In article ,
Yousuf Khan writes:
Moon was produced by head-on collision | Astronomy.com
http://www.astronomy.com/news/2016/0...d-on-collision

The UCLA press release is at
http://newsroom.ucla.edu/releases/mo...forming-planet
and the article in _Science_ is at
http://science.sciencemag.org/content/351/6272/493.full

I couldn't find a preprint, but there might be one somewhere.

The Theia Theory used to talk about a glancing blow between the early
Earth and a Mars-sized Theia, perhaps at 45 degrees. New data suggests
that Theia might have actually hit head-on with the Earth. That's
because the percentage of Oxygen 16/17/18 isotopes between the Earth and
Moon are nearly identical,

Specifically, the new data show the isotope ratios (actually a ratio
of differences) is the same within 2 ppm. A paper last year
http://science.sciencemag.org/content/344/6188/1146

found apparent differences at the 18 ppm level. The new paper
suggests that was sample selection, i.e., the rocks analyzed earlier
were not characteristic of the whole Moon.

indicating that they came from the same source.

If the isotope ratios are identical, _either_ Earth and Theia had
identical ratios to begin with (possible but not very likely) _or_
the present Earth and Moon have identical fractions of Theia
material. That last would require a head-on collision with everything
getting mixed; a glancing collision would put a larger fraction of
Theia material into the Moon.

I'm very much an outsider here, but this looks to me like a highly
technical insider dispute that most of us shouldn't worry about.

My question is, if the isotopic evidence which was used to create the
Theia Hypothesis in the first place, due to a previously perceived
difference in isotopic abundances,

The isotope ratios are _at least_ very similar, now seemingly
identical within measurement accuracy. This tends to support the
Theia hypothesis. If the Moon formed on its own and was somehow
captured by the Earth, it would be expected to have very different
isotope ratios.

so now that that isotopic evidence has disappeared, why keep the
Theia Hypothesis? There were other theories prior to that, which
were discarded due to the isotopic evidence.

That evidence being the _similarity_ of ratios, which is now stronger
than before. The argument now seems to be about the details of the
collision, but as I say, I'm an outsider.

For a different hypothesis to be viable, it will have to be
consistent with not only the isotope ratios but also the element
ratios and other evidence. As with all scientific hypotheses, it
will have to be testable (falsifiable) to receive scientific
consideration.

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