
  • by User 956 (568564)
    Scientists believe that initially dark material on one side of Iapetus may have come from other moons orbiting Saturn in the opposite direction ... As it absorbed more sunlight, its surface water evaporated, and vapor was transported from the dark side to the white side of Iapetus.

    that's not a scientifically-described result of synchronous rotation. That's apartheid.
  • "I'll meet you on the Dark Side of Iapetus" just doesn't have the same flow...
    • "I'll meet you on the Dark Side of Iapetus"

      I belive the word is "see" rather than "meet", also there is nothing in the lyrics that states what moon they were talking about. /pedant
    • obligatory Eddie and the Cruisers

      The dark side's callin' now, nothin' is real
      She'll never know just how I feel
      From out of the shadows she walks like a dream
      Makes me feel crazy, makes me feel so mean

      Ain't nothin' gonna save you from a love that's blind
      When you slip to the dark side you cross that line
      On the dark side, oh yeah
      On the dark side, oh yeah
  • I nominate Natalie Portman in Hot Grits as a baseline for this study....

    "Thermal segregation then proceeded in a runaway process as the dark side lost its surface ice and got darker still. Now the leading hemisphere is as dark as a tarred street and the trailing hemisphere resembles freshly fallen snow."

    So, we can terraform hot planets by tarring the streets and thus creating freshly fallen snow?...or does this mean that if we tar the streets then Natalie Portman will sled away with me in the new snow?

    Do w
  • by Tablizer (95088) on Tuesday October 09, 2007 @02:27AM (#20908015) Homepage Journal
    Some other mysteries are coming together. There are more data on the signature mountain ridge that gives Iapetus its "walnut" appearance. In some places it appears subdued. One big question that remains is why it does not go all the way around...And the ridge looks too solid and competent to be the result of an equatorial ring around the moon collapsing onto its surface. The ring theory cannot explain features that look like tectonic structures in the new high resolution images.

    So the collapsed ring theory (posted earlier on /.) is falling out of favor? So the walnut mystery remains. Giant impacts can sometimes mess up a moon's shape, but usually the odd damage is on the opposite side of the impact, not a circumference ring ridge. Whoever can pose a physical scenario that can cause such a feature (outside of orbiting ring collapse) may somebody have it named after them.
           
    • The ridge is looking more and more like a tectonic structure. If you have red-blue 3D glasses, you can look at the topography around the western end of the "ridge": http://photojournal.jpl.nasa.gov/catalog/PIA08379 [nasa.gov] In this area, the ridge is not continuous, but is broken up into a series of mountains. These mountains resemble mountains seen elsewhere in the solar system that are formed from thrust faulting, again suggesting that this feature is tectonic, rather than something depositional, as per the rin
      • by Tablizer (95088)
        The ridge is looking more and more like a tectonic structure.

        Perhaps, but nobody has proposed any realistic scenario with it that would produce such a ring. My hat-ass guess would be that it used to have a much faster rotational spin that induced volcanism at the equatorial bulge, perhaps with the help of a now-gone partner producing tidal effects. It would be interesting to see a simulation of the damage caused by such close and spinning bodies.
                 
        • ...faster rotational spin that induced volcanism at the equatorial bulge, perhaps with the help of a now-gone partner producing tidal effects.

          Now that I think about it, if the orbit with the parter was highly elliptical, then the tidal friction could get pretty strong. It would pull and then push repeatedly for each orbit. Io IIRC is heated more due to ellipticity (although it has no co-moon near it).

          Done! Where's my Nobel? ...What?...Evidence? Simulation? Math? Naw, that's for the little people :-)
  • Well, evidently, the other moons are diesel powered and Lapetus is flying into the smoke trail - cough, cough...
  • Everyone knows that Iapetus is dark on one side to indicate the presence of a large black monolith.
    • Everyone knows that Iapetus is dark on one side to indicate the presence of a large black monolith.

      If we send Cassini down to investigate I doubt it will have the intelligence to blurt out that final message.

  • wtf? (Score:2, Informative)

    by sergiolopes (883984)
    From TFA: "230 degrees Fahrenheit or 127 Kelvin"

    Not even NASA gets this right???
  • Is this when ITapetus blinks, then? ....I'll get my spacesuit.
  • 2001 References?? (Score:4, Insightful)

    by oni (41625) on Tuesday October 09, 2007 @09:11AM (#20910261) Homepage
    No references to the book 2001, A Space Odyssey yet? You guys are slipping. In the mid '60s, when A.C. Clarke wrote the book, he asked asked astronomers (mainstream scientists, not UFO nuts) "if you had to pick one object in the solar system that appeared artificial, what would it be?" They all picked Iapetus. At the time, the blurry photos we had from ground-based telescopes could tell us that it was 50% light and 50% dark, but nothing else. It was a big mystery, even after the Voyager flybys. For that reason, Clarke used Iapetus as the sight of the monolith stargate (the movie version used Jupiter).

    We're really lucky to live in a time when all these mysteries are solved.
    • Re: (Score:3, Interesting)

      by Teancum (67324)
      While that may be solved, the equatorial ridge on Iapetus is just as confounding, perhaps even more so.

      Crazy individuals like Richard Hogland are suggesting things like it is a large spacecraft aka Death Star due to this and other physical structures on this satellite of Saturn, but even from a pure geological/scientific viewpoint there are many more questions to be asked about this than have been answered.

      It will be interesting to see if any follow-up mission to Saturn will ever happen after the Cassini mi
      • by oni (41625)
        It will be interesting to see if any follow-up mission to Saturn will ever happen

        I think it's unlikely to happen for 50 years or more, and that's kind of sad.

        But who knows, maybe there'll be some major technological advance that'll make Cassini-like probes common place. Maybe we'll build a space elevator, or maybe we'll start using nerva rockets. Heck, if we (the US) can manage to get Ares built, think of the upper stage booster we could mount on that puppy! We could have landers the mass of Cassini - ma
        • by Teancum (67324)
          It is too bad that New Horizons can't get to Saturn in a "grand tour" arrangement with Pluto, but I do understand that such situations are rather unusual. The Voyager opportunity was rather unusual in terms of how the planets seemed to line up for a perfect low delta-v opportunity to visit all of them. I'm not sure how hard a Jupiter-Saturn-Pluto arrangement would be, but I can't imagine it happens that often either... I'm not ready to do the math to predict the resonance of those three bodies at the mome
    • Jupiter ... Jupiter ... oh yeah, "and beyond the infinite".

      That was the point in the movie's script where the writers said, "hey, I think we did a great job so far. Just scribble something down for the rest and we'll call it day."
  • by kidcharles (908072) on Tuesday October 09, 2007 @02:25PM (#20915039)
    I'll bet you these guys [solcomhouse.com] are from Iapetus.
  • This picture [nasa.gov] shows a bright field with dark material down in the craters. That suggests to me that the impacts excavated a bright veneer to uncover dark material beneath, which is at odds with the image caption and the idea that the dark material is debris from other satellites. But I guess that's why I'm not a surfaces person.
  • by PPH (736903) on Tuesday October 09, 2007 @09:20PM (#20920231)
    Its like millions of years of bug splats on your windshield.

Little known fact about Middle Earth: The Hobbits had a very sophisticated computer network! It was a Tolkien Ring...

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