Scientists still aren't entirely sure how the Moon formed, though plenty of hypotheses have been discussed. Now, new findings around metal deposits in lunar craters could mean we need to rethink those hypotheses again.
In short, the findings suggest there are more metals like iron and titanium in the Moon's craters than we thought there were – and that might point to a rich store of metal material just under the lunar surface.
How much metal the Moon is holding in comparison to Earth is one of the key clues astronomers have when trying to figure out how our satellite came to exist. Any adjustments in those readings can tell us more about the Moon's origins.
While the most prevalent hypothesis has been that the Moon was spun off from Earth's crust as a result of a massive collision with a third body, our planet's crust has less iron oxide in it than the Moon does – something that scientists have been trying to explain for some time.
This new research posits that there's even more metal material lurking just below the lunar surface, which wouldn't make sense if it had spun off from the relatively metal-poor Earth crust. That casts doubt on the preferred origin story.
"It really raises the question of what this means for our previous formation hypotheses," says space scientist Essam Heggy, from the University of Southern California.
"By improving our understanding of how much metal the Moon's subsurface actually has, scientists can constrain the ambiguities about how it has formed, how it is evolving and how it is contributing to maintaining habitability on Earth."
The latest study is the result of radar readings from the Miniature Radio-Frequency (Mini-RF) instrument on board NASA's Lunar Reconnaissance Orbiter, currently circling around the Moon.
In the course of looking for ice, Mini-RF was busy measuring the dielectric constant – an electrical property of lunar soil within craters – when researchers noticed that the level rose as the craters got bigger, up to craters 5 kilometres (3.1 miles) in diameter. In even larger craters, the dielectric constant levelled off.
Follow-up analysis using data from other instruments and spacecraft confirmed that the larger craters contained more metal, quite possibly because they dig deeper into the Moon's surface. This suggests that below the surface, which is relatively metal-free, there's a lot more to find.
"This exciting result from Mini-RF shows that even after 11 years in operation at the Moon, we are still making new discoveries about the ancient history of our nearest neighbour," says planetary scientist Noah Petro, from NASA's Goddard Space Flight Center.
"The Mini-RF data is incredibly valuable for telling us about the properties of the lunar surface, but we use that data to infer what was happening over 4.5 billion years ago!"
Of course there's still a lot of uncertainty when it comes to peering back that far and trying to work out the complexity of how the Moon first fell into Earth's orbit. Further research is already underway to ascertain if this link between more metal and bigger craters also holds true on the Moon's southern hemisphere.
It's possible that the Moon was created from material much deeper inside Earth than previously thought, or that these extra metals are the result of a molten lunar surface gradually cooling down.
There are quite few potential possibilities and many questions remain, but publishing findings like this helps to remove some of the unknowns about the Moon and where it comes from, which should lead to better hypotheses in the future. That applies to other moons in the Solar System too, as well as our own.
"Our Solar System alone has over 200 moons – understanding the crucial role these moons play in the formation and evolution of the planets they orbit can give us deeper insights into how and where life conditions outside Earth might form and what it might look like," says Heggy.
The research has been published in Earth and Planetary Science Letters.