More musings on moon's origin
New analysis of soil from the Moon backs up the theory that it formed after a giant impact between Earth and another object, but still hasn't settled the debate.
U. Wiechert and colleagues from Switzerland and the US report on their re-analysis of Apollo mission Moon soil samples in this week's issue of Science.
They used a new high-precision technique to establish that the concentrations of oxygen isotopes in the Moon's soil are similar to those of Earth. These findings confirm earlier, less precise analyses of Apollo mission samples.
"This observation is consistent with the Giant Impact model," they write, "provided that the proto-Earth and the smaller impactor planet (named Theia) formed from an identical mix of components."
But it is this proviso that has Curtin University earth scientist, Professor Simon Wilde, unconvinced.
Earlier this year, Professor Wilde and colleagues published an article in Nature reporting on their discovery of 4.4-billion-year-old mineral crystals in Western Australia.
The 'Giant Impact' theory holds that a Mars-sized object slammed into the Earth sometime between 4.4 to 4.45 billion years ago - but any bombardment that formed the Moon must have occurred when the Earth was still molten, before crystals could form.
So if Wilde and his colleagues' findings are accurate, the Moon must have formed well before 4.45 million years ago, or by a different process altogether.
While he describes the new data by Wiechert and colleagues as "pretty impressive", Professor Wilde remains "agnostic" on how the Moon formed.
"The study is internally consistent but it relies on the assumption that Theia and the proto-Earth were formed from the same materials," he told ABC Science Online.
"Wiechert and team argue that the Mars-sized Theia originated from a region of the solar system that was the same distance from the Sun as the Earth."
"This will remain a big problem for those that don't like the Giant Impact model," said Professor Wilde, adding they could use the same assumption to argue that the Moon was formed independently from, but in the same region as, the Earth.
Emeritus Professor Ross Taylor of the ANU, who was among the first to analyse Apollo samples back in July 1969, acknowledges the new study doesn't totally rule out the idea that the moon formed independently.
"This is not a definitive answer in this respect, it's not the smoking gun," he said.
However Professor Taylor argues that the Giant Impact model is the only model that allows for the fact that the EarthMoon system spins more rapidly than its neighbours.
And, he said, there was a lot of supporting evidence from computer modelling for the idea that the impact came from a massive object orbiting at the same distance from the Sun as the Earth.
"There's no other model in town," he said.