A study published in the journal Science provides new evidence that the Moon formed from the catastrophic collision of the proto-Earth with a hypothetical planet-sized body named Theia.
This is an artist’s depiction of a catastrophic collision between two celestial bodies; such an impact between the proto-Earth and Theia likely formed the Moon. Image credit: NASA / JPL-Caltech.
Most planetary scientists support the giant impact hypothesis which proposes that the Moon was created out of the debris left over from an impact between the Earth and the planetesimal Theia (sometimes called Euryphaessa or Orpheus), about 4.5 billion years ago.
Efforts to confirm that the impact had taken place had centered on measuring the ratios between the isotopes of oxygen, titanium, silicon and others.
These ratios are known to vary throughout the Solar System, but their close similarity between Earth and Moon conflicted with theoretical models of the collision that indicated that the Moon would form mostly from Theia, and thus would be expected to be compositionally different from the Earth.
Now Dr Daniel Herwartz from the Geowissenschaftliches Zentrum in Germany and his colleagues have used more refined techniques to compare the ratios of 17O/16O in lunar samples, with those from Earth.
They initially used lunar samples which had arrived on Earth via meteorites, but as these samples had exchanged their isotopes with water from Earth, fresher samples were sought.
These were provided by NASA from the Apollo 11, 12 and 16 missions. These samples were found to contain significantly higher levels of 17O/16O than their Earthly counterparts.
“The differences are small and difficult to detect, but they are there,” explained Dr Herwartz, the lead author of the study.
“This means two things: firstly we can now be reasonably sure that the Giant collision took place; secondly, it gives us an idea of the geochemistry of Theia.”
“Theia seems to have been similar to what we call E-type chondrites (Enstatite chondrites).”
“If this is true, we can now predict the geochemical and isotopic composition of the Moon, because the present Moon is a mixture of Theia and the early Earth. The next goal is to find out how much material of Theia is in the Moon.”
Most models estimate that the Moon is composed of around 70 to 90 per cent material from Theia, with the remaining 10 to 30 per cent coming from the early Earth.
However, some models argue for as little as 8 per cent Theia in the Moon.
The new findings indicate that a 50:50 mixture seems possible, but this needs to be confirmed.
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Daniel Herwartz et al. 2014. Identification of the giant impactor Theia in lunar rocks. Science, vol. 344, no. 6188, pp. 1146-1150; doi: 10.1126/science.1251117
