Where did the Moon come from?
The origin of our Moon has long been a mystery. Although there are 3 plausible theories, they rely on knowing about what the Moon is made from.
It was not until samples of the Moon were brought back during the NASA Apollo missions in the 1960s and early 1970s that one theory became most likely.
Theories
There are 3 likely theories as to how the Moon formed:
- Accretion theory - The Moon was created at the same time and from the same material as the Earth or perhaps simply broke off as the Earth was created.
- Impact theory - The Moon formed during an impact between the Earth and another small planet, about the size of Mars. The debris from this collision collected in an orbit around Earth to form the Moon.
- Capture theory - The Moon was a wandering body that formed elsewhere in the solar system, captured by Earth's gravity as it passed close by.
Evidence from the Moon
Formation of the Moon from the debris of a giant impact.
The Apollo missions brought back over a third of a tonne of rock and soil from the Moon. Analysis of this material revealed that it was similar, but not quite the same as rock from Earth.
If the Moon had been created elsewhere and was captured by the Earth's gravity we would expect its composition to be very different from the Earth's.
If the Moon was created at the same time, or broke off the Earth, then we would expect the type and proportion of minerals on the Moon to be the same as on Earth. But they are slightly different. For example, the minerals on the Moon contain much less water than similar terrestrial rocks. The Moon is also rich in material that forms quickly at high temperature.
This suggests that the impact theory is the most plausible.
Toolbox
Our fossil insect collection includes Rhyniognatha hirsti, the world's oldest fossil insect, dating back some 400 million years.