About 4.5 billion won years ago, a primitive version of the Earth covered in molten lava orbited the sun. Shortly after its appearance, it was hit by a smaller object the size of Mars, called Theia, in an explosive event. Theia was blown to pieces by the impact, while a large chunk of the Earth was sent into space.
The gravity of much of the rest of our planet has caused this matter to swirl around Earth. In a surprisingly short amount of time, perhaps less than 100 yearssome of that material stuck together and formed the moon.
Or at least, this is the popular theory of lunar origin. Now, however, there is new evidence that the moon was indeed created from the debris of this cosmic impact billions of years ago. The discovery of certain gases inside the moon supports this idea and also provides us with important new details about how it may have happened.
While completing her PhD at the Swiss Federal Institute of Technology (ETH) in Zurich, Patrizia Will studied six lunar meteorites recovered by NASA from Antarctica in the early 2000s. In these rocks , she and her colleagues found helium and neon trapped in tiny glass particles, which are formed during volcanic eruptions on the lunar surface when magma is pulled up from the moon’s interior. These gases, called noble gases because they are relatively dormant, are native to Earth and likely inherited by the moon “during its formation,” Will said. The study was published in the journal Scientific advance.
Previous work has hinted at the giant collision hypothesis. Moon rocks show striking similarities to Earth rocks, suggesting a common origin. However, there are key differences: Moonstone has lighter version of clofor example, pointing to a dramatic event early in the history of our two worlds that separated some matter.
Most scientists now agree that the event was a giant collision. “We pretty much hypothesized a colossal impact,” said Sujoy Mukhopadhyay, a geochemist from the University of California, Davis, who was not involved in Will’s study. “It’s still the best theory on the table.”
After the collision, a disk of matter is displaced by the impact — possibly a donut of vaporized rock called a synestia, measured temperature thousands of degrees—May have formed around our planet. The amounts of neon and helium detected in lunar samples support the hypothesis that the moon formed in this homozygous state, since the relative abundances of these gases suggest that they come from the Earth’s mantle and are blasted into space by impact before merging into the interior of our satellite. If these gases were transported through space into the moon by the solar wind instead, we would expect much lower amounts in the meteorites analyzed.
“It’s really exciting work,” says Mukhopadhyay, noting that no previous studies have been able to find evidence of such native gases in lunar rocks. “The concentrations are so low that it’s difficult to detect,” said Ray Burgess, a geochemist from the University of Manchester and a reviewer of Will’s research. “It’s a big step forward.”
