Moon 'wasn't created by giant collision'

Posted by Kate Taylor

A new chemical analysis of lunar material collected by Apollo astronauts in the 1970s appears to disprove theories that the moon was created through a massive collision between Earth and a Mars-sized object 4.5 billion years ago.

A comparative analysis of titanium from the moon, Earth and meteorites, led by Junjun Zhang of the University of Chicago, indicates that the moon’s material came from Earth alone.

"Just like in humans, the moon would have inherited some of the material from the Earth and some of the material from the impactor, approximately half and half, says associate professor Nicolas Dauphas.

What we found is that the child does not look any different compared to the Earth. It’s a child with only one parent, as far as we can tell."

The team chose to study titanium because it tends to remain solid or liquid, rather than becoming a gas, when exposed to tremendous heat - making it less likely to become incorporated by the Earth and the developing moon in equal amounts.

It also contains different isotopic signatures, with different objects in the newly forming solar system absorbing those isotopes in different ways, leaving clues as gto where the moon came from.

Zhang initially found variations in the titanium isotopic composition between the lunar and terrestrial samples. She then corrected the results for the effects of cosmic rays, which could have changed the titanium isotopic composition of the lunar samples.

The analyses greatly reinforce previous work by other researchers who came to the same conclusion after comparing terrestrial and lunar oxygen isotopes.

Unfortunately, ruling out the collision theory doesn't leave much in its place. Other possibilities are that the moon arose via fission from a molten, rapidly rotating Earth following a giant impact, or through collision with an icy body lacking entirely in titanium; maybe one with the same composition as Earth. However, there are big problems with each of these theories.

"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," says Dauphas.