A new analysis of lunar rocks brought home by Apollo mission astronauts has shed fresh light on the violent birth of the moon. Researchers in Germany have found small but distinctive chemical signatures that suggest the moon formed when a giant planetary body slammed into the early Earth 4.5bn years ago.
Scientists have several theories for how the moon may have formed, but the “giant impact hypothesis” has been the leading explanation for some time. A cataclysmic impact between the Earth and a Mars-sized planet, known as Theia, would have scattered rock and dust from both bodies out into space, and these fragments would then have coalesced to form the moon.
But there was a problem with this scenario that left some researchers in doubt: variants of chemical elements on Earth seemed identical to those on the moon. If the moon had formed in a huge collision, then lunar rock should differ from Earth rock, because the former would contain material from Theia.
Scientists at the University of Göttingen analysed rocks brought back by the Apollo 11, 12 and 16 missions and found small but significant differences in the ratios of oxygen isotopes in moon and Earth rocks. On average, the difference amounted to only 12 parts per million. “The differences are very small but they are significant,” said Andreas Pack, a geochemist who worked on the rocks.
The study, reported in Science, solves one of the most enduring puzzles over the birth of the moon by confirming that, as would be predicted from a giant impact, the Earth and moon have slightly different chemical signatures. “This strengthens the evidence for a giant impact. That is very much the most likely process that formed the moon,” Pack said. “It was a long-lasting debate because we hadn’t found any isotopic difference between the Earth and the moon.”
The results give scientists some much-needed clues about the chemical make-up of Theia. The moon rock is similar to a class of minerals called enstantite chondrites. “This is the first time it has been possible to say something more about the giant impact hypothesis,” said Addi Bischoff, a senior author on the paper.
“The next goal is to find out how much material of Theia is in the moon,” said Daniel Herwartz, the first author of the study. Previous models of the birth of the moon had led scientists to believe it was around 70% Theia. But calculations by the German researchers point to a more even split, with the moon perhaps being half Earth and half Theia.
Earth’s moon is unusual in the solar system. There are more than 150 moons orbiting other planets, and most were either captured by their parent planet, or formed alongside the planets they orbit. The violent collision that created the moon is thought to explain the nature of its orbit and why it has less water and volatile compounds.
Last year, a separate study of lunar rocks brought back by Apollo astronauts found that traces of water locked inside the moon were originally from Earth. Scientists at Brown University drew the conclusion after running tests on water in droplets of volcanic glass trapped inside crystals in the rocks.