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The Dark Side of Iapetus

Posted by kdawson on 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."

8 of 73 comments (clear)

  1. opposite direction moons by Harmonious+Botch · · Score: 1, Interesting

    "... 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.

    1. Re:opposite direction moons by stevesliva · · Score: 3, Interesting

      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.

      --
      Who do you get to be an expert to tell you something's not obvious? The least insightful person you can find? -J Roberts
    2. Re:opposite direction moons by rde · · Score: 4, Interesting

      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.

    3. Re:opposite direction moons by E++99 · · Score: 2, Interesting

      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.

    4. Re:opposite direction moons by Tablizer · · Score: 2, Interesting

      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 that easily that I can see. I see 3 general scenarios at the moment:

      1. The colliding object hit in the same direction that the proto-Earth was already rotating.

      2. The colliding object hit in the opposite direction of proto-Earth's rotation, but hit hard enough to reverse or angle the Earth's spin. But orbital pressure from nearby planets "corrected" it over time. Being at an angle may be sufficient, but being completely opposite may have hindered moon creation. It would be interesting to see such simulations.

      3. The colliding object hit in the opposite direction of proto-Earth's rotation, but because of the opposite direction of movement, the debri didn't orbit long and fell back to Earth due to tidal forces, leaving NO moon. But if this happened, then we wouldn't be talking about the moon. Thus, perhaps the collision was somewhat aligned with the existing rotation of the proto-Earth, otherwise we wouldn't be talking about the moon. (Perhaps there were other collisions that didn't generate moons because of this.)

      In short, the colliding object may have to be somewhat compatible with the existing rotation to create a moon, so a selection process reduces the chance of a counter-rotating moon forming via collision. Something that hits at the opposite direction of rotation generates more heat and less momentum than the other way around. Heat would just create debris clouds and plasma, which is not enough to form a moon. Collisions that "take advantage" of existing rotation will toss more debris into higher orbits.

      --
      Table-ized A.I.
  2. He's heading towards that small moon by adamkennedy · · Score: 2, Interesting

    obiwan> That's no moon...

    http://saturn.jpl.nasa.gov/multimedia/images/image-details.cfm?imageID=2763

  3. A really strange moon in multiple ways by Tablizer · · Score: 5, Interesting

    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.

    --
    Table-ized A.I.
  4. Re:2001 References?? by Teancum · · Score: 2, Interesting

    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 mission ends within my lifetime, as there certainly have been some very amazing discoveries that deserve a strong follow-up investigation. It is amazing now that we have discovered such an amazing variety of worlds to explore even within our own Solar System, which is now beginning to help us to explain this hunk of rock that we live on right now as well.

    Or more specifically, when you try to do a statistical comparison with a sample size of one, you tend to have all kinds of wild and crazy theories that get thrown around using that data point. Now that we have nearly 100 different worlds to compare the Earth to in terms of geological formation, structures, hydrology, and weather patterns; we certainly can start to make some much more informed guesses about what is happening right now here and what we can expect in the future.