How was the Moon formed? The most-widely-accepted theory suggests that billions of years ago, our young planet was struck by a body as large as Mars, forming the Moon. However, new observations of the levels of metals on our companion world suggest this origin story may need updating.
Roughly 4.5 billion years ago, Theia — a Mars-sized world — is thought to have collided with the upper crust of the Earth, a region of our planet relatively poor in metals. This impact kicked large quantities of material into orbit around the Earth, which coalesced into the Moon.
This idea would suggest that metals would also be rare on the Moon. However, radar observations of the Moon suggest relatively large quantities of metals in some craters on our planetary companion.
Examination of dust at the bottom of lunar craters revealed significant concentrations of metals, including iron and titanium oxides. Analysis of the material, using the Miniature Radio Frequency (Mini-RF) instrument on the Lunar Reconnaissance Orbiter (LRO), suggests these metals were brought up from beneath the lunar surface by meteors striking the Moon.
Interestingly, larger, deeper craters showed greater concentrations of metals than smaller basins. However, this was not a simple relationship for larger impact sites. For craters measuring between two to five kilometers (one to three miles) in diameter, larger craters contained higher concentrations of exposed metals. However, the amount of metals seen in craters between five and 20 km (three to 12 miles) in diameter did not change significantly with size.
“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 neighbor,” said Noah Petro, the LRO project scientist at NASA’s Goddard Space Flight Center.
Oddly, although the bright highlands of the Moon are metal-poor, areas of the Moon (particularly the dark seas or maria), contain even greater concentrations of iron oxide (rust) than is found on Earth. This idea, long-known to researchers, presents a significant challenge to the Theia hypothesis.