Making our Moon

FRED WATSON is Australia’s Astronomer-at-Large.

SINCE THE APOLLO lunar landings of the 1960s and ’70s, we have understood that the Moon was formed early in the Solar System’s history as a result of something colliding with the Earth. An earlier hypothesis that the Moon was an asteroid captured by our planet’s gravity was discarded following the discovery that the lunar rock and soil samples returned by the Apollo astronauts have identical chemistry to those on Earth. The idea then became accepted that early Earth suffered an impact by an object about the size of Mars that ejected a massive plume of debris, which subsequently coalesced to form the Moon. So confident were planetary scientists of this scenario that they even gave the impacting object a name: Theia, after the mother of Selene (the Moon) in Greek…

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Australian Geographic (Digital) - 1 Issue, July/August 2019

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