Our beautiful Moon, serenely circling our world as it illuminates our evenings and tugs at our tides, is thought to have been born from a violent and catastrophic collision between a freshly-formed Earth and a Mars-sized wayward protoplanet almost four and a half billion years ago. This “giant impact hypothesis” is generally accepted because it provides causation for many aspects of why the Moon is how we see it today. But there have always been some discrepancies with the theory—one of which being the difference in distribution and amount of some heavy metals in the Moon’s composition based on when the impact event occurred during Earth’s formative years.
Now, observations with the Miniature Radio Frequency (Mini-RF) instrument aboard NASA’s Lunar Reconnaissance Orbiter (LRO), in orbit since 2009, indicate the Moon’s crust is much richer in heavy metals like iron and titanium than previously thought—perhaps clearing up some questions on its origin.
Via a news release from Johns Hopkins University’s Applied Physics Laboratory (JHUAPL):
Using Mini-RF, the researchers sought to measure an electrical property within lunar soil piled on crater floors in the Moon’s northern hemisphere. This electrical property is known as the dielectric constant, a number that compares the relative abilities of a material and the vacuum of space to transmit electric fields, and could help locate ice lurking in the crater shadows. The team, however, noticed this property increasing with crater size.
For craters approximately 1 to 3 miles (2 to 5 kilometers) wide, the dielectric constant of the material steadily increased as the craters grew larger, but for craters 3 to 12 miles (5 to 20 kilometers) wide, the property remained constant.
(See the published paper from the LRO’s Mini-RF instrument team here.)
Discovery of this pattern opened a door to a new possibility. Because meteors that form larger craters also dig deeper into the Moon’s subsurface, the team reasoned that the increasing dielectric constant of the dust in larger craters could be the result of meteors excavating iron and titanium oxides that lie below the surface. Dielectric properties are directly linked to the concentration of these metal minerals.
If their hypothesis were true, it would mean only the first few hundred meters of the Moon’s surface is scant in iron and titanium oxides, but below the surface, there’s a steady increase to a rich and unexpected bonanza.
“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. 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!”
— Noah Petro, LRO project scientist at NASA’s Goddard Space Flight Center
The team emphasizes that the new study can’t directly answer the outstanding questions about the Moon’s formation, but it does reduce the uncertainty in the distribution of iron and titanium oxides in the lunar subsurface and provide critical evidence needed to better understand the Moon’s formation and its connection to Earth.