How Come We Have A Moon?

But I didn’t want one! Stop your whining, Earthlings. We have a serious question to answer, courtesy of Tamara:

What’s the moon like below its surface, moving into the interior? And what’s the current thought on its formation?

Well, we do know a lot about the Moon’s insides the same way we know about the Earth’s; just like the Earth has earthquakes, the Moon has moonquakes. These tell us about the Moon’s interior. From back when we landed on the Moon, we planted the Apollo seismic experiments and saw 28 moonquakes. From these, we learned that the Moon has a crust about 60 km deep, a deep mantle, and a tiny core that’s less than 25% of the Moon.

But now let’s get to the really interesting one: how come we have a Moon at all? Of all the other rocky planets out there, Mercury and Venus have no Moons, and Mars just has a couple of captured asteroids. But Earth is special because we have a huge Moon! So let’s pause to take a good look at it, and then let’s figure out how we got to have one:

Well, the Moon has a bunch of interesting things about it that let us figure out the answer. These are:

  1. The Moon has the same density as the Earth’s Mantle.
  2. Surface rocks on the Moon (brought back by Apollo) have the same composition as rocks on the Earth. But…
  3. There is practically no iron on the Moon, whereas there’s a bunch of it on the Earth (mostly in the core).
  4. Rocks on the Moon have been “baked” by the Sun, and have virtually no moisture in them.

There are a bunch of discredited theories about how the Moon formed, but the one that works is awesome, and is called the giant impact theory.

Basically, a huge chunk of rock smacked into the Earth when it was still very young, and caused a big chunk of the Earth’s mantle to fly off around it. The chunks that didn’t fall back to Earth coalesced because of gravity and formed the Moon; the giant impact possibly created the Pacific Ocean on Earth. So the reason we think we got a Moon is just that we got “lucky” enough to get hit by a giant rock!

I was going to show you a computer simulation video of the formation of the Moon, but oh, my God, is it boring. (If you have 4 minutes and really want to watch it, go ahead and click the above link anyway, but I warned you.) It’s much more entertaining to watch Cookie Monster sing about eating the Moon, and if you’ve made it this far, you deserve the entertainment.

P.S. — Do not try to eat the Moon.

Comments

  1. #1 Brian
    May 1, 2008

    Another factor worth mentioning with this post is the effect the Moon has on the Earth’s length of day. When the Moon first formed, it was very close to the Earth, and it’s been moving farther away ever since. Just like a spinning skater moving her arms out, when the rotating Moon get farther away from us, the angular momentum must be conserved. That means the rotation speed slows down. Since the Earth and Moon are tidally locked, this affects the Earth as well. Geologic evidence tells us that the length of day on the Earth was much shorter in the past. For example, one Earth day was 18 hours long 900 million years ago. This means that a year could fit more days in it back then with 481 days. This so-called “lunar retreat” is even measurable today by bounding lasers off the lunar surface reflectors left there by Apollo and measuring the time it takes to receive the reflection. Given its present rate of retreat of 3.8 cm/year, the Moon will eventually reach an orbit of 560,000 km (compared to 384,000 km today) in 15 billion years, at which time the Earth-Moon system will truly be “synchronous”, which means both bodies are locked face-to-face. The length of day then would then be infinite. We’d only show one face to the Sun. Luckily, our solar system probably won’t last 15 billion years!

  2. #2 Brian
    May 1, 2008

    By the way, there were way more than 28 moonquakes. There were 12,558 seismic events detected during the 8-year Apollo Passive Seismic Experiment. 7245 of these are deep moonquakes, 1744 were meteroid impacts, 28 were shallow moonquakes, 9 were artificial impacts, and the rest were from unidentified sources. The discovery of the deep moonquakes was completely unexpected, and many scientific questions remain about them, although most people think they’re caused by tidal forces. The shallow moonquakes could potentially be a problem for future lunar astronauts, and their mechanism is not understood at all.

  3. #3 Brian
    May 1, 2008

    What’s that comment about the Pacific Ocean coming from the lunar-forming impact? I’ve never heard of such an idea. If the Earth had no plate tectonics, maybe I’d buy the hypothesis, but that’s not the case. The Pacific Ocean basin is formed from mid-ocean ridges through ongoing processes we can study today. The ocean basins have grown and shrunk over time. Currently, the Atlantic is growing, and the Pacific is shrinking. By the way, some people have proposed that the northern lowlands on Mars formed from a large impact, which is conceivable given that Mars has no plate tectonics to erase such a scar.

  4. #4 ethan
    May 1, 2008

    See, I told you. My friends are smart. ;-)

  5. #5 Henry Cate
    May 1, 2008

    “When the Moon first formed, it was very close to the Earth, and it’s been moving farther away ever since.”

    I think Peter Ward and Donald Brownlee make the point in “Rare Earth” that if you do the math you find with the moon so close you have land tide, that the gravitational pull of the moon move land up and down. My memory is they were talking something on the order of hundreds of feet!

  6. #6 ethan
    May 1, 2008

    It’s pretty interesting that our Moon, over time, spirals outwards, away from us. But Mars’ Moons, Phobos and Deimos, are spiralling inwards, and are expected to collide with Mars in a few hundred million years.

  7. #7 Brian
    May 1, 2008

    Actually, only Phobos is spirally inward to Mars. Deimos is spiraling outward. This happens because Phobos’ angular speed is less than its mean motion. The opposite it true for Deimos and the Earth’s moon; their angular speed is greater than their mean motion.

  8. #8 Brian
    May 1, 2008

    “spiraling”, not “spirally”.
    Oh, and I don’t think most people still think Phobos will crash into Mars intact. It’ll pass through the Roche limit before that and break up, perhaps giving Mars a temporary ring until the pieces burn up in the atmosphere or fall to the surface. Kim Stanley Robinson’s excellent book “Red Mars” does feature Phobos crashing to Mars due to a terrorist act on a space elevator using Phobos as an anchor point. Good stuff!

  9. #9 Brian
    May 1, 2008

    By the way, I love the Cookie Monster movie. I’ve been reaching deep into the back of my mind to see if I remember that sketch from my Sesame St watching days, but I either never saw it or just forgot about it.

  10. #10 ethan
    May 1, 2008

    I think this is the only time I remember seeing Cookie Monster’s bedroom. He is pretty stylin’ with the monster nightcap and all. He’s also pretty tough to eat in a vacuum like that. Your sesame street watching days are going to be returning fairly soon as Henry gets to be old enough…

  11. #11 Tamara
    May 2, 2008

    Thanks for posting the luminous response, along with the great visuals. This and the information from the comments is so interesting to ruminate about and has sparked some enlightening exploration…

  12. #12 Cardinal
    September 8, 2008

    To the Men of Science:
    What if the moons were like watchtowers in a time before the existance of human life? What makes people think that the only form of life lives off water? Or light? Even Egyptians had carvings of “other life forms”. When we start where we first begun in “recorded time” not theoredic time, we’ll find out more than i think we’re ready to believe. What gives man the mere right to create and destroy a planet as he see fit. Whether it is or isn’t a planet our universe is older than any living being. All of a sudden we’ve got enough information about a time before we existed. Pluto is the name of this something we all seen. Nobody really knows what it is but before we say what it isn’t we shouldn’t cast our opinions on other individuals negatively because of our status or degree.Food for thought.

  13. #13 lauren
    January 12, 2009

    omg my friend thinks the moooon is made out of cheese :)when its made out of rock hahahah

  14. #14 kellly
    January 12, 2009

    i love the moon i might ask it to marry me lol

  15. #15 moorishia el ethereal ra
    February 12, 2010

    the moon is really an orbiting satalite created from the earth by an elder called shabazz or shabazar and a lost group of mixed meddle eastern decendants!!fertile cresent!!! mu=mare=marsh=primortal(pangia=asia)=atlantis=lemuria=(bethleheim-means the house of the god lehmu!!!not the house of bread!!!) sumaria=babylon=egypt=nubia=kush=hebrew=arab=hindi-kush mountains=hindi!!another story!! eve thing why you only see one side of the moon!!!? watch closely the moon moves as it were alive ,but then if you are one with the universe you know it is not the need of oxygen that classifies all things as living in thin=s universe!!

  16. #16 amanda
    March 1, 2010

    Hi Ethan, just been looking at your site because my 7 year old asked me why we have a moon. He found your video link explaining how the moon was formed very interesting. Thank you for makin an interesting site… even i learned something new!!

    Amanda

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    I learn more science from your blog than anywhere else. Great information that is easily understandable. Thanks.

  22. #22 mike ramsey
    October 13, 2010

    Nice to be educated in such a light hearted way…so did men really land on the moon in the late 60′s or was it just a tv studio?

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    January 6, 2011

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  25. #25 jessica
    January 7, 2011

    Well the oil leak is a serious problem because seeing as all of the oil is compressed beneath the surface of the Earth, when a hole is made, the oil will come shooting out because the pressure has been released and there’s nothing we can really do to stop that easily. The man on the moon and robot on Mars are easier because we’ve sent things out before and those two were the same but modified to fit the requirements. We’ve never really had an oil leak of this scale before so scientists don’t really know what to do about it. But yeah, it’s annoying.
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  26. #26 live2009
    January 18, 2011

    This post really does help some things, but there’s still a premise there that prevents it from being entirely convincing: to wit, that the tiny amount of CO2 currently in our atmosphere has an outsize importance on why our planet is the temperature it is. This may well be true (and I’m inclined to believe it is, for no other reason than that the people who seem to think most scientifically believe this is the case), but since I can’t really grok the science myself, I have trouble believing it in my heart of hearts on say-so, you know?http://www.itunes.com

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    January 19, 2011

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  29. #29 backlinks
    April 26, 2011

    I think it’s pretty cool how the mood was made. Did you know that the moon is getting further and further away. So everyday you look at it, is officially the closeest you’ll ever see it again?

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  30. stop that easily. The man on the moon and robot on Mars are easier because we’ve sent things out before and those two were the same but modified to fit the requirements. We’ve never really had an oil leak of this scale before so scientists don’t really know what to do about it. But yeah, it’s annoying.

  31. requirements. We’ve never really had an oil leak of this scale before so scientists don’t really know what to do about it. But yeah, it’s annoying.

  32. #32 iPhone Photography
    July 16, 2011

    I think the Muppets video was the icing on the cake. Thanks!

  33. #33 points
    August 18, 2011

    Moon is a fascinating thing.. you never really think about it.. it seems so dry and lifeless.

  34. #34 Lauren
    January 20, 2012

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    January 20, 2012

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  36. #36 BeiberFreak21
    January 20, 2012

    The video was like suuuper Bo-ring!

  37. #37 Fri
    July 3, 2013

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