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Moon Science

The Moon's Two Sides Look So Different Thanks To 4.5 Billion-Year-Old Physics (forbes.com) 96

StartsWithABang writes: 4.5 billion years ago, a giant object collided with our proto-Earth, kicking up debris that eventually coalesced into the Moon. While the near side contains dark maria and lunar lowlands, the far side is almost exclusive heavily cratered, high-mountainous regions. This was a mystery for a long time, but it appears that heating from the hot, young Earth caused a chemical and crustal difference between the two faces.
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The Moon's Two Sides Look So Different Thanks To 4.5 Billion-Year-Old Physics

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  • by ILongForDarkness ( 1134931 ) on Sunday November 22, 2015 @07:16PM (#50982247)

    but couldn't the far side have craters and the near side few because something big, like I don't know the earth, blocks one side and not the other?

    • by Michael Woodhams ( 112247 ) on Sunday November 22, 2015 @07:33PM (#50982351) Journal

      As seen from the moon, the Earth is only about two degrees across, so the proportion of projectiles blocked by it would be miniscule. Even that small effect is reduced (possibly beyond zero) by 'gravitational focusing': projectiles which come towards the moon from the direction of the Earth which would otherwise have missed can be deflected by Earth's gravity such that they hit. (And this happens more often than projectiles that would have hit being deflected so they miss.)

      Here [oxfordjournals.org] is a paper I found on gravitational focusing.

      • "As seen from the moon, the Earth is only about two degrees across"

        Now.

        At the time of the late heavy bombardment, the moon was a _lot_ closer than it is now. Close enough that the earth was a fairly effective shield.

        It also means that tidal forces and heating effects would have been a lot stronger too.

    • No, they explain this in the article. This might account for maybe 1% difference due to the actual distance between the Moon and Earth, but that is about it.

      Ironic, I was watching "The Universe" on Netflix earlier, the exact episode that covered most of this. Good stuff.

      • by Anonymous Coward

        Although the Moon was much closer to the Earth in the past, perhaps 20x closer. The article suggests that the Moon formed just outside of the Earth's Roche limit. However, the articles don't explain the difference in the number of craters on the two sides of the moons. I would guess that most of the visible craters were formed prior to the maria, although I'm not sure why the events that formed the maria occurred almost at the end of (or after) the heavy cratering periods.

      • by rasmusbr ( 2186518 ) on Sunday November 22, 2015 @08:01PM (#50982477)

        Another interesting aside is that many have tried to explain gravity by postulating that the universe is full of tiny particles that fly about randomly in all directions and that gravity works because bodies block the particles from hitting one another.This is sometimes called the screening theory of gravity.

        If you make some reasonable assumptions you will find that two nearby bodies would block particles from hitting one another, creating forces that follow the inverse square law...

        These theories also predict that planets will de-orbit and crash into their stars, and that moons will similarly crash into their plants. But hey, no theory is perfect.

        • The Illuminati wise tail about the formation of the moon. If this were the case, then the earth would have coalesced with iron deposits on one side after the collision and the orbit lock would have been to both celestial objects likely resulting in a solid iron core in the earth, and we probably wouldn't be here. I would have expected they would have better engineered a load of bull-crap to feed us based upon being German/English Blue Blood/Mongol origins of this organization. Why not? They did a hell of

        • Similar to the theory of light bulbs as darkness absorbers?
          • Similar to the theory of light bulbs as darkness absorbers?

            In a sense yes. Attractive forces like gravity and magnetism have always been a challenge to explain in terms of direct contact.

            The motivation for the screening theories is that some people, or perhaps most people, have a deep-seated intuition that all of physics ought to be reducible to direct contact interaction.

            You could say that one of the prime motivations of the early scientist was to prove that there were no non-direct contact forces at work in the universe, except for the force of God himself.

      • Ironic, I was watching "The Universe" on Netflix earlier, the exact episode that covered most of this. Good stuff.

        No, it would be ironic if you had watched "The Universe" to find out about this topic and they explained the concept of irony to you instead!

    • by gsslay ( 807818 )

      If only TFA specifically mentioned this and explained why not.

      Oh wait, it does. You could indeed have missed something; in this case Reading TFA.

  • by Anonymous Coward on Sunday November 22, 2015 @07:24PM (#50982297)

    Another, more plausible explanation is that the moon was actually an ancient artificial space station, placed in a very specific orbit around the earth. Over time, debris has collected on it, giving it its current appearance. The fact that it always faces the earth, and orbits the earth once every month is very intriguing. This isn't something we'd expect to happen were the moon a natural creation. That suggests that it likely was created as an artificial space station which has since become, for lack of a better term, dusty over a long span of time.

    • by Anonymous Coward on Sunday November 22, 2015 @07:39PM (#50982375)

      That's no space station. It's a moon!

    • Not only that, but without the moon, Earth's day would not be even close to 24 hours. (Lunar tides over billions of years have modified the length of the day.) Think how inconvenient that would be! I think you're on to something.

      • The earth's day has always been 24 hours, however the defined period of the 86400 seconds within it may have changed over the millenia.... :-)

    • by ClickOnThis ( 137803 ) on Sunday November 22, 2015 @07:57PM (#50982451) Journal

      The fact that it always faces the earth,

      Nothing remarkable about that. It's due to tidal locking [wikipedia.org] and is quite common. [wikipedia.org]

      and orbits the earth once every month is very intriguing.

      Yea, and Lou Gehrig died of Lou Gehrig's disease. [cue creepy music]

    • by Anonymous Coward

      Did you read the article? The moon was formed much closer to the than it is now, perhaps 20x closer. The authors did some calculations that suggest that the moon likely became tidally locked within a few hundred days of its formation due to its proximity to Earth at the time of its formation.

      And as for it being curious that the moon orbits the earth once every month, that is plain silly. The month was originally defined in terms of the time it takes the moon to go through its phases. And if you're que

      • by ACE209 ( 1067276 )
        But what about the distinct whooshing sound the moon makes when passing over your head? Can you explain that?
      • The article mentions that the moon was tidally locked from the start but does not explain why. At first I was a bit suspicious because that is a huge assumption. The paper in http://iopscience.iop.org/2041... [iop.org] explains that in more details.

        Also, another reason for having 12 months is that this number is, like 60 and 360, quite remarkable because it has a lot of divisors. Simply speaking 12 is the smallest number that can divided by 2, 3, 4 and 6 while 60 can also be divided by 5 and 10. That is why we still

    • That would be neat, however if it were a station I would bet its inexplicable mass (probably much lighter) would be driving astrophysicists crazy. I assume that all of the equations are working out as this isn't the case.

    • Don't forget that humans are descended from the crew of the space station and are divided into two factions. The mutineers and loyal crew. The AI of the station is just waiting for a human to return.
  • I read the article and it mentioned the hot side faced Earth. Not that heat radiating tens of thousands of miles in a vacuum heated it thousands of degrees.

    More than like the force of the bang pushing the material left it run towards the back facing Earth not to mention gravitational pulls swelled the molten core towards Earth. And a 100 degree heat difference from the side facing the sun as well might of had a very small role.

    1 billion years ago the Earth had 100 to 1,000 foot tides as the Moon and the Ear

    • I think that you misunderstood the article. It's not that the hot side faced the Earth, it's that the heat of the Earth (around 2700 Kelvins) kept that side of the Moon (orbiting considerably closer than it does now) hot longer than the side facing away.
      • And the "article" was misconstruing the actual research. The actual study didn't say RADIANT heat from the Earth somehow magically went through the vacuum of space. The latent heat of the impact could very well keep many silicate minerals in a gaseous form on early Earth though.

        The study indicates that mathematically, higher levels of reflected solar radiation ("Earthshine") - which again, could be possible with post-impact silicate gas atmospheric compositions - contributed to the side that always faces t

        • by ceoyoyo ( 59147 )

          "The actual study didn't say RADIANT heat from the Earth somehow magically went through the vacuum of space."

          That's kind of what radiant heat does.

          • Only the Sun is allowed to send those magic rays!

          • Oops, my bad. It was quite late and I meant to type "RADIATING" heat, as in heat being transferred through a medium, as should be obvious from the rest of the comment.

            • by ceoyoyo ( 59147 )

              I was being a smartass, but if you insist... your comment is incorrect throughout. The paper refers to "earthshine" which today is dominated by reflected sunlight. But the point of that paper is that right after the collision that created the moon, earthshine was dominated by radiated heat from the Earth, not reflected radiation from the sun, meaning that the earthshine was much more intense than today. The takeaway from the paper is literally the opposite of what your post said:

              "The actual study [did] s

    • by Michael Woodhams ( 112247 ) on Sunday November 22, 2015 @08:29PM (#50982581) Journal

      1 billion years ago the Earth had 100 to 1,000 foot tides as the Moon and the Earth were much closer

      My initial response is "I don't think so." My second response is to calculate, so here goes:
      Current distance to moon = 384,400 km = 4 x 10^8m
      Current rate of increase in distance to moon = 3.8 cm/year = 4 x 10^-2 m/year.
      If this rate were constant over a billion (10^9) years, then a billion years ago the distance to the moon was 4 x 10^-2m/year*10^9year = 4 x 10^7 m closer, or 10% closer. Tidal effect strengths are inverse-cube in distance, so a billion years ago, lunar tides would have been about 30% larger than now.

      This doesn't come close to "100 to 1000 foot tides."

      • My facts were quoted from a college level biology book. It was believed the tides were much bigger.

        • I've shown the claim is implausible, not impossible. I'd be interested to see what evidence there is to support it.

          I don't see the need for the claim from an evolutionary point of view: there is no reason I am aware of to suppose that current tides are insufficient to drive organisms to evolve into terrestrial niches.

      • by Namarrgon ( 105036 ) on Sunday November 22, 2015 @11:31PM (#50983247) Homepage

        Good, but the rate of increase in distance to the moon isn't constant (it was faster in the past), and it's thought [cornell.edu] that the moon formed at a distance of only about 20 to 30 thousand kilometers.

        By your maths, 4B years ago would've put the moon at 60% its current distance, but at formation it is more likely to have been only 6% of current distance. Assuming similar mass to today, 60% closer implies more like 4.6x the current tidal force - but 6% distance might be 4,600x stronger forces (probably more, given that the distance to the Earth's surface was even closer). How this translates into actual tidal sizes is left as an exercise for someone who knows more than I do.

        Of course back then there probably wasn't much water around, given terrestrial temperatures in the thousands of degrees, but there may have been some impressive magma tides instead.

        • What happened 4 billion years ago is not the point - the claim was made in the context of the 'conquest' of the land by multicellular life, which was only about 0.5 billion years ago (not 1 billion, as I used in my analysis, and so tides were likely only about 15% higher then.) I agree that tidal conditions were very different four billion years ago, and that my linear extrapolation would not apply so far back.

          10% (for a billion years, or 5% for 500 million) is a small enough change in distance that we woul

          • Wait, I thought that everything ever published that tells us exactly how evolution happened is totally accurate? Is someone suggesting that there might be some minor mistakes in those textbooks? What's next? Are we going to hear some nonsense about the global warming models not quite predicting today with 100% accuracy?

      • by ceoyoyo ( 59147 )

        The rate won't be constant. The moon is gaining momentum by stealing it from Earth by raising tides. The closer the moon is, the larger tides is raises, which means it steals more momentum. The actual relationship is likely complex, but it's probably at least quadratic.

  • by berchca ( 414155 ) on Sunday November 22, 2015 @08:15PM (#50982541) Homepage

    I wish we still had physics like that today... these new-fangled ones just don't coalesce like they used to.

    • Ever since they passed all those laws requiring us to conserve energy we've been stuck with the same laws of physics.
    • We freed the banking from the law of supply and demand and the profits of the financial sector have boomed to 25% of all profits earned by all enterprises. But still we are still hampered by rest of the economy saddled with physical process of delivering goods and services in the real world of Euclidean geometry and physics. Ages ago, before we understood the real cost of energy a small band of elite "scientists" passed all sorts of laws, "conservation of energy" "conservation of angular momentum" and the
  • This theory would require the moon to be tidal locked very early in its history. I didn't think that was true.

    • by delt0r ( 999393 )
      it easily could be locked very quickly if it formed close to earth, just outside the roche limit for example. Of course if it liquid it is even harder to define "locked".
  • it appears that heating from the hot, young Earth caused a chemical and crustal difference

    Heating from hot young things tends to cause chemical and crustal differences in my underwear.

    Sorry. It's Sunday night and my fantasy team is in the toilet and I've been drinking.

  • by Anonymous Coward

    Forbes articles display just a blank page for anyone running browser protections, a clear sign that Forbes' web admins are totally clueless since they can't even get static content through to viewer eyeballs.

    • by Anonymous Coward

      Hang on, you run settings/extensions that purposefully disable and remove functionality from a website and then blame the web developers for not giving you a satisfying experience? Do you also remove the tires from your car and blame the manufacturer for the bumpy ride?

  • How do they know the earth was first fully formed and only then collided with something large causing the moon to form? I can imagine it was a bit of a jumble at the time but this claim seems a bit arbitrary. Why the need for a collision with something large? Was it something larger than the moon?

    • by Teun ( 17872 )
      Fortunately we send a few missions to the moon that brought back soil samples.
      With these samples and a lot of remote sensing it became fairly certain that the moon is made up of similar material as the earth's mantle or crust.
      An (logic) extrapolation of this points to the moon being ripped out of the earth after the separation between core and mantle had already taken place.

      Even the creationists have it from their own source: http://biblehub.com/genesis/2-... [biblehub.com]
    • How do they know the earth was first fully formed and only then collided with something large causing the moon to form? I can imagine it was a bit of a jumble at the time but this claim seems a bit arbitrary. Why the need for a collision with something large? Was it something larger than the moon?

      It comes basically from the various theories on the formation of the moon. It could have been created in place, spun off of the earth, captured while it passed by, the result of a collision with another object, etc. So there were lots of different theories, and then there was the actual evidence dealing with the chemical make up of the moon, the orbital mechanics, the conservation of angular momentum, etc. Some theories were better than others and explaining the various facts. Some were much worse than the

  • This article uses way too many exclamation points (I counted 12). Putting exclamation points at the end of every other paragraph doesn't automatically make something interesting and exciting. It just looks like the writer has no confidence in capturing the reader's attention without them!

  • Is Earth's magnetic field caused by the moon?
  • Seriously? The moon is gravitationally locked now, sure, but the Earth (and moon) still being liquid/hot when it slowed to a lock? I don't think so. For this to be plausible the moon would have had to coalesce, in an orbit, with nearly zero spin angular momentum, which seems absurdly unlikely. Otherwise, like a bird on a rotisserie, it would have been "roasted" pretty much equally on both sides. So maybe, but I doubt it.

  • by thegarbz ( 1787294 ) on Monday November 23, 2015 @09:13AM (#50984483)

    Well I guess Slashdot did finally stop linking to Medium. Though it looks like StartsWithABang just moved his personal blog to Forbes and now reposts everything he writes from there.

  • This is old news. Really, really, really old news.

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