The Mystery of Moon’s Origin Resurfaces

A new study casts doubt on the theory that the moon was born of a collision between Earth and a Mars-sized object

A new chemical analysis of lunar material collected by Apollo astronauts in the 1970s conflicts with the widely held theory that a giant collision between Earth and a Mars-sized object gave birth to the moon 4.5 billion years ago. According to a University of Chicago press release ‘a comparative analysis of titanium from the moon, Earth and meteorites, published by Junjun Zhang of the university and four co-authors, indicates the moon’s material came from Earth alone’.
“Just like in humans, the moon would have inherited some of the material from Earth and some of the material from the impactor, approximately half and half,” says Nicolas Dauphas, co-author of the study, which appears in Nature Geoscience.

“What we found is that the child does not look any different compared to the Earth,” Dauphas says. “It’s a child with only one parent, as far as we can tell.”
The research team based its analysis on titanium isotopes—forms of titanium that contain only slight subatomic variations. The team selected titanium because the element is highly refractory. This means that titanium tends to remain in a solid or molten state rather than becoming a gas when exposed to tremendous heat. The resistance of titanium isotopes to vapourisation makes it less likely that they would have been incorporated by Earth and the developing moon in equal amounts.

“When we look at different bodies, different asteroids, there are different isotopic signatures. It’s like their different DNAs,” Dauphas says. Meteorites, which are pieces of asteroids that have fallen to Earth, contain large variations in titanium isotopes. Measurements of terrestrial and lunar samples show that “the moon has a strictly identical isotopic composition to the Earth,” he says.

Solving the conundrum of the moon’s origin afresh will prove challenging because all of the alternative scenarios for the moon’s formation have drawbacks. “We thought we knew what the moon was made of and how it formed, but even 40 years after Apollo, there is still a lot of science to do with those samples that are in curatorial facilities at Nasa,” Dauphas says.

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