Rethinking the Moon’s Violent Origin

By Mika McKinnon | September 13, 2016 4:15 pm

(Credit: Wikimedia Commons)

Our moon has a violent past that just got a little more fearsome.

Planetary formation is not for the weak of heart. Building a planet from countless grains of dust and tendrils of gas is a violent process of colliding, coalescing, and melting into bigger and bigger rocks. Our Earth was a decently-sized world when it took one final hit: a Mars-sized object smashed into our home planet.

We used to think this object, Theia, struck a glancing blow and was captured into Earth orbit as our new moon. With this theory of a relatively low-energy impact, Theia peeled off a chunk of the Earth as molten debris and vaporized a bit more with the energy of impact. The material coated the new moon in a mantle of terrestrial geological material around an alien core, geologically similar yet distinctly separate.

But that story doesn’t hold up under the cold light of geochemical analysis.


(Credit: Kun Wang)

The Apollo astronauts brought back a precious cargo of 842 pounds of rock, pebbles, and dust from the moon. Scientists painstakingly analyze these samples to build a picture of our nearest neighbor.  During recent reanalysis with high-precision instruments, researchers found a curious quirk.

Lunar rocks are almost, but not quite, like those on Earth. They’re too similar to be formed from completely different parent-rocks, and must share a common past. And yet lunar rocks have a slight over-abundance of a particular chemical isotope, potassium-38, so their pasts cannot be identical.

Researchers Kun Wang and Stein B. Jacobsen developed a new origin story for the moon to explain this potassium riddle. Their research was published inNature Geoscience yesterday.Instead of a glancing blow, what if Theia hit the Earth straight on? In this theory, the higher-energy impact disintegrates Theia completely, and strips Earth of its outer layers of crust and mantle. The debris melts into a planetary stew, the implacable power of angular momentum drawing it out into a dense, hot disk.

Earth’s brief ring is more reminiscent of Venusian hellscapes than Saturn’s delicate structures. It’s hot enough to vaporize rock and rain lead, with atmospheric pressures more than ten times higher than we find at sea level. In this stew, the biggest fragments of coalesce into moonlets, growing into a moon. The lightest potassium elements disproportionately condense out on the young moon, leaving the heavier elements to rain down on Earth.

It’s a wild story for how we got our moon. But right now, it’s the story that makes the most sense.

This article originally appeared in
CATEGORIZED UNDER: Space & Physics, top posts
MORE ABOUT: solar system
  • ld_elon


    So no sense can really be made of m00n gods origins..

    Fits perfect.

  • darryl

    Pffftt! Everyone knows the moon is made of cheese.
    The seismic velocities of 1.2-1.9 km/s match that of Muenster, Cheddar and Provolone. No Earth rocks come close to this.


    • Bill Caper

      Provolone Darryl. Just look at the night sky.
      Muenster is too yellow and I refuse to believe it can be (white) cheddar. Though, to be fair, I have no scientific basis (yet) for that part of my analysis.

    • Lorie Franceschi

      Come on guys, it is made of white Swiss, that is why there are so many holes in it.

    • Absolut John

      We all know that the moon is not made of green cheese – but what if it were made of barbecue spare ribs? Would ya eat it then? I know I would. I’d have seconds, then polish it off with a tall cool Budweiser. I would do it. Would you? Its a simple question… Would ya eat the moon if it were made of ribs??! It’s not rocket science – just say yes and we’ll move on….
      – Will Ferrell as Harry Carray – SNL 1997

  • Stephen

    Imagine an icy Theia; the source of our oceans… Bowen’s Rx Series reflects the two chemistries. The opening & closing of the ocean basins the result of the collision. The continental material is the pendulum…


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