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Moon Space Science

Earth May Once Have Had Two Moons 139

Posted by samzenpus
from the that's-no-moon dept.
AaronW writes "According to a story at space.com, Earth may once have had two moons. The smaller moon, estimated to be 750 miles (1200km) wide and only 4% of the mass of the larger moon, crashed into the far side of the larger moon which caused the features we see today on the moon. The surface of the far side of the moon is quite different than the side facing the earth, having a different composition and a much rougher terrain."
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Earth May Once Have Had Two Moons

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  • by elrous0 (869638) * on Wednesday August 03, 2011 @03:32PM (#36975968)

    I plead ignorance on the topic, but this explanation strikes me as a little far-fetched considering that it seems more reasonable to me to conclude that the far side of the moon being so different might more reasonably have something to do with the fact that it's permanently facing away from earth. Wouldn't the far side get somewhat more meteor impacts and somewhat more exposure to cosmic radiation, for example? It would seem to me that the earth-facing side would be at least somewhat shielded by earth, compared to the far side--and that over a very long period of time this could make for a difference in geology. For that matter, wouldn't the gravitational field of the earth also have some effect on lunar geology over extremely long periods of time (effecting the two sides somewhat differently), much as the lunar field effects earth's oceans in the very short term?

    Perhaps someone more familiar with lunar geology than your humble narrator could explain why these differences are thought to be unrelated to its orientation to earth, and need to be explained instead by a hypothesis as radical as a moon impact.

    • by arisvega (1414195)
      Hard to say without more quantitative analysis- this moon hasn't always had a 'far side'. There where times where its rotation was not tidaly locked with Earth -i.e. it has not been always showing Earth the same face, this is something that needs time to happen.
    • by mr1911 (1942298)
      I'm sure that's it.

      Stupid scientists publishing papers always overlook the obvious answers in favor of their "two moons colliding" theories.
    • Also pleading ignorance, as I am a biochemist and a moon is a couple of magnitudes away from the scale of things I am used to deal with - from basic physics, though, I don't see how facing/not facing the earth would make a geological difference. Why it should come from another moon impacting there and not from a collision with any old asteroid seems a bit far fetched for me.
      • by gcnaddict (841664) on Wednesday August 03, 2011 @03:54PM (#36976252)
        The explanation given by the paper is that it would've been far more likely for a trojan satellite (one which shares the orbit of our known Moon with the Earth) to have gently crashed into the Moon at a rate of just a few, perhaps one or two, miles per second, which is a collission speed so gentle as to be in absolutely miniscule ranges of probability with an asteroid impact. The net effect of such a slow impact wouldn't be a crater; rather, it would be roughly the same as mashing a clump of dirt on a bigger ball of clay.

        As for the far-side bit, the moon wasn't always tidally locked. Tidal locking happens with lots of time.
        • I was aware how tidal lock works - we get to look beyond our lab walls and offices every now and then ;) Thanks, though, for the part about the collision speed. Gotta have a look at the paper - that is actually an interesting aspect.
        • by blair1q (305137)

          1-2 miles per second doesn't make a crater?

          that's 4000-8000 miles per hour.

          that's certainly crater-worthy.

          • by gcnaddict (841664)
            Are you a astrophysicist?
          • by swillden (191260)

            1-2 miles per second doesn't make a crater?

            that's 4000-8000 miles per hour.

            that's certainly crater-worthy.

            No doubt.

            4% of the moon's mass moving at 1600 meters per second would have a kinetic energy of 3.7e27 J, equivalent to 45 billion of the most powerful thermonuclear bombs ever detonated, and all of that energy would be delivered in a little less than 1/10 of one second. It would also accelerate the moon by 60 meters per second -- 134 miles per hour.

            That seems like it'd make a crater to me.

        • by geekoid (135745)

          MPS not MPH.

      • The bottom line is that we saw from the Apollo missions and tests conducted on the few Moon rocks that were brought back that the Moon has been tidally locked in it's current orientation for nearly 3.4 billion years (or at least that is what NASA was claiming back in 2005, they may have changed their estimates since). This is assuming that the accretion process and the cooling process to create the Moon did not occur naturally in a tidally locked configuration, which computer models have shown that if an ob
        • by Gerzel (240421)

          The trajectory depends on how hard it hits and at what angle as well as how fast it was moving away to begin with.

          Until I see the numbers run in some reasonably good simulations I'm not saying anything.

        • by DavidYaw (447706)

          The bottom line is that a 750 mile object of 4% the mass of the Moon, colliding at the far side of the Moon (as suggested in this article) would have had a very noticeable effect on the Moon's orbital trajectory around the Earth, providing an eccentric elliptical orbit, make it non-tidally locked, and most importantly would be sending the Moon on a spiral towards us, rather than away from us as we are currently seeing.

          If the 750 mile object were in the same orbit as the moon, it would have the same orbital velocity as the moon. With a very small difference in velocity between it and the moon, it wouldn't change the moon's orbit much at all. Since the theory is that the smaller moon would have started in either the L4 or L5 Earth-Moon Legrange points, they would have had the same velocity.

    • by vlm (69642)

      From the moon, calculate the percentage of the sky hemisphere covered by the earth. Its really small. Assuming a random distribution of meteors from every direction, the shadow of the earth isn't that impressive.

      Another way to look at it, is as a thought experiment, imagine magically all the meteors came from the sun, as a magical point source, rather than randomly everywhere. What percentage of the time does the moon spend in a lunar eclipse? Answer is practically zero.

      Note that in low earth orbit, "mo

      • by bberens (965711) on Wednesday August 03, 2011 @03:48PM (#36976160)
        When you consider the gravitational field rather than line of sight then the "shadow" that Earth casts is really quite large.
        • by vlm (69642)

          When you consider the gravitational field rather than line of sight then the "shadow" that Earth casts is really quite large.

          How so? I agree the gravity field of the earth screws around with "lots" of incoming trajectories. But that still means it hits just about as often.

          Lets say, for the sake of argument, that "the system" is contained within a cubical volume of space, a tenth of an A.U. on a side. Something passes thru that cube. No matter how much it curves (more or less) the percentage volume of space represented by the moon is a constant within that cube. So the percentage of impacts is roughly constant.

          • by dgatwood (11270)

            How so? I agree the gravity field of the earth screws around with "lots" of incoming trajectories. But that still means it hits just about as often.

            Because gravity sucks. Earth's gravity is pulling material away from the side facing Earth and towards the side that is away from it. In your model, the number of objects that hit the moon might be the same, but the impacts are now skewed towards objects that would have been farther away in the absence of gravity, and thus, the face of the moon that receives t

            • by Kagura (843695)

              How so? I agree the gravity field of the earth screws around with "lots" of incoming trajectories. But that still means it hits just about as often.

              Because gravity sucks. Earth's gravity is pulling material away from the side facing Earth and towards the side that is away from it. In your model, the number of objects that hit the moon might be the same, but the impacts are now skewed towards objects that would have been farther away in the absence of gravity, and thus, the face of the moon that receives the most impact events isn't the same.

              And objects just a little farther away that wouldn't have impacted anything are now skewed slightly enough towards the earth to reach the moon's orbit and impact the moon. GP was right, earth's gravity doesn't "vacuum clean" or cause a reduced chance of impacts on the moon.

              • by dgatwood (11270)

                And objects just a little farther away that wouldn't have impacted anything are now skewed slightly enough towards the earth to reach the moon's orbit and impact the far side of the moon.

                FTFY.

                It is only equal when the moon is roughly 90 degrees from Earth's orbital plane. At that point, the side of the moon facing in the direction of Earth's orbit is going to simply get different impacts.

                When the moon is ahead of Earth, objects flying towards the moon hit the back side of the moon anyway.

                When the moon is b

          • by blair1q (305137)

            And, anything that is coming past Earth towards the moon is going to be lensed into the moon. Earth's gravity is dangerous.

            Maybe that's why there are more big craters on this side.

            • by RockDoctor (15477)
              Object passing the Earth but not stopping there (colliding) will be lensed towards the Moon. But because the Moon is quite small, most of them will miss because they're lensed too much. Seen from the Moon, the annulus on the sky of close encounter trajectories (with the Earth) that will result in a collision that would not otherwise have happened is quite a narrow annulus.

              Look at the full Moon in the sky ; now quadruple it's diameter and you've got a first approximation to the view of the Earth form the Mo

      • by rubycodez (864176)
        But the real situation is different. Objects in orbit around the sun go in the same direction. Things get disturbed in outer orbit and get thrown inward, or are in orbits that go far out then back around sun. For things moving the same direction around sun, the Earth forms large shield covering small remote Moon almost half the time , and the other half the time the same face of moon, opposite the earth-facing side, is open to impacts.
      • by perpenso (1613749) on Wednesday August 03, 2011 @03:51PM (#36976194)
        I don't think you can use the observable earth as an indication of the protection offered. I would expect that the earth's "gravity well" (sorry, a more proper term escapes me at the moment) would be more relevant. That said I am quite suspicious of your overall approach. It seems to be commonly accepted that Jupiter provides the earth with significant protection, now consider the percentage of the sky hemisphere that jupiter occupies.
        • I would expect that the earth's "gravity well" (sorry, a more proper term escapes me at the moment) would be more relevant.

          Is it the Hill sphere? I think I remember that being the volume where an object's gravitational attraction is the dominant influence on other objects.

        • by Convector (897502)

          "Gravitational cross-section", perhaps?

      • by dgatwood (11270)

        Assuming a random distribution of meteors from every direction, the shadow of the earth isn't that impressive.

        First, your initial assumption is begging the question, IMO; the statement that the distribution is random is equivalent to the statement that impacts will be equally distributed, therefore you're saying that impacts will be equally distributed because they are equally distributed.

        You would actually expect the majority of meteor impacts in an orbital system to be caused by a fast-moving planet or m

        • Thus, there should, in theory, be slightly more impacts on the side of the moon in the direction of its travel, but the difference should be relatively tiny, probably to the point of being undetectable.

          Either this explanation works, or it doesn't. If it does, then slightly more is true: the density of impacts on the near side of the moon should increase as you move away from the closest point facing the earth.

          That's trivial to see from your picture. When the moon is directly in front of the earth's

    • by cybergrue (696844)
      I agree. The prevailing theory of why the Moon's sides are so different is because the tidal lock caused the magma flows on the near side. This smoothed things out on the near side while as you stated, the far side was exposed to more meteor impacts. Also, the magma flows are thought to be relatively (in geographic and astronomical terms) recent and possibly ongoing, hence erasing any signs of older impacts under the lava.

      What probably happened here is someone decided to model what would happen if the

      • Hm, how far into the moon's life did tidal lock set in? Was there enough volcanic activity left for this mechanism to work? With the recent reports on rather young volcanic structures, that might be - but as I said above, I am kinda clueless regarding anything exceeding basic astronomy.
    • by alta (1263)

      I just wish they'd figure away to turn the moon around for a while. I'm kinda getting tired of the view, and if there's something different on the other side, i sure as hell want to look at THAT for a while.

  • Serious guys. You learn read book now. Thanksbye.

    • by Anonymous Coward

      Clearly, the have crashed into the far side of the had, leaving behind a rough grammatical landscape.

    • A number of explanations have been proposed for the far side's highlands, including one suggesting that gravitational forces were the culprits rather than an impact from Francis Nimmo at the University of California, Santa Cruz, and his colleagues.

      Maybe I'm reading this wrong, but are they suggesting that Francis Nimmo and his colleagues hit the moon?

      Damn. That Francis Nimmo is so fat...

  • 2 moons? (Score:2, Insightful)

    by margeman2k3 (1933034)
    That's no moon...
    • Oh wait, yeah it's just a small moon.

    • by slick7 (1703596)

      That's no moon...

      The moon is hollow, it rings like bell. Also, legend has it that not only did Earth have two moons, but it no moons too. And like the [parent] said, that's no moon!

    • That's no moon...

      Because the bloody IAU reclassified it as a dwarf moon or "Lunaoid".

  • "Earth Two Moons Once May Have Had" ....

    • by YodasEvilTwin (2014446) on Wednesday August 03, 2011 @03:44PM (#36976114) Homepage
      You mean "Two Moons, Earth May Once Have Had" or "Had Two Moons Once, Earth May Have." Yoda often places the predicate (minus the helper verb or all verbs) before the subject -- "Lost a planet, Master Obi-Wan has" -- or otherwise rearranges phrases. He doesn't jumble words around randomly.
      • Obviously, you had to be summoned, YodasEvilTwin... The Master's sentence structure is actually closely related to Latin sentence structure - which actually pretty much allows for near random jumbling. It has a preferred mode, though, which is Yodaesque.
  • Next time, take better care of your natural satellites.

  • Details are still sketchy at this point. Reports that the second moon was inhabited by a super-intelligent race using crystals to seal away an ancient evil, and was accessible from Earth via a "Big Whale," could not be substantiated.

  • These folks include my Christian, Jewish and Muslim friends, whose religions revolve around the dis-ambiguity that mother earth has always had one moon and not two...otherwise which moon would my friends' holy books be referring to?

    • So a story based on apparently a single study (unproven) that "suggests" that the earth MAY have had (in other words, this *could* be a plausible explanation for why our one moon looks like this). And you already *have* accepted it while criticizing those who won't?

      The study hasn't even, as far as I can tell, been reviewed or in some way shown to be plausible, however that works with studies on essentially history.

    • As I do not know many Jews or Muslims, I can only speak for this atheist's Christian friends, who have no problem at all with science. On the large scale, literalist idiots are basically an American problem. Hell, the Vatican has an observatory, with some damn good scientist running it.
      • by NevarMore (248971)

        Hell, the Vatican has an observatory, with some damn good scientist running it.

        Is the telescope pointed at the playground next door or the window in the nunnery shower?

    • by pnewhook (788591)
      The two moons would have been millions of years before any religious texts were written.
    • by rubycodez (864176)
      interesting, but they would not see contradiction as this tiny moon became part of The Moon while our earth was still being formed, 10 to 100 million years after whatever caused our present moon to get scooped out of the Earth, long before life appeared. So just a part of creation of our Moon, really.
    • You don't need to look to something so esoteric to find missing facts in sacred texts. For example, I don't recall much about micro-organisms in the various bits I've read; however there is a fair amount about various hygienic practices.

      Is there really any need to nit-pick at peoples beliefs? Nobody's beliefs are literally right anyways.

    • As longtime watchers of QI know, comedian Rich Hall already has a hard time accepting the entire concept of there being two moons around earth, even without this revelation. He'll be insufferable now. Stephen won't be able to shut him up.

    • by Reapman (740286)

      How so? just curious where a second moon would somehow invalidate their beliefs. Oh wait this is just flamebait to get a religious discussion going and get us completely off topic. My bad!

    • by geekoid (135745)

      Since it would have crashed into the larger moon many, many, years before your religions existed, it's n surprise it's not in your texts. Like Dinosaurs, or other planets.

  • but what have now?

    can earth has more?
  • Ungh, ones NUFF man!! Nuh UNGH!!

  • by Anonymous Coward

    "It is entirely plausible for a Trojan moon to have formed in the giant impact, and for it to go unstable after 10 million to 100 million years and leave its imprint on the moon," study coauthor Erik Asphaug, a planetary scientist at the University of California, Santa Cruz, told SPACE.com. Imagine "a ball of Gruyere colliding into a ball of cheddar."

    shudder

  • Did it have a shadow of a jumping mouse on?

    There is no Dark Side of the Moon really, matter of fact its all dark.

  • It's a known fact that the real far side of the moon is just a Hollywood-caliber set that was created on some government base. Why? Because the real far side of the moon contains a Jetsons-like society of humans who have no concept of a debt ceiling.
  • by MoldySpore (1280634) on Wednesday August 03, 2011 @04:05PM (#36976398)

    No. The second Moon was the home to the Lunarians [wikipedia.org]. The Lunarians are a race of beings from a world destroyed which became the asteroid belt, and are identified by a moon-shape crest on their foreheads. They created this artificial moon, resting until a time they believe their kind can co-exist with humans. But the whole second moon thing got screwed up for everyone, because after Cecil whooped Zeromus' ass, the Lunarians decided to throw that moon into gear and get the hell away from us.

    Thanks for nothing, Lunarians.

  • Seems that we regularly 'discover' that Uranus has another moon or two every now and then. http://en.wikipedia.org/wiki/Moons_of_Uranus [wikipedia.org] I think we know who the culprit is. I would advise you to be a bit more responsible for what Uranus does. Don't think we haven't noticed. Bathing might help.
  • Sadly, it was destroyed during an accidental "mooning" maneuver the Earth was trying to direct at Venus over some perceived sleight from the previous drunken weekend at the Solar System Club*.

    *Membership required.

    • by Cogita (1119237)

      Sadly, it was destroyed during an accidental "mooning" maneuver the Earth was trying to direct at Venus over some perceived sleight from the previous drunken weekend at the Solar System Club*.

      *Membership required.

      Note: This was unrelated to the events preceding Pluto's expulsion, which was the result of his fraudulent claims to be an only child.

  • by hedley (8715)

    I've seen it. It's rubbish.

  • I read the article, but could not find any mention of observations or physical evidence which supports this hypothesis better than, or even as well as, any generally accepted theory. The article only seems to describe a simulation that the authors cooked up. It would seem that such a collision would have some significant residual effects on the orbit of the moon. I could not find any mention of the time period in which this hypothetical collision occurred. Would there be any geological evidence for it o

    • by Convector (897502)

      In the actual Nature article, the authors mention that a test of their model would be to find evidence that material with a different composition accreted on the far side. They also say that evidence for accreted material should show up in the gravity measurements of the upcoming GRAIL [mit.edu] mission. But these would be pretty subtle distinctions.

    • by geekoid (135745)

      Correct. They ran some numbers and a small moon crashing into the moon fits the data as we know it now; however it doesn't eliminate other possible causes.

      This is just another data point. As always with science, new data may change what we know, or more likely, refine it.

  • I still cling to the "Moon is made of cheese" theory, though the thought of two moons colliding does seem intriguing, especially if one was made of Colby and the other Monterey-Jack.
  • I always knew there was something special about the Dark Side of the Moon. "There is no dark side of the moon really. Matter of fact it's all dark." Queue the "get off my lawn dirty hippie" remarks.
  • But Stephen Fry had told us that there are two moons right now! The Moon and Cruithne [youtube.com]... of course in a later episode he claimed there's 4 or 5...
    • by geekoid (135745)

      Funny clip, however Cruithne is Near earth asteroid, and not a moon. It does NOT orbit the earth.

  • Earth May Once Have Had Two Moons

    And one of them had the face...of Jackie Gleason!

    I've seen pictures.

  • by Rydia (556444)

    They could've just asked me. I was there when the second moon left, after all!

  • by log0n (18224)

    Isn't Cruithne our 2nd moon?

    • by log0n (18224)

      Nm I'm a bit late to the game.. saw the above posts re: Cruithne

    • by Sulphur (1548251)

      Isn't Cruithne our 2nd moon?

      It stopped in the asteroid belt when funding ran out.

      No bucks, no Buck Rogers.

  • Such a collision lends itself to catastrophism, a theory of large changes that happen relatively frequently.

    This would have serious consequences for the evolution of life on the surface.

    Nothing would be more disasterous for prelife than a 750 mile wide object impacting the surface of the moon, which at that point in time would be less than 30KM from the earth. The impact would have created impactors almost certainly miles in size that would be ejected into the local gravitational well around the earth.

    For

    • by RockDoctor (15477)

      Such a collision lends itself to catastrophism, a theory of large changes that happen relatively frequently.

      Speaking as a geologist ... dump the "relatively frequently". If it's happening "relatively frequently", then it's a normal event, even if it's catastrophic.

      "Catastrophism" is the acceptance that events do (have) happened in geological history for which we have no direct observational experience. So for a large part of human history, large volcanic eruptions were unexpected catastrophes, until peopl

      • by hackus (159037)

        Well,

        "It would have reduced the time available from around 900000000 years to 899900000 years."

        True, but I am already shocked how fast life evolved, so even 100,000 years to myself is even more shocking under the conditions in the local moon/earth neighbourhood after something like this happens.

        It must have been an awe inspiring site to see the impact from the surface of the earth. Well, in a pressure suit anyway. ;-)

        What I can't figure out is the m

        • by RockDoctor (15477)

          but I am already shocked how fast life evolved,

          I enjoy knocking people over with a landscape where you get to see 500 million years of geological activity, after climbing up a few million years of such activity. We're still looking at around twice that amount of time. That is a lot of time.
          There is a geological term, "deep time" ; it's a gatekeeper to understanding geology in the same way that appreciating "astronomical distances" is to understanding astronomy. Which is why I repeatedly take aspirant geol

  • Well, obviously the far side of the moon didn't have to be finished off. No one was going to see it.

  • ... you saw the hole in the moon.
  • I am not a lunar scientist, but if we're talking about a slow-moving collision with another orbiting body coming from either L4 or L5, wouldn't that have been a collision with the leading or trailing sides of the moon?

    It seems...particular...that one of these impacts would have occurred and would have *just* put enough spin on the moon that the moon turned this side away from earth, and then stopped?

    Or are they asserting that this body was so low-density that whatever layer it accreted onto the moon then in

  • And at one point atlantis rose from the earth, taking with it all the humans, to another planet.....leaving behind that big crater in the bottom of the atlantic....
    but I guess this too, we will never really know or be able to prove

  • One of the problems with moon-related theories is that there really is hardly any evidence left for us to go on. There's a prevailing theory that a mars-sized planet struck earth, and the rebound caused a the moon to glob off. How do we know this is, well, plausible? First, some reasonable assumptions are made about the pre-moon earth, then a computer simulation is run that applies the theory, and then the final results of the simulation are compared to the present time. The prevailing moon theory is th

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