There is a hypothesis that says a part of the Earth was split away and became the Moon. Is there any scientific evidence for this claim?

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    $\begingroup$ This is possibly worth migrating over to skeptics.stackexchange.com.... $\endgroup$
    – Rory Alsop
    Jun 2, 2011 at 12:37

2 Answers 2


There is quite a bit. Here are the ones that come to mind immediately:

  • Similar surface ages - The oldest rock on the earth and the oldest rocks returned from the moon are the same age. This implies similar creation time.

  • Isotopic composition - The ratios of various atomic isotopes are basically the same indicating that the two bodies were created from the same original material

  • Relative densities - The moon is overall composed of material that is lower in density than that of the Earth. This is to be expected from the impact scenario as it is the lighter materials from the Earth's crust that would have been blasted off to form the moon.

  • Lack of lunar volatiles - The moon is missing a lot of the lighter, low boiling point volatiles that are expected to be trapped in its rocks. This too is to be expected from the collision scenario as they would have been vaporized.

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    $\begingroup$ +1 Not to mention the fact that the moon is enormous; it doesn't exactly orbit Earth so much as wobble around it while orbiting the sun. $\endgroup$
    – Jon Purdy
    Jun 30, 2011 at 21:32
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    $\begingroup$ Might be beating a dead post, but I feel I should point out that this is in answer to "parts of the moon came from a primordial Earth," NOT that "the moon split from Earth." These are VERY different things, the first (and what the answer is) being the "Big Splash" hypothesis where a Mars-sized object struck Earth and formed the moon, versus the fission hypothesis where Earth was spinning really fast and spawned the moon (or the latest "Big Burp" where an early explosion within Earth blasted material out to form the moon). $\endgroup$ Aug 1, 2011 at 19:32
  • $\begingroup$ @Stuart: Good point. The fission hypothesis is so far from the front of my mind that I didn't event think that that was what the OP might be asking about. In fact, it never even occured to me until you mentioned it. $\endgroup$
    – dagorym
    Aug 2, 2011 at 15:14
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    $\begingroup$ @dagorym: I run a blog called "Exposing PseudoAstronomy," so potential misinterpretations and an eye on falsified ideas that some people still cling to are always in my mind. $\endgroup$ Aug 3, 2011 at 2:52
  • $\begingroup$ Lest anyone reading the above comments think the fission hypothesis is dead: It may be overshadowed by the giant impact scenario, but it has not been ruled out entirely. Things get muddled too when you consider there is a gray area in between wherein a spun-up post-impact Earth undergoes fission. $\endgroup$
    – user10851
    Jan 29, 2013 at 18:27

The fission hypothesis does not really explain why the moon rotates at the same rate as it's orbit of the Earth. The scenario of a external force like another planet or giant comet hiting the Earth and braking off a piece that created the moon would better explain the velocities of rotation. As they say the second hypothesis is not ruled out, but the first just makes better sense when you study the rate of velocity of its orbit around us and the rotation on its axis.


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