Scientists studying samples of lunar rock from the Apollo space programme have discovered evidence to support the theory that the Moon was created from the debris of a massive cosmic collision between the Earth and an object as big as Mars.
Concentrations of oxygen isotopes in the lunar rock reveal a distinct chemical signature of the original planet-sized object that was supposed to have collided with the Earth about 4.5 billion years ago, a study has found.
Until now scientists have been unable to find substantial isotopic differences between lunar and terrestrial rock. This called into question whether the Moon was indeed created in a collision rather than being bits of the primordial Earth that had been flung out into orbit during its early formation.
However, the analysis of lunar rock samples from three Apollo missions half a century ago has revealed distinct isotopic differences with terrestrial rock. Scientists believe these differences are remnants of the original Mars-sized object, named Theia, the mythical Greek Titan who was the mother of the moon goddess Selene.
“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,” said Daniel Herwartz of George August University in Gottingen, Germany.
Most computer models of the collision suggest that between 70 and 90 per cent of the material making up the Moon comes from the original Theia, with the remaining 10 or 30 per cent coming from terrestrial debris flung out from Earth during the glancing blow of the impact.
Until now, the giant-impact hypothesis to account for the existence of the Moon has lacked the smoking gun of distinct isotopic differences between the rocks found on Earth and those found on the Moon.
However, the study, published in the journal Science and based on the analysis of three rock samples brought back on Apollos 11, 12 and 16, not only shows significant differences in the isotopes of oxygen, but indicates that Theia belonged to a class of rare meteorites known as 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,” Dr Herwartz said.
“The next goal is to find out how much material of Theia is in the Moon,” he said.
Mahesh Anand, a planetary scientist at the Open University in Milton Keynes, who was not involved with the study, said the findings highlight the unique nature of the Earth-Moon system – most of the other 150 moons in the Solar System are either captured planetesimals or were formed directly from the debris of the planet they orbit.
“It is an exciting story but is derived from just three lunar rock samples. We have to be cautious about representativeness of these rocks of the entire Moon and so further analysis of a variety of lunar rocks is required for further confirmation,” Dr Anand said.
“This research brings back into limelight some of the unanswered questions surrounding the giant impact hypothesis that will require additional analysis of lunar and terrestrial samples to gain further insights into the origin of the Moon,” he said.