Slashdot Mirror

← Back to Stories (view on slashdot.org)

Study Explains Why Lunar Craters Are Bigger On the Near Side

Posted by samzenpus on 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.'"

12 of 85 comments (clear)

  1. earlier paper links by Trepidity · · Score: 4, Informative

    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", the initial report of the observations

    "The Crust of the Moon as Seen by GRAIL", reconstructing the crust thickness and composition from the observations

    --
    10 PRINT CHR$(205.5+RND(1)); : GOTO 10
  2. Not really solving the puzzle. by Hans+Lehmann · · Score: 4, Insightful

    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?

    --
    09 F9 11 02 9D 74 E3 5B D8 41 56 C5 63 56 88 C0
    1. Re:Not really solving the puzzle. by iggymanz · · Score: 2

      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.

    2. Re:Not really solving the puzzle. by DMUTPeregrine · · Score: 4, Interesting

      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.

      --
      Not a sentence!
    3. Re:Not really solving the puzzle. by InfiniteLoopCounter · · Score: 2

      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. 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 earth, leaving one side completely different in makeup from the other side. One side is tidal locked to the Earth, so it always faces us as you already know. You can see some of this by looking at the smooth section of moon vs the rough section as well.

    4. Re:Not really solving the puzzle. by InfiniteLoopCounter · · Score: 4, Informative

      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 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.

  3. Thin crust, thick crust by mrbester · · Score: 5, Funny

    Anybody else unaccountably hungry?

    --
    "Wait. Something's happening. It's opening up! My God, it's full of apricots!"
  4. I thought this was kind of obvious by localroger · · Score: 5, Interesting

    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.

    --
    Brackets contain world's first nanosig, highly magnified:[.]
    1. Re:I thought this was kind of obvious by khallow · · Score: 2

      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 Saturn in its almost circular orbit. In particular, this has resulted in the "Tiger Stripes", a region of substantial volcanism and very new surface (some parts being less than a thousand years old amd considerably warming that the rest of the moon's surface).

      Perhaps the lunar mares were at one time the most stressed regions of the Moon while it was cooling much as the "Tiger Stripes" are thought to be for Enceladus.

  5. Gravitational slingshot by thisisauniqueid · · Score: 2

    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.

  6. Two tides by Okian+Warrior · · Score: 2

    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 moon should experience this same effect. I would expect tidal forces to draw mass away from the center, resulting in more mass on the far side.

  7. Nevermind that... by excelsior_gr · · Score: 3, Funny

    Are there any news of a monolith?