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Martian Moons May Have Formed Like Earth's

Posted by Soulskill on from the carved-out-of-green-cheese dept.

sciencehabit writes: Astronomers have long believed that Mars snatched its two moons — Phobos and Deimos — from the asteroid belt. That would explain why the objects look like asteroids—dark, crater-pocked, and potato-shaped. But computer simulations by two independent teams of astronomers (abstract 1, abstract 2) indicated that Mars's moons formed much like ours did, after a giant space rock smashed into the planet and sprayed debris into orbit.

10 of 50 comments (clear)

  1. Wou would have thought. by nospam007 · · Score: 3, Funny

    "Mars snatched its two moons — Phobos and Deimos — from the asteroid belt."

    So there really _are_ (astronomical) body snatchers from outer space.

    1. Re:Wou would have thought. by Ken_g6 · · Score: 2

      Yeah, except TFA is about how Mars isn't (much of) one. Jupiter is a big body snatcher, and Neptune may have snatched a big body (Triton), but it looks like Mars is innocent.

      --
      (T>t && O(n)--) == sqrt(666)
    2. Re:Wou would have thought. by antiperimetaparalogo · · Score: 3, Informative

      "Mars snatched its two moons — Phobos and Deimos — from the asteroid belt."

      So there really _are_ (astronomical) body snatchers from outer space.

      I am full of Fear and Dread right now...

      --
      Antisthenes: "Wisdom begins by examining the words/names." - excuse my English, i am (slightly...) better with my Greek!
    3. Re:Wou would have thought. by Vulch · · Score: 3, Interesting

      Basically you need a third object to get involved, for instance it's thought that Triton was one of a pair of similar sized objects in orbit around each other. Triton was (relatively) going backwards at the time of a close approach so landed up in orbit around Neptune, it's partner got slung away. That also seems to explain why Triton is in a retrograde orbit.

    4. Re:Wou would have thought. by Rockoon · · Score: 2

      In the case of small bodies (such as spacecraft) the atmosphere can substitute for the third body.

      No, it can't.

      You have found a way to lose enough energy for the object to remain in the system, but have not found a way to then add the energy necessary to put the object into a stable orbit (one that doesnt intersect your "solution" atmosphere.)

      To be quite clear: If the orbit intersects the atmosphere this time around, and you dont add energy at some point immediately after that, then it will again intersect the atmosphere the next time around, and the time after that.... it will only take a few orbits until your "captured" object slams into the planet/moon that you claim is "capturing" it.

      --
      "His name was James Damore."
  2. Aphelion vs Parhelion by TWX · · Score: 4, Informative

    Phobos and Deimos both have nearly circular orbits, with aphelions and parhelions that are not very far apart:

    Phobos:
    9,234 km min
    9,376 km average
    9,518 km max


    Deimos:
    23,453 km min
    23,458 km average
    23,463 km max


    Compared to our Moon:
    384,400 km min
    363,104 km average
    405,696 km max


    I can see how one could say that Phobos and Deimos, like our Moon, have extremely regular orbital distances, but given that the science that has stated that our Moon was caused by an impactor is still itself being both refined and challenged, I wonder if it's a little premature to conclude that based on orbital characteristics alone the two Martian moons derived from the same sort of event as our Moon. After all, many of the planets have orbits that are very near circular, but we do not interpret their existence in a similar fashion.

    --
    Do not look into laser with remaining eye.
    1. Re:Aphelion vs Parhelion by ceoyoyo · · Score: 2

      "many of the planets have orbits that are very near circular, but we do not interpret their existence in a similar fashion."

      We do actually. It's pretty well accepted that the planets around the sun coalesced from a protoplanetary disc surrounding the young sun. The impact hypothesis for moon formation is similar: a big impact causes debris to be thrown into an orbiting disc around the planet and one or more moons then coalesce out of it. The alternative, capture of a separately orbiting body, isn't seriously considered for the origin of the planets.

    2. Re:Aphelion vs Parhelion by painandgreed · · Score: 3, Informative

      I can see how one could say that Phobos and Deimos, like our Moon, have extremely regular orbital distances, but given that the science that has stated that our Moon was caused by an impactor is still itself being both refined and challenged, I wonder if it's a little premature to conclude that based on orbital characteristics alone the two Martian moons derived from the same sort of event as our Moon. After all, many of the planets have orbits that are very near circular, but we do not interpret their existence in a similar fashion.

      The impactor theory for the origin of the moon is being refined, but AFAIK, it really hasn't been challenged seriously since the early 90's. Before that there were many competing theories for the origin of the moon from forming at the same time as the earth, captured by there earth, formed from impact, and a few others. Meanwhile there are various criteria such scenarios must meet dealing with angular velocity of the moon, composition, etc. In the early 90's computer modeling got to the point that they could do such for impactor theory and resulted with a model of an impact of another body of similar composition that would collide with the proto-Earth, split off a glob that would become the moon while leaving it's own iron core to explain the Earth's relative large one. At that point, while not perfect, the impactor theory was basically doing better over all in the criteria than the other options. I remember seeing the presentation and video of the computer simulation while an undergrad in physics in the early 90's. I've been keeping up with the subject when I see it, and it has been modified, mainly that two moons were formed and then they recombined in a rather low energy collision to form the moon. I have not heard any serious competition by the other theories since then.

  3. Density matches theory by Yergle143 · · Score: 3, Interesting

    Here are some relevant space object mean densities. Mars 3.93 g/cm3, Phobos 1.87 g/cm3, Deimos 1.47 g/cm3, Mercury 5.42 g/cm3, Luna 3.34 g/cm3, Earth 5.51 g/cm3, Ceres 2.07 g/cm3, Vesta 3.45 g/cm3, Europa 3.01 g/cm3, Comet 67P/Churyumov–Gerasimenko 0.47 g/cm3. If Phobos and Deimos formed from a violent collision it might be expected that they would be dense rocky objects like our moon or Vesta. However it seems that these moons more resemble the icy object end of the density spectrum. Did they form during a wetter Martian era?
    I think the take home message is that some exploring of the Martian moons is in order; a sample return mission would be much simpler than a Mars return with an interesting scientific purpose.
     

    1. Re:Density matches theory by ceoyoyo · · Score: 3, Interesting

      The impact hypothesis nicely explains why the moon is less dense than Earth: the impact preferentially threw up light elements from the crust and upper mantle, not heavy elements that would have sunk to the core. The densities of Phobos and Deimos are also less than that of Mars, but because they're so small, and are probably more like orbiting gravel piles, their densities are also consistent with small asteroids.