From the European Association of Geochemistry
New isotopic evidence supporting moon formation via Earth collision with planet-sized body
A new series of measurements of oxygen isotopes provides increasing evidence that the Moon formed from the collision of the Earth with another large, planet-sized astronomical body, around 4.5 billion years ago. This work will be published in Science* on 6th June, and will be presented to the Goldschmidt geochemistry conference in California on 11th June.
Most planetary scientists believe that the Moon formed from an impact between the Earth and a planet-sized body, which has been given the name Theia. Efforts to confirm that the impact had taken place had centred 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 a group of German researchers, led by Dr. Daniel Herwartz, have used more refined techniques to compare the ratios of 17O/16O in lunar samples, with those from Earth. The team 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; they were found to contain significantly higher levels of 17O/16O than their Earthly counterparts.
Dr Herwartz said “The differences are small and difficult to detect, but they are there. 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**.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 it is composed of around 70% to 90% material from Theia, with the remaining 10% to 30% coming from the early Earth. However, some models argue for as little as 8% Theia in the Moon. Dr Herwartz said that the new data indicate that a 50:50 mixture seems possible, but this needs to be confirmed.
The team used an advanced sample preparation technique before measuring the samples via stable isotope ratio mass spectrometry, which showed a 12 parts per million (± 3 ppm) difference in 17O/16O ratio between Earth and Moon.
Dr Daniel Herwartz will present “The elevated Δ17O composition of the Moon relative to the Earth” to the Goldschmidt conference, Sacramento, California, on 11th June at 09.45, Eastern Time. The Goldschmidt conference is the world’s leading annual conference on geochemistry, http://goldschmidt.info/2014/
Please mention this Goldschmidt presentation in any story which results from this press release.
The elevated Δ17O composition of the Moon relative to the Earth
D. HERWARTZ, A. PACK, B. FRIEDRICHS AND A. BISCHOFF
We present the first isotopic evidence for giant impactor material in lunar rocks. The Moon presumably formed from the debris of a giant collision between two proto-planets (‘giant impact hypothesis’). Most numerical models of the collision predict that the Moon dominantly formed from impactor material. Thus, the Moon should inherit the isotopic composition of the impactor. So far, however, no isotopic difference between the Earth and the Moon has been resolved. We have measured the triple oxygen isotopic composition of APOLLO basalts using an improved protocol . We show that the Δ17O isotopic compositions of lunar APOLLO basalts is elevated by 12 ± 3 ppm relative to the Earth. We also show that enstatite chondrites (EC) comprise an even higher Δ17O of 51 ± 6 ppm relative to the Earth. Thus, EC cannot be the sole building blocks of the Earth. Instead EC may resemble the composition of the giant impactor. If so, the Moon may be composed of ~40% impactor material, consistent with recent numerical models of the collision [2,3]. In an alternative scenario the lower Δ17O composition of the Earth could reflect a late veneer with low Δ17O. Addition of 0.5% carbonaceous chondrites would be sufficient to lower the Δ17O of silicate Earth by 12 ppm.
*The paper on which this presentation is based will be published in Science on 6th June 2014 (details below). The full paper will be available after the embargo at: http://www.sciencemag.org/lookup/doi/10.1126/science.1251117