The Moon’s Mystery: Scientists Debate How it Formed
How was our planet’s satellite formed? Scientists are still searching for the answer.
by Robert Irion
It has taken centuries for scientists to settle on a creation story for our moon, the most popular of which is depicted on the July cover of National Geographic magazine. But as I learned at a recent lunch with Erik Asphaug, a planetary scientist at Arizona State University, the debate is still far from finished.
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…