Moon was formed after planet-sized body hit Earth 4.5 billion years ago

A new series of measurements of oxygen isotopes have provided evidence that the Moon was formed after Earth collided with another large, planet-sized astronomical body, about 4.5 billion years ago.

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.

Most models estimate that the Moon it 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. Dr Herwartz said that the new data indicate that a 50:50 mixture seems possible, but this needs to be confirmed.

The study has been published in the journal Science.