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Phobos and Deimos Once a Single Moon?

Posted by michael on from the family-ties dept.

blamanj writes "Phobos (fear) and Diemos (panic), the twin moons of Mars have caused astronomers grief for years, as conventional hypotheses about the moons either violate physical laws or have difficulty accounting for their observed orbits. Now a new hypothesis conjectures that they were once a single moon, that broke apart in an ancient catastrophe."

6 of 60 comments (clear)

  1. S. Fred Singer INFO by MonkeyBoyo · · Score: 3, Interesting

    I don't know how "new" this theory is. Here is some info on S. Fred Singer.

  2. Doubtful by CheshireCatCO · · Score: 4, Interesting

    Right, there's a lot here that makes me dubious of the claim. First off, I should point out that I've worked on the capture problem for Mars's moons. (The results haven't been published, although the did land a grant.)

    First off, why is synchronous orbit a hint as to their breakup? There's no reason that synchronous orbit is preferred, either as a capture point or as a point for breakup. In fact, synchronous orbit is an unstable equilibrium: a slight perturbation drives everything away from it. (Which is why Phobos is heading inward and Deimos outward.)

    Also, he needs to explain why a larger moon orbited there happily (without perturbation!) for billions of years before breaking apart. In the very least, we're witnessing Mars's moons at a very unusal time, and such coincidence make me (and most astronomers) nervous.

    Also, Phobos has drifted inward since any such breakup. Why isn't it breaking up more? Unless there's some internal strength (in which case, why did it break up then?), it should.

    To be honest, I sort of question his background for this. Besides the fact that he's not an astronomer, he wants to put a base on Deimos? The surface gravity on those moons is virtually non-existant. (For Deimos, being smaller, it's under 1 cm/sec^2, I believe.) No one could even walk around properly. (Although, if he hollowed it out and made a colony ship out of it, we could launch it to Tau Ceti... But it might encounter some hostile, three-eyed aliens.*)

    I'd be happy to hear him explain his idea to a group of dynamicists. Hell, I'll volunteer. But I'm very skeptical for now.

    (* Kudos to anyone who catches *that* reference.)

    1. Re:Doubtful by Tablizer · · Score: 3, Interesting

      Which is why Phobos is heading inward and Deimos outward.

      I am curious as to why they are drifting. Anybody have the scoop?

      --
      Table-ized A.I.
    2. Re:Doubtful by CheshireCatCO · · Score: 4, Interesting

      Glad you asked!

      Let's start with moons outside of synchronous orbit. These moons raise a tidal bulge on their planet. (The one on Earth is most apparent in the oceans. Or, rather, at their edges. But there's a bulge in the rock, too.) Now, the planet is spinning and it isn't a perfect fluid. So it will tend to carry the bulge forward with it, before the bulge can move back to under the moon where it wants to be. A balance is struck between these two competing forces where the bulge rides somewhere ahead of the moon.

      The moon, then, feels a tug forward in its orbit. This tends to give it angular momentum, so that it drifts outward. (Angular momentum increases as you go out from the central object.) The planet, meanwhile, is being pulled backward so that its spin slows down. (As it must, to conserve angular momentum in the system.) This is why Earth's day in lengthening and why the Moon has drifted about 60 Earth radii from where it formed over the past 4.5 billion years.

      What happens of the moon is *inside* synchronous orbit? The opposite happens: the moon moves ahead of the bulge and gets pulled back. So it drifts in.

      I'll leave it as an exercise to the reader to work out what retrograde (backward orbiting) moons do. Triton is an example, by the way.

    3. Re:Doubtful by WolfWithoutAClause · · Score: 2, Interesting
      Besides the fact that he's not an astronomer, he wants to put a base on Deimos?

      No, he's right, or in orbit around it. It makes a lot of sense. There's probably ice on Deimos and/or Phobos. If so, that's rocket fuel; the space equivalent of oil. And Deimos is ideally placed for this- it's high up above Mars (but not so far that you can't go down), and close in delta-v terms to the Earth, ideal for sending fuel back to Earth orbit to fuel Mars and Lunar missions. It's also a great source for rock for use for radiation shielding in LEO. And don't imagine for a moment that a Deimos base precludes a Mars base- it enables a Mars base.

      The surface gravity on those moons is virtually non-existant. (For Deimos, being smaller, it's under 1 cm/sec^2, I believe.) No one could even walk around properly.

      Yeah right, real important, no walking.

      --

      -WolfWithoutAClause

      "Gravity is only a theory, not a fact!"
  3. Still There? by Markus+Registrada · · Score: 3, Interesting
    What, is that moon still there? Sometimes it seems like things just go around and around the drain forever.

    Seriously, it's funny how astronomers always think that nobody will touch these rocks that are just sitting there in handy orbits. It's the same with Cruithne, the asteroid that co-orbits earth. They always say it will join Earth again in 600 years (or whenever), and it never seems to cross their minds that we might have found something more useful to do with it by then.

    Deimos will probably be more useful, though, than Phobos, as a counterweight to attach to the end of the big elevator down to the surface. We might have to move Phobos out of the way -- making the elevator shimmy this way and that so that Phobos just misses colliding each time past is asking for trouble.