The Moon is shrinking!

By Phil Plait | August 19, 2010 12:46 pm

The Moon is shrinking!

Well, a little: new results from the Lunar Reconnaissance Orbiter indicate that over recent geological time, the Moon has shrunk by approximately 100 meters in diameter!

Here’s the evidence, or at least one piece of it:

LRO_gregory_scarp

[Click to unshrinkenate.]

That image shows the Gregory scarp, a cliff across the surface of the Moon. Scarps like these have been known for centuries — I’ve observed many myself using a backyard telescope — but it was always thought they were big and restricted to just some areas on the Moon. LRO, though has found many smaller scarps, and also importantly that these scarps are distributed globally, all across the surface of our nearest neighbor in space.

What does this have to do with shrinkage?

lunar_thrust_faultThis diagram shows how scarps are formed: thrust faults. If the Moon shrinks, even a little, the surface shrinks too, and gets compressed. This causes stress under the surface, building pressure. Eventually that pressure exceeds the strength of the rock above it, and you get a fault. One side of the fault slams upward, forming the scarp.

We see these sorts of features on many bodies in the solar system, including the Earth, where their origin is from tectonic plate motion. As the continental plates move past each other, friction allows the pressure to build up. There’s no such drift on the Moon — the surface is essentially one big piece — so something else must be causing it.

Shrinkage makes the most sense. The interior of the Moon is still warm from the leftover heat from its formation billions of years ago. Hundreds of kilometers of rock makes a pretty good insulator, so the Moon has cooled very slowly. As the interior cools, it shrinks, and the surface collapses down as well. Pressure builds, then snap! While a lot of these scarps are very long and dozens of meters in height, LRO’s sharp vision (it can see objects less than a meter in size!) can spot scarps that are only a kilometer or two in length and a few meters high. That sort of feature is not possible to see from Earth. They’re too small.

So this scarp formation has been going on essentially since the Moon formed, but the question is: does this process still continue today?

Maybe. The scarps seen are relatively young, something like 800 million to a billion years old. Dating them is difficult, but again LRO’s keen eye helps us out:

LRO_scarp_craters

In this image, the scarp seen cuts right across some small craters. Geologists can use the number of craters to get an age for the landscape, which is how they have been able to get a rough estimate of how old the scarp is. Also, despite the lack of air and standing water, the Moon undergoes erosion. Solar wind, micrometeorites, and even the thermal expansion and compression caused by the Moon’s day/night cycle all work to grind up surface rock into dust. The scarps look quite fresh, limiting their age.

Another interesting bit of evidence is that the Apollo missions put seismographs on the Moon, which recorded quite a few moonquakes. Several dozen of these quakes were shallow, which may be related to scarp formation (since the thrust faults are near the surface).

So, does this mean the process is still going on? It’s not really possible to say as yet. A billion years is a long time, and it’s difficult to know if any small scarps are younger than that. As LRO scans more and more of the lunar surface, more of these types of landscape features will be seen.

And finally, what does this mean for the ultimate size of the Moon? Well, you have to put this into perspective: the Moon’s diameter is about 3475 kilometers. This shrinkage is only about 0.003% of that. In other words, you’d never, ever be able to see that by comparing the Moon’s diameter now as it was, say, a billion years ago. So we’re not in any danger of the Moon collapsing down into a tiny little ball!

What it does mean is that despite it being perhaps the most well-studied object in the sky, our Moon is still capable of surprising us. Not only that, but some of its secrets are actually rather big, but so well-hidden we need to study the Moon pretty carefully to uncover them. Happily, we’re doing just that. There’s not a scientist on the planet who doesn’t like surprises, especially when they’ve been sitting right there in the sky for us to find.

Credits: Credit: NASA/Goddard/Arizona State University/Smithsonian


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CATEGORIZED UNDER: Astronomy, Cool stuff
MORE ABOUT: LRO, Moon, scarps

Comments (61)

  1. What it does mean is that despite it being perhaps the most well-studied object in the sky, our Moon is still capable of surprising us.

    Of all the objects I love to look at through the telescope, the moon is still right up there at the top! If for no other reason, and to paraphrase Gene Cernan, it’s a place!

  2. QuietDesperation

    I keep telling them to wash it in *cold* water.

  3. Off by a factor of 2: “the distance between the moon’s center and its surface shrank by about 300 feet” translates to a shrinking diameter if 200 meters.

  4. Pete

    @QuietDesperation – no! Cold water makes the shrinkage worse!

  5. NY POST headline: The Moon is shrinking in the cold…. like a frightened turtle!

  6. AZSkeptic

    Yeah, but when is it going to explode out of our solar system with Martin Landau and Barbara Bain on it?

  7. “What it does mean is that despite it being perhaps the most well-studied object in the sky, our Moon is still capable of surprising us.”

    I guess the question is “the most well studied object in the sky” with WHAT?

    Sure, we’ve studied the moon a lot, but not with anything like the LRO Camera (LROC). I mean, this sucker is showing us the astronauts’ foot trails!

    Or any of the other instruments, for that matter.

    So I would say that if we spend hundreds of millions to get that kind of firepower in orbit at the moon, we better NOT be surprised that we find out something new.

    Just a devil’s advocate point of view on this….

  8. BJN

    “Relatively young” seems a tad misleading. A billion years is nearly a quarter of the age of the Earth and Moon. The shrinking would date back to the first complex multicellular life on Earth.

  9. Mark Beadles

    Rest assured, if the Moon did ‘collapse down into a tiny little ball’ Mike Brown would demote that sucker.

  10. MattF

    I expect the creationists to try to back up the clock and assert that if the Moon is shrinking, and if it’s really all those millions of years old, then it should have been big enough to smack dinosaurs in the head. Or something.

    Another theory for the extinction of dinosaurs: Blunt force trauma!

    Maybe the winged ones even flew up there and died.

    So we need to go back to show the creationists that there aren’t any pteranodon fossils on the Moon. Of course, to assert that there aren’t any, we’ll have to do a lot of exploring. (Just don’t let on that we’re doing real science there, too, or the creationists will shut us down.)

  11. I, for one, would like to hear Neil Adams’ comment on this.

  12. Andrew

    I seem to remember that, before the general acceptance of plate tectonics, one of the early theories that attempted to explain the formation of geological features on the Earth – mountains, for example – and also to explain why earthquakes occur, was that they were a result of cooling and contraction – http://en.wikipedia.org/wiki/Geophysical_global_cooling ?

  13. Cynic View

    It’s time to haul Wallace back before he eats up all the cheese.

  14. Torbjörn Larsson, OM

    This why you should always go with the plate tectonics option in the land contract.

    I’ve observed many myself using a backyard telescope

    News at noon: Bad Astronomer denies reality. Says he “can’t see the purpose of looking in nearest mirror, I know these noble features don’t shrinkenate”.

    More seriously though, a ~ 0.003 % linear volume shrinkage would imply a temperature difference dT ~ 3*3*10^-5/10*10^-6 ~ 10 K (assuming concrete as ‘rock’ and small volume change [Wikipedia on coefficient of thermal expansion]; also assuming solar constant et cetera constant).

    If the Moon core was ever near fluid, and the first spherical mode prediction of a remnant mantle convection riser giving the fossilized magnetic field says that, it still likely is! The Moon suddenly became a hotter subject.

  15. OtherRob

    It was in a pool!

  16. Zucchi

    We really need to go look at this stuff from ground level, on foot.

  17. Julie T

    I think this is actually the work of Grue and the Minions…muahahahaha!

  18. amphiox

    The moon is shrinking because it’s cooling!

    So AGW must be a lie! Ha!

  19. I heard the pyramids were also stolen replaced with inflatable replicas. Gruuuu!!!!!!!!!!!!!!!!!

    I always wondered just how cool the moon (& Mars) would really be in the center. I guess not as cool as we thought.

  20. Thomas Siefert

    So we’re not in any danger of the Moon collapsing down into a tiny little ball!

    I’m just waiting for the next chain email with the subject : The Moon is collapsing down into a tiny little ball!

  21. Darth Wader

    I see a major fault in your story.

  22. Rik

    I wonder what Neal Adams has to say about this.

  23. Rider

    Has anyone consulted Neil Adams?

  24. The Moon is in danger! I believe that the Earth will be the end of the world. Because we have troubles, then the Earth will be disaster by the nature (rains, earthquakes, winds, and others, etc.) Also the Earth will be threatened by the Sun. Do you believe that 2012 would be waiting for us?

    From Colombia

  25. Thank goodness the darn thing is moving away from us at a fast clip of 3.8 centimeters per year. The idea of using the small craters “to get an age for the landscape” is a fascinating one.

  26. Gavin Flower

    300feet is approximately 90metres

    1 inch = 25.4 mm = 0.0254m (legal definition of an inch)

    1 foot = 12 * 0.0254m = 0.3048m

    300feet = 300 * 0.3048m = 91.44 metres exactly

  27. alfaniner

    “M-O-O-N” — That spells “shrinkage”.

  28. Kevin F.

    Just like in Despicable Me!!!!!

  29. Pat Montana

    @29 alfaniner: LOL! Way to take a “Stand” on the issue!

  30. The moon is collapsing…the moon is collapsing, run for your lives! ;o)

  31. Justin

    Well I don’t doubt the moon will shrink with age, I’m skeptical about it being a recent phenomenon though.

    The hypothesis goes that the absence of craters along the faults suggest that the faults are recent and that could be one mechanism, but the only one? For example, in the good old days engineers would strength a surface by shot peening, you know the hammer tone finishes they try to emulate with paints these days. What if the moons crust is stronger in areas where craters proliferate and faults naturally occur where craters are scarce. It would make more sense to me than claiming the “The moon has just started shrinking”

    You know what’s coming now, emails telling us the moon will be the size of a peanut before the end of the century.

  32. ND

    Ok who dropped the micro black hole into the moon?!

  33. Sridhar

    @MattF: There is a new Tamil song with lyrics to the effect “His fame is so tall that it brushes against the moon”. Could not help getting reminded of it by your comment!

    Actually, that song has a fair amount of scientific imagery in its lyrics (Endhiran songs, for the Tamilians amongst you). Remember Phil talking about another Pop singer with good science in her lyrics.

    Maybe that is the best way make science cool & accessible to all?

  34. ray

    @gavin flower
    i thought the same thing. however his point was that the radius shrank by 300 feet… on both sides of the center.

    so that gives a diameter shrink of 600 feet or 180m.

  35. Messier Tidy Upper

    Well, I’m surprised by this. I knew there had been (is ongoing?) shrinkage on Mercury which is a similar world so I guess maybe I shouldn’t have been – but I am. ;-)

    The idea of recent geological activity – even shrinkage like this – happening on our Moon is surprising given how long ago I thought it had cooled down and how ancient even the youngest moon rocks recovered by Apollo have turned out to be.

    I second the call for a human lunar return mission preferably including a permanent lunar base (& while we’re at it, let’s build a Lunar Farside telescope into the bargain!) to investigate this and the many other Lunar surprises and undiscovered facets we’ve yet to find on the nearest, most accessible unearthly astronomical body.

    Gee I wish we had a plan ready and under construction to go back there ASAP.. Oh wait we *did* Ares-Constellation – but then Obama cancelled it. :-(

  36. How do you know the shrinkage isn’t caused by all that cold water that’s on the moon?

  37. Tempus Vernum

    Huh, I don’t know why it amazes me that the interior of the Moon is still hot but I really had no idea that was the case.

    Even the Moon is still capable of surprises now and then.

  38. Nigel Depledge

    Justin (33) said:

    What if the moons crust is stronger in areas where craters proliferate and faults naturally occur where craters are scarce. It would make more sense to me than claiming the “The moon has just started shrinking”

    It looks like you have not understood what Phil was saying.

    Something must cause the faults to occur. The only possibility is movement of one part of the surface relative to another part. Your idea that the crust happens to be softer where there are fewer craters (and why are there fewer craters in those places?) does not work.

    The faults are seen all over the moon. They are many ages. The one that Phil highlights is relatively young – which we can tell because there are few craters there. But the faults also occur at places that have many craters (sorry, I don’t have a direct reference for this, it is implied in what Phil has said).

    No-one is claiming that the moon has “just started” shrinking. It has been shrinking (very slowly) for a long time. The new discovery means that (a) we can be relatively confident that the cause of the faults is shrinkage – after all, it must be a global phenomenon of some kind – and (b) it may still be shrinking (it might not, but it might).

  39. Nigel Depledge

    Messier Tidy Upper (36) said:

    I second the call for a human lunar return mission preferably including a permanent lunar base (& while we’re at it, let’s build a Lunar Farside telescope into the bargain!) to investigate this and the many other Lunar surprises and undiscovered facets we’ve yet to find on the nearest, most accessible unearthly astronomical body.

    Lunar Farside Radio Telescope. To make the Square Kilometer Array look tiny. Now that would be something!

    Gee I wish we had a plan ready and under construction to go back there ASAP.. Oh wait we *did* Ares-Constellation – but then Obama cancelled it

    Don’t start this again. We had enough of it on the previous moon thread.

  40. Jeff

    Excellent,

    Now THIS is the type of blog post I love to see, but then again I’m a professor, so this type of back to basics principles with diagrams and hypothesis, are science at its best, I think. Keep these types of posts coming, I love them.

    This illustrates the difference between a true scientist like Phil and a phony one (not one at all) like Richard C Hoagland. RCH has become such a big mouth joke it isn’t funny or entertaining any more, just ludicrous. Hate comparing these two, but since they are both media figures, they come to mind. Of course, we all remember Carl Sagan and he, too, was a true scientist and good science popularizer.

  41. JMW

    From the catalogue of useless facts…

    So by my calculation, things on the moon weigh 0.015% more now than they did a billion years ago.

  42. Sticks

    Obviously the moon has not had it’s Jimmy Dean breakfast yet

    ;-)

  43. bazza

    Metres are a unit of measurement.

    Meters are a device used to measure things.

  44. JoeSmithCA

    Well that proves it, everything shrinks and gets smaller with age including the moon. I wonder if the moon will get flabbier and develop a gut too or is all that racing around the earth keeping it in shape?

  45. JoeSmithCA

    @Bazza

    Nope, we here in theee uniiited states done call them things meeteerrzzz!

    http://en.wikipedia.org/wiki/International_System_of_Units#Spelling_variations

  46. Gary Ansorge

    43. JMW

    Dang, you beat me to the punch.

    SO, if the moon shrinks, I suppose earth does too? Maybe that explains why the dinosaurs were so big. Lower surface gravity in the old days which, of course, explains why human sized angels could fly around then.(snark)

    Gary 7

  47. rw23

    > Yeah, but when is it going to explode out of our solar system with Martin Landau and Barbara Bain on it?

    Along with Barry Morse, and let’s not forget Catherine Schell. Especially let’s not forget Catherine Schell!

  48. Chris

    Haha, I enjoyed OtherRob’s seinfeld reference in #15

  49. ONLY BARK OF MOON IS SHRINKING NOT WHOLE MOON IS SHRINKING

  50. the interior of the moon has voids left from the cooling and solidifacation from the molten blob that formed 4 billion years ago. from time to time these voids collapse causing moon-quakes and reducing the size but not the mass of the moon.

  51. ronny z

    Baloney …. go to the site “neil adams” he is a geologist that has awesome video on how the planets including our are actually expanding. Great site to show the kids on how planets were formed and yes matter can be created. Ever hear of water. two gases combined makes matter. They say the one portion slips under the other, I say the one on top is expanding.

  52. Nigel Depledge

    OK, I’ll bite . . .

    Ronny Z (57) said:

    . . . matter can be created.

    Well, duh. E=mc2

    Pump in enough energy and you get matter. The LHC will be doing just this – if you collide protons with enough energy, you can get all kinds of amazing particles formed. However, they pretty much all decay away within microseconds (well, the long-lived ones last microseconds; the faster-decaying ones tend to decay in nanoseconds or picoseconds. But you get the idea).

    However, there is simply no way that any planet is hot enough to do this. The amount of energy required to create millions of tons of matter (which is what would be required to make a planet get even a tiny bit bigger) is phenomenal. It has to come from somewhere, but there is no source of energy anywhere near the solar system that is adequate.

    Ever hear of water. two gases combined makes matter.

    Erm … ever hear of the three states of matter – gas, liquid, solid?

    In your example, you are combining matter with matter to get … matter. Big deal. It doesn’t prove anything (except that water isn’t an element, but that’s irrelevant to your point).

  53. TheDisappointed

    Ronny z —–‘Neal Adams’ is not a geologist, he is a cartoonist…And don’t forget the moon is also moving away from the earth. If the earth and moon are ‘growing’ in place then the gravitational attraction should increase and move them closer together. This silly ‘theory’ has more holes it it than a swiss cheese (or is that a moon cheese)?

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