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The Moon Is Shrinking Like a Wrinkled Apple

Posted by Soulskill on from the that's-no-apple dept.

astroengine writes "New observations by NASA's Lunar Reconnaissance Orbiter have uncovered a number of previously unknown, recently formed 'lobate scarps' — raised cliffs about 9 meters high and several kilometers long — over the lunar surface. These scarps form along thrust faults where compression forces the moon's crust to rise. Up until now it was thought these lobate scarps only occurred around the lunar equator, but the high resolution LRO imagery suggests they are ubiquitous, regardless of latitude. As the moon is geologically inactive, what could be creating these features? It would appear the moon's surface is acting like the skin of an apple surrounding the shrinking, dehydrated flesh of the fruit; the lunar crust (skin) is wrinkling as the body of the moon (the flesh) shrinks due to cooling contraction inside the moon's core."

7 of 116 comments (clear)

  1. Tides? by egburr · · Score: 4, Interesting

    My first thought was couldn't this be more of a tidal effect than due to shrinking? After all, look at what the orbiting mass of the moon does to our oceans. Wouldn't the mass of the earth have a similar effect on the moon? Even if it is tide-locked so the same face always faces the earth, surely there's some slight wobble to that that would cause stress.

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    1. Re:Tides? by Froze · · Score: 3, Interesting

      I think the idea is that tidal distortions would be almost exclusively limited to the equatorial regions, this appears to be radially isomorphic, indicating that it is not the result of tidal stress.

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    2. Re:Tides? by ewskau · · Score: 3, Interesting

      The article does not mention this at all, but if the moon is shrinking then its rotational period must be getting shorter (angular momentum). There does not seem to be an indication that the period of the moon is decreasing, suggesting its either too small of an effect, not there, or not being looked for.

    3. Re:Tides? by mea37 · · Score: 3, Interesting

      Interesting point. However, I think tidal locking makes it a little more complicated than that.

      I'm not sure what happens when a tidally locked satelite's diameter gradually changes, but given that tidal locking is an equalibrium state it seems reasonable to suspect that the tidal lock is preserved.

      If so, then as the moon's rotation would naturally tend to speed up, the Earth would pull back on it. This would reduce the increase in rotation, but to preserve angular momentum it would also have to increase the orbital period - meaning the moon would move to a lower orbit with both its period of orbit and its period of rotation slightly reduced.

    4. Re:Tides? by osu-neko · · Score: 5, Interesting

      This would reduce the increase in rotation, but to preserve angular momentum it would also have to increase the orbital period - meaning the moon would move to a lower orbit with both its period of orbit and its period of rotation slightly reduced.

      You were right before the dash: it will increase the orbital period, not reduce it as you said (contradicting yourself) after the dash. This will push the moon into a higher orbit, not a lower one. And indeed, the moon is moving 38mm further away every year, although this is primarily due to the same effect slowing the Earth's rotation rather than the Moon's.

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  2. Geologically inactive? by Hatta · · Score: 4, Interesting

    That the moon is undergoing these kinds of changes shows that the moon is geologically active. There may be no convection going on in its core, but this is still geological activity.

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  3. Similar to early theories of Earth geology by penguinchris · · Score: 2, Interesting

    I would like to point out that before tectonics was relatively well understood and accepted (this only happened in the late 1960's), among the various models proposed to explain geological structures on the earth such as mountain belts was this exact idea.

    So personally, as someone interested in the history of the field of geology (and a geologist myself), I think this is really pretty awesome. Whoever came up with this idea before really had a great idea - I can't recall when it was but it was likely mid-late 1800's, so too bad he's not still around to see that his theory was plausible.

    It was understood well before the 1960's that this couldn't explain the earth's structures - it was not a seriously considered theory for very long for several reasons - but at least the idea is sound.

    And to comment on those who are saying that this proves the moon is geologically active, I think this is a pedantic point which depends on how you define "geologically active", and that's the kind of thing that has an obvious simple answer to start with but then gets complex when you have situations like this.

    As a geologist I would still define the moon as being inactive. Active to me would imply influences besides simply gravity (although gravity is of course a major driving force in earth-type tectonics, it's not the only factor). If you subdivide the earth into active and inactive areas, even the inactive areas will occasionally have things like earthquakes happen, due to intra-plate stresses or whatever. But you won't get volcanic activity or major tectonic activity in those areas, just like you won't on the moon.