Moon formed after Earth was hit by planet-sized body

Scientists have found new evidence to support the theory that Moon was created by a gigantic collision between Earth and a mysterious planet-sized object some 4.5 billion years ago.
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, researchers said.
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 from the University of Goettingen, 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.
"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," Herwartz said.
"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," said Herwartz.

Most models estimate that the Moon is composed of around 70 per cent to 90 per cent material from Theia, with the remaining 10 per cent to 30 per cent coming from the early Earth. However, some models argue for as little as 8 per cent Theia in the Moon.
Herwartz said the new data indicates 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 difference in 17O/16O ratio between Earth and Moon.

The finding was published in the journal Science and will be presented at the Goldschmidt geochemistry conference in California.