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Moon NASA

Study Explains Why Lunar Craters Are Bigger On the Near Side 85

Posted by samzenpus
from the some-craters-are-bigger-than-others dept.
An anonymous reader writes "A new study of asteroid craters on the moon has uncovered some big differences in the composition of the crust on the two sides of the moon. 'While massive impact basins pockmark the moon's near side, its far side contains considerably smaller basins. The discrepancy in crater distribution has puzzled scientists for decades. To investigate what may have caused this difference, the team obtained data from NASA's twin GRAIL probes, which orbited the moon from January to December 2012. During its mission, the probes circled the moon, making measurements of its gravity. Zuber and her colleagues used this data to generate a highly detailed map of the moon's crust, showing areas where the crust thickens and thins; in general, the group observed that the moon's near side has a thinner crust than its far side.'"
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Study Explains Why Lunar Craters Are Bigger On the Near Side

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  • earlier paper links (Score:5, Informative)

    by Trepidity (597) <delirium-slashdot@hacki s h . o rg> on Sunday November 10, 2013 @06:38PM (#45386773)

    The paper this press release is about doesn't seem to be online, but two papers from the past few months analyzing this GRAIL data (with some of the same authors) are available:

    "Gravity Field of the Moon from the Gravity Recovery and Interior Laboratory (GRAIL) Mission" [pitt.edu], the initial report of the observations

    "The Crust of the Moon as Seen by GRAIL" [ucsc.edu], reconstructing the crust thickness and composition from the observations

  • by Hans Lehmann (571625) on Sunday November 10, 2013 @06:40PM (#45386777)
    OK, there are larger craters because the crust is thinner on this side, but why is the crust thinner on this side? Mere happenstance, or is it caused by orbital mechanics or some other reason?
    • by iggymanz (596061)

      but the moon itself believed to have been formed from a collision of a planet with the mass that became the earth. in that model, the reason why composition would be uneven as the moon tidal lock with one side facing the earth that in fact due to earth's gravity acting on object of uneven composition.

      • but the moon itself believed to have been formed from a collision of a planet with the mass that became the earth. in that model, the reason why composition would be uneven as the moon tidal lock with one side facing the earth that in fact due to earth's gravity acting on object of uneven composition.

        Almost. The moon was thought to be formed in stages [youtube.com]. First by a proto-planet called Thea roughly about the size of mars hitting the earth not full on, leaving debre in space. Over time the debree formed a ring to minimize energy between the earth and the ejected material. The ring then had paricles clump together that rolled on more dust to form 2 objects. These 2 objects with different material makeups (probably from the different amounts of Earth or Thea in them) then collided after some passes around the

        • by icebike (68054)

          Wait, that's a cute theory, but it doesn't explain anything.

          First that theory assumes the the moon became instantly tidally locked from near the moment of its creation, which seems highly unlikely for a body born of an impact, followed by re-impact. (The debris impacts on the far side would occur more often, because the near side would not be shielded by the earth, but that works ONLY once the proto-moon is tidally locked.).

          I'm not sure there is much in the way of evidence for exactly when the moon became

          • by InfiniteLoopCounter (1355173) on Sunday November 10, 2013 @09:54PM (#45387807)

            First that theory assumes the the moon became instantly tidally locked from near the moment of its creation, which seems highly unlikely for a body born of an impact, followed by re-impact. (The debris impacts on the far side would occur more often, because the near side would not be shielded by the earth, but that works ONLY once the proto-moon is tidally locked.).

            I'm not sure there is much in the way of evidence for exactly when the moon became tidally locked.

            This is the current leading theory. Yes, it is very recent, but in the video I linked to you'll see lots of famous physicsts that you should recognize.

            It's been a while since I was in physics studying this kind of thing, but it seems to me that since it is a smaller body and formed around the earth at a much smaller distance from the earth and then moved out, that there would be only a handful of parameters that would determine how long a tidal lock would take. First would be the small mass of the moon and smaller iron core, which would lead to faster tidal lock than say a planet around a star. Second would be the distance from the Earth (smaller the distance the faster it would occur). Thirdly, the impact between the 2 'proto-moons' would directly influence the rotation rate and axis of rotation of the moon (although from a disk around the earth, the eccentricty with respect the earth would be minimal). Since the moon was very close to the earth around its formation, and it formed in orbit around the earth, I would assume that a tidal lock would have occured very soon after its formation. Probably somewhere there is a simulation to show this.

            Here's an article [space.com] that explains why the composition is likely to be different for more fluid materials upon the theorised collision of the 2 'proto-moons'. This explains in principal that the less solid objects were drawn to the near side (as per the original article linked to in the summary), leaving a thinner crust on the near side. The only new interesting take-home is that the period of bombardment of comets and asteroids (due to the stabilization of the orbits of Jupiter, Saturn, Neptune, and Uranus throwing them towards the inner solar system) was likely slightly less violent than predicted before.

            • Another thing to remember is that the moon was much closer in the past and is slowly drifting away from the Earth. Tidal influences on the moon and earth system would have been much greater in the period it is thought the moon was formed.

          • The chance of any object large enough to leave a crater visible from earth on the moon being shielded by earth, is very small. While the earth is roughly four times the diameter of the moon, it's at such a distance that it covers a very small part of the total area from which objects from space will hit it.

            Just look at the sky at night at a full moon. Calculate roughly how much area the moon covers if it's high up in the sky and multiply the surface area by 8 (4^2 then divide that number by two because

            • by icebike (68054)

              The chance of any object large enough to leave a crater visible from earth on the moon being shielded by earth, is very small. While the earth is roughly four times the diameter of the moon, it's at such a distance that it covers a very small part of the total area from which objects from space will hit it.

              You seem to forget that the impactors of that time were all left over debris from a large body impacting the earth, and this junk was orbiting in a loose ring mostly within the orbital path of the Earth. These weren't random asteroids or comets coming in from the oort cloud or some such place.

              When the earth precedes the moon as the earth orbits the sun, it will sweep all the big impactors in its path, and only a few near-miss smaller impactors will deflected into the moon. Similarly, When the moon leads

    • by DMUTPeregrine (612791) on Sunday November 10, 2013 @06:50PM (#45386839) Journal
      Probably some relation to orbital mechanics, Luna likely became tidally locked to the Earth before it had fully cooled. The tidal forces on Luna would have affected how the materials settled. I'm not an expert on this by any means, so I can't calculate what exactly would happen.
      • by Anonymous Coward

        You would think that the near side would have the thicker crust, though.

        • The Earth-Moon system orbits around a point which is not at the center of the Earth, sort of like a barbell with small and large ends will balance on a point which is not in the center of the big end. The point of rotation is still inside the Earth, just not at the center of the Earth.

          This is why we have two tides [roughly] each day - the side near the moon gets attracted to the moon, while the other side gets swung around on the outside and experiences centrifugal force, pulling it away from the center.

          The

          • Yes, that's to be expected, but why there wouldn't also be more mass on the near side, and a low-mass region around the border is what I don't know enough to explain.
          • This is why we have two tides [roughly] each day - the side near the moon gets attracted to the moon, while the other side gets swung around on the outside and experiences centrifugal force, pulling it away from the center.

            I heard it explained as the tidal bulge near the moon experiencing the strongest pull, the Earth in the middle a lower pull, and the bulge on the other side an even lower pull than the Earth. Does that do away with centrifugal force being needed to explain it, or is just another way of putting it?

    • The Lunarians did it! [baenebooks.com]
    • My guess is that non-force that is centrifugal. As the moon split away from the earth, as stated in the Giant Impact hypothesis, the crust would cool. These bits of denser solid crust would then be forced to the far side of the moon, much like a centrifuge forces denser particles to the bottom of a test tube.

      Well, that's my theory anyway, it's probably a load of bollocks.
      • How about this: Over the ages, the far side has been pelted by meteors a lot more than the near side. This has sandblasted away the big craters and has served as a source of raw material for thickening the crust on that side.
    • OK, there are larger craters because the crust is thinner on this side, but why is the crust thinner on this side? Mere happenstance, or is it caused by orbital mechanics or some other reason?

      One way to make a crust thinner is to hit it a lot.

      Works on pizza and iron, don't see why it wouldn't work on moons...

  • ...but that one's far away.

  • by mrbester (200927) on Sunday November 10, 2013 @07:00PM (#45386903) Homepage

    Anybody else unaccountably hungry?

  • I would hazard a guess that if the core is more liquid than the crust then the Earth's gravity might tend to draw it towards the near side more when it gets hit on that side than the far side. Hits on the far side might be more restrained as there is not as much expansion pressure. Over millions of years the effect could become quite pronounced.

    Just talking through my hat, of course, and probably full of it.

    • Tidal effects don't work like that. It's a stretching, not a one directional pull. It would elongate the core, not pull it towards Earth relative to the rest of the moon. The oceans on Earth experience two daily tides for that reason, the oceans bulge both towards and away from the moon. (So does the crust, but less obviously.)

      Moreso, the closer you are to the centre of mass, the less tidal effects you experience. So the moon's core would experience less tidal force than the crust.

      • Earth rotates, the moon not so much, so while there is a high tide and a low tide in the oceans of the Earth due to the moons orbit, the gravitational pull of the Earth on the moon is always in the same direction. There may well be some stretching in the moon from centripetal forces and indeed there are other forces not being taken into account such as the rotation of the Earth-Moon system around the sun or the Solar system around the Galaxy.

        I don't think you know what you are talking about any more than I

        • Tidal effects don't require axial rotation, only the orbit. For example, a tether in orbit will always point towards and away-from the centre of gravity of its primary. They quickly become tide locked because of their exaggerated shape. The two ends experience a force towards, and away from, the centre of gravity of the primary. The same thing happens with a spherical object, but takes longer. That's why the moon is tidally locked.

          Essentially, the radius of orbit is the only line where gravitational and cen

  • by Anonymous Coward

    If the moon is cheese

    Thin and original crust

    Means it is pizza

  • fewer rocks strike the Earth facing side of the moon because the earth is in the way

    • See my comment elsewhere in this thread. The earth is only a very small part of the moon hemisphere. It's effectively a little less than three times as big "in the moon sky" and the rest of the hemisphere. It is actually about four times as big, but to make it easier, I'm also including the half of the hemisphere you can't see in the comparison how much it would shield you.

      It's like saying a thousand people throwing pebbles at you simultaneously have less chance hitting your body because you are holding u

  • Here's an interesting visualization [youtube.com] of a possible early evolution of the solar system. It's a bit surprising just how sudden the late heavy bombardment event was.
  • by localroger (258128) on Sunday November 10, 2013 @07:28PM (#45387051) Homepage
    If the Moon was created by a glancing collision between the Earth and a Mar-sized protoplanet, which seems to be the going theory nowadays, then the Moon was created in Low Earth Orbit during the very heavy bombardment phase of the LHB. Once the Moon became tidally locked -- which would have happened pretty quickly at such proximity -- Nearside was shielded from most further bombardment by the Earth. So Nearside is kind of a fossile from the heaviest epoch of the LHB, while Farside continued to get pelted as the big stuff was swept up, and finally got the fine dusting of the last scraps evening it out. It would also have continued to accumulate crud, which Nearside wouldn't, thus the thicker Farside crust.
    • by khallow (566160)

      Nearside was shielded from most further bombardment by the Earth

      The Earth isn't much of a shield. The vast majority of the junk that passes inside the orbit of the Moon misses the Earth. Really, if a side is sheltered to any degree from impacts, it would be the trailing side of the Moon. Googling around, that results in an interesting theory from a Mike Martinez::

      It would really be interesting to find out why the back-side is so different from the front side facing earth. My own 'little ' theory is that the back side was once the leading side of the moon, the side facing orbital direction very early on---and when the moon was much closer to the earth. I believe that the moon was possibly heading into heavy 'traffic' on it's orbital swings at a much faster pace--maybe taking only days to complete one orbit instead of it's 29 days now. Later on--maybe another few 100 million years or more. other massive hits on the moon occurred--swinging it slowly to it's present position.

      My pet theory is that the early Moon was in a situation similar to Enceladus, a moon of Saturn. Here, Enceladus experiences liquid water volcanism due to the stress heating from moving towards and away from Sat

  • only different. Southern hemisphere of Mars is something like six kilometres higher in elevation than the northern half. The Electric Universe people have a lot to say about such things - none of which involve impact events: hexagonal craters? Craters within craters? Crater chains?
    • by danda (11343)

      Yes.

      --
      When the first space probes returned images of the Moon, they revealed a surface heavily pockmarked with craters and riddled with long-sinuous channels (or rilles). Scientists seeking to interpret these features were constrained by the traditional geologic toolkit. The "debate" over the lunar craters only included two possible causative agents: volcanism, or impact. Eventually, a consensus was reached that meteoric impacts were the primary source of lunar craters.

      But more than forty years ago, the Bri

  • by thisisauniqueid (825395) on Sunday November 10, 2013 @08:38PM (#45387439)
    Wouldn't asteroids hit the moon a lot harder if they glanced past the earth, due to gravitational slingshot? Seems like a good reason for a bigger crater.
  • When the earth and moon collided it was not a high speed affair, or at least not as high speed as most celestial collisions, and happened much later than most theories have it doing so. Part of the moons crust at the time peeled off during the collision and formed most of the land masses we now have on earth. Most of the detritus of the collision continued to rotate about the earth and moon causing many many meteor strikes on both planets. Like I said it's just a theory.

  • by excelsior_gr (969383) on Sunday November 10, 2013 @10:03PM (#45387859)

    Are there any news of a monolith?

  • Gravity is not the only or the strongest force in the universe.

    Most likely the craters were caused by plasma discharges, ie electrical arcs.

    Too much to go into here. Do yourself a favor. Google "electric universe". Read the book "The Electric Sky".

  • I imagine having the earth act like a gravity focus, greatly increasing volume of space and velocities that would have missed the moon completely... need to make a few diagrams to get this across well.

    I can't see why this would not be pretty obvious.

    Of course, I play Kerbel Space Program so have more intuitive feel for these systems.. buy it, learn it, teach it to your children. How else will they dodge space trash?

How many NASA managers does it take to screw in a lightbulb? "That's a known problem... don't worry about it."

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