All About Space

How the Moon wasn’t made


It was flung off a rapidly spinning Earth What is it?

An idea which was popular for decades was that the material which makes up the Moon was once part of Earth. It suggested the Moon separated from Earth while it was semi-molten and spinning rapidly, and many saw the Pacific Ocean as a void created by the departing material. Darwin backed up the idea with solid and accurate calculatio­ns.

Why it’s wrong

By the 1930s calculatio­ns showed that Earth would have had to spin at an inconceiva­ble rate to throw off enough material to form the Moon.

It was made elsewhere and captured by Earth What is it?

Many of the moons in our Solar System are thought to be captured objects – Phobos and Deimos around Mars are good examples. It isn’t inconceiva­ble that Earth could have captured the Moon, as this would explain why the Moon and Earth appear to have different densities.

Why it’s wrong

For Earth to capture a large Moon, both objects would have to travel slowly – a collision was more probable. It’s also unlikely that Earth’s gravity would’ve been able to hold the Moon for so long.

It was formed at the same time as Earth What is it?

The co-accretion idea is that Earth and the Moon formed together in the early Solar System from the debris around the newborn Sun. Support came from observatio­ns of double stars. If two distinct stars could form out of a nebula, then it seemed at least possible that two worlds might coalesce side by side in orbit around a single star.

Why it’s wrong

While the oxygen isotopes may be the same, the densities of Earth and the Moon and the amounts of iron on each are different.

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