Earth's two moons

There's a big difference between the side of the Moon we can see, and the side we can't. Although it seems pretty pockmarked to the layperson, "our" side of the Moon is actually the smooth half. On the dark side, there's huge mountain ranges and much bigger craters.

There are lots of theories that seek to explain this disparity. The newest: Earth once had two moons. And the smaller of the pair eventually crashed into its larger sibling on the side that faces away from Earth. From the BBC:

Dr Martin Jutzi from the University of Bern, Switzerland, is one of the authors of the paper. He explained: "When we look at the current theory there is no real reason why there was only one moon.

After spending millions of years "stuck", the smaller moon embarked on a collision course with its big sister, slowly crashing into it at a velocity of less than three kilometres per second - slower than the speed of sound in rocks.

... In a commentary, Dr Maria Zuber from the Massachusetts Institute of Technology (MIT) in Cambridge, US, suggests that while the new study "demonstrates plausibility rather than proof", the authors "raise the legitimate possibility that after the giant impact our Earth perhaps fleetingly possessed more than one moon".

In other words, this isn't so much a proven thing, as the scientific equivalent of a plot bunny. The researchers hope to inspire studies that would either prove them wrong, or lend credence to their ideas. This could end up being the start of something big. Or it could eventually be regarded with about as much respect as the suggestion, "What if Moon were cookie?" We'll have to wait to find out.

Image: A mini-guide to our wonderful Moon, a Creative Commons Attribution Share-Alike (2.0) image from dingopup's photostream

• [Paul Giachetti]

  I think there’s a simpler solution: Since the side that faces us always, well, faces us, it’s pretty much shielded by either the Earth itself or it’s gravitational pull. If you twirl a Basketball around on a rope and throw a rock at it, chances are it’s either going to hit the side facing out, or the twirler (you). 

  • [relawson]

    So the moon is our… dirt shield? ;)

    I read this the other day somewhere else and went looking for desktop pictures. I will share the one I chopped up and threw together for my dual screens (3840×1200).  and the source image, too…

    Enjoy!

  • [Andrew Singleton]

    Occum for the win.

    Nice deductive reasoning. Anyone mind testing this with a tethered ball and lots of nerf objects to fling at the pair?

• [taras]

  The dark side of the moon changes, and is sometimes the side facing us. The correct term is far side.

  Also, this reads strangely to me – I always thought the far side had smaller and shallower craters (though I’m just going by images.)

  • [jim80y]

    No it doesn’t! The same side of the moon always faces the earth.

• [Bradley Hall]

  What’s a plot bunny?

  • [MDwebguy]

    I was wondering about the ‘plot bunny’ reference as well.  Thanks for asking!

    • [Maggie Koerth-Baker]

      I’ve always heard it used in fiction writing as a good idea that could turn into a story, but isn’t one yet. So, you might offer another writer a plot bunny and see what they do with it. Or you might be reading something else and think up a plot bunny that you file away for yourself to use as a story concept later. 

  • [billstewart]

    Is it the one eating rice cakes on the moon?

• [Steve Pankow]

  Time for a new shirt design!

  • [Spriggan_Prime]

    Needs more plot bunnies.

• [MDwebguy]

  I thought the far side of the moon is where the aliens hang out.

  That’s the problem with growing up; your most quixotic and unlikely notions are dispelled, one by one, until the day comes when you find yourself yelling at the neighborhood kids to keep off your lawn and you realize you used to be one of those kids.

• [nosehat]

  It seems to me that an enormous impact, like a smaller moon hitting a larger one, would produce a very smooth surface, not a rough one, since everything would be liquefied.  

  The article says the impact may have been “slow”, “less than the speed of sound”.  But no matter how slow their relative speeds, something with the mass of a small moon will still have a huge amount of kinetic energy, and all of that would be converted to heat on impact, and all this heat would be at the region of impact–the surface.  It seems like the surface melting would be a given.

• [Glen Able]

  Can anyone confirm that the moon has always had the same face pointed towards us?  I sort of understand the idea of tidal locking, but I don’t know if that precludes any additional rotation over long periods.

  • [Donald Petersen]

    Somebody will have better information than I do, but I expect the same side has been facing us for a long enough time for there to have been many more impacts on the far side.  Surely on the order of a billion years or two.

    But I expect the fine folks at MIT and the University of Bern have considered such things.

• [t3kna2007]

  There is no dark side of the moon really. Matter of fact it’s all dark.

• [Bucket]

  Before the later Apollo missions were cancelled, some people at NASA wanted one of the later missions (Apollo 20, IIRC) to land on the far side. I would imagine rocks collected from such an expedition would have confirmed or denied this theory pretty quickly.

  However, that would have been a risky mission and would have required putting small satellites in lunar orbit to maintain communication. Which would have been way cool, actually, but expensive.

  • [mccrum]

    “Which would have been way cool, actually, but expensive.”

    Because the rest of the money spent getting there in the first place was relatively cheap?  I mean, sure by then they were trying to justify Apollo 17 and why some old rocks were pretty cool but I think if you’d left it to the guys actually doing stuff, chatting with the CM every once and a while would have been enough, Houston would just have had to suffer.

• [liquidstar]

  If I’m not mistaken, (and I certainly may be)  this theory is being proposed at least in part to account for the “mountainous” terrain on the far side of the moon.   This is interesting, as it is probably due to the automated alien machines mining mostly on the far side.  It’s funny to see a spurious scientific theory being proposed to account for this, a distinct wobble in academic circles.

• [john832]

  Good Lord! Are you one of those people who comment on articles about airliner crashes to say that black boxes aren’t actually black, and are usually painted bright orange to make them easier to find? “Dark” means “hidden” (in addition to “not illuminated”), and the far side of the moon has often and long been called the “dark side”.

• [Spriggan_Prime]

  I saw a recent ‘documentary’ and the rough patches on the dark side of the moon were due to a large space ship from a race of warring metamorphic robotic aliens crashing into it. There was also a bunch of the robots hidden in the moon dust. Something about a government conspiracy and the 60s moon landing. It was in 3-D!

  The explosions were awesome.

• [masamunecyrus]

  Well, assuming that the Moon accreted from debris left from a huge collision, there were once billions or trillions of moons. And then those moons merged to form bigger moons. At some point, of course there were two moons. And then those, too, merged to form one moon.

• [baronkarza]

  Our pal Ban Bern knows all about the time when the Earth had 2 moons. What happened to it? We sold it, long ago, to the planet Kababa:
  Back when the earth had two moons 
  It seemed as though eclipses were incessant 
  Every seven years they were both full 
  Sometimes when one was full the other was a crescent! 
  We sold it to the people of Kababa 
  For the Berlin wall and all the tea in China 
  For the gross national product of America 
  For the cash crop of North Carolina 
  Back when the earth had two moons 
  Nobody had any money 
  Nobody ever had a fever 
  No one ever slept alone either 
  Shoes were the only transportation 
  No one ever heard of litigation 
  There was hardly anybody who could spell 
  But every body slept pretty well 

  But we sold it to the people of Kababa 
  For cars and roads and offices 
  For Warner Brothers, Exxon and Sony 
  For hamburger meat and baloney 
  Ah, but now we’re better off–don’t you see 
  With CNN on the TV 
  Child hookers in the Philippines to screw me 
  Karaoke for every wannabe 
  We got fast food and condoms and hair spray 
  We got the right to vote to take all of our rights away 
  We got life and the means to spend it 
  We’ve even got the bomb in case we want to end it 

  But if you ever find yourself wishing 
  The moon was back in its original position 
  There’s a rumor that the deal was never signed 
  And Kababa isn’t difficult to find 
  Go to Jupiter, take the first right, fly north north north for 30 billion light years 
  Remember that space is curved and you can’t miss it 
  Remember that space is curved and you can’t miss it