Lunar Subsurface May Be Richer in Metals than Previously Thought
Using data from the Miniature Radio Frequency (Mini-RF) instrument onboard NASA’s Lunar Reconnaissance Orbiter (LRO), a team of U.S. researchers has characterized the dust found at the bottom of the Moon’s craters and found evidence that the lunar subsurface might be richer in metals, like iron and titanium, than thought.
Substantial evidence points to the Moon as the product of a collision between a Mars-sized protoplanets, named Theia, and young Earth, forming from the gravitational collapse of the remaining cloud of debris. Consequently, the Moon’s bulk chemical composition closely resembles that of Earth.
Look in detail at the Moon’s chemical composition, however, and that story turns murky.
For example, rocks in the lunar highlands contain smaller amounts of metal-bearing minerals relative to Earth.
That finding might be explained if Earth had fully differentiated into a core, mantle and crust before the impact, leaving the Moon largely metal-poor. But turn to the Moon’s maria and the metal abundance becomes richer than that of many rocks on Earth.
This discrepancy has puzzled planetary scientists, leading to numerous questions and hypotheses regarding how much the impacting protoplanet may have contributed to the differences.
In the new study, University of Southern California researcher Essam Heggy and his colleagues found a curious pattern that could lead to an answer.
Using the Mini-RF instrument, they sought to measure an electrical property called dielectric constant within lunar soil piled on crater floors in the Moon’s northern hemisphere.
They noticed this property increasing with crater size. For craters 2-5 km (1-3 miles) wide, the dielectric constant steadily increased as the craters grew larger, but for craters 5-20 km (3-12 miles) wide, the property remained constant.
“It was a surprising relationship that we had no reason to believe would exist,” Dr. Heggy said.
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 scientists 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.
If their hypothesis were true, it would mean only the first few hundred meters of the lunar surface is scant in iron and titanium oxides, but below the surface, there’s a steady increase to a rich and unexpected bonanza.
Comparing crater floor radar images from Mini-RF with metal oxide maps from LRO’s Wide-Angle Camera, Japan’s Kaguya mission and NASA’s Lunar Prospector spacecraft, the team found exactly what it had suspected.
The larger craters, with their increased dielectric material, were also richer in metals, suggesting that more iron and titanium oxides had been excavated from the depths of 0.5-2 km (0.3 to 1 mile) than from the upper 0.2-0.5 km (0.1 to 0.3 miles) of the lunar subsurface.
These results follow recent evidence from NASA’s Gravity Recovery and Interior Laboratory (GRAIL) mission that suggests a significant mass of dense material exists just a few tens to hundreds of kilometers beneath the Moon’s enormous South Pole-Aitken basin, indicating that dense materials aren’t uniformly distributed in the Moon’s subsurface.
“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,” Dr. Heggy said.
“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.”
The study was published in the journal Earth and Planetary Science Letters.
E. Heggy et al. 2020. Bulk composition of regolith fines on lunar crater floors: Initial investigation by LRO/Mini-RF. Earth and Planetary Science Letters 541: 116274; doi: 10.1016/j.epsl.2020.116274