The Moon's Mystery: Scientists Debate How it Formed
How was our planet's satellite formed? Scientists are still searching for the answer.
This illustration of the moon being formed appears on the July cover of National Geographic magazine.
Illustration by Dana Berry, National Geographic
Before the giant impact model gained traction nearly four decades ago, three other models were in contention. One said the moon condensed from the same whirling cloud of dust that created Earth. But this "binary" model couldn't explain why the moon, far from being a smaller twin of Earth, is much less dense than our planet, with no iron core.
A second model held that the young molten Earth spun so rapidly that it split apart, flinging a giant blob of magma into space. But Earth's spin today and the moon's orbit don't fit the pattern predicted by the "fission" model.
In the third model, Earth's gravity lassoed the moon as it wandered through from some distant part of the solar system. This "capture" scenario was appealing until the Apollo astronauts brought their moon rocks back home. The minerals in them turned out to be similar to those in Earth's mantle—not exotic at all.
The giant impact model avoided all these problems. When it came along in the 1970s, the model fit an emerging view of how the solar system as a whole had formed. In that view, gaseous and rocky protoplanets grew within a disk around the young sun, competing for space, for tens of millions of years. Collisions were inevitable.
As Earth got bigger, it absorbed several Mercury-size or Mars-size objects. The final major blow was an impact so fierce that it left a permanent reminder in orbit around us. According to the impact model, the moon coalesced mostly from the shattered debris of the impactor, a rocky protoplanet similar to Earth. Because the impactor's own iron core sank into Earth's core, the moon is all rock.
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