As is so often the case, a scientific team recently made an impressive discovery while searching for something completely different. In this case, what started as a hunt for ice in polar lunar craters led to a discovery that could change our view of the Moon's formation.
Team members of the Miniature Radio Frequency (Mini-RF) instrument on NASA’s Lunar Reconnaissance Orbiter (LRO) spacecraft found new evidence suggesting that the Moon's subsurface is richer in metals than previously thought.
The young Moon's formation
Evidence has long pointed to the Moon as the product of a collision between a Mars-sized protoplanet and young Earth. The ensuing gravitational collapse is thought to have formed the Moon out a cloud of debris leftover from the collision. Due to this, scientists hypothesize, the Moon's chemical composition closely resembles that of Earth.
Looking in detail at the Moon's chemical composition, however, throws a spanner in the works. For example, there is a large discrepancy between the low amount of metal-bearing minerals on the Moon's bright plains, the lunar highlands, compared to the abundance of metal in the Moon's maria — it's large, darker plains.
The difference has puzzled scientists for years, leading to various hypotheses about how the impacting protoplanet may have contributed to the contrasting levels of metal-bearing minerals on the Moon.
Now, the Mini-RF team revealed they found a curious pattern that might lead to an answer. The finding, published July 1 in Earth and Planetary Science Letters could help draw a clearer connection between Earth and the Moon, the NASA research team says.
Looking for ice and finding metal on the Moon
“The LRO mission and its radar instrument continue to surprise us with new insights about the origins and complexity of our nearest neighbor,” Wes Patterson, Mini-RF principal investigator from the Johns Hopkins Applied Physics Laboratory (APL) in Laurel, Maryland, and a study co-author said in a NASA press release.
Using Mini-RF, the researchers measured an electrical property within the soil on crater floors in the Moon's northern hemisphere, known as the dielectric constant — a number that compares the relative abilities of a material and the vacuum of space to transmit electric fields.
How much metal is on the Moon?@NASA's Lunar Reconnaissance Orbiter has found evidence that increased levels of iron and titanium oxides lurk below the lunar surface >> https://t.co/ztScYSowl2@NASAMoonpic.twitter.com/HZkydOd2UU— NASA Marshall (@NASA_Marshall) July 2, 2020
The team was hoping the number could help locate ice lurking in the crater shadows. However, they noticed that the property increased with crater size. When craters reached a certain size — 3 to 12 miles (5 to 20 kilometers) wide — the property remained constant.
This led the team to a new hypothesis: as meteors that form larger craters also dig deeper into the Moon's subsurface, 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 specific metal minerals, NASA explains.
If the hypothesis were true, it would mean that while large parts of the Moon's surface might lack iron and titanium oxides, below the surface there is an abundance of minerals.
An abundance of lunar metal-bearing minerals
In order to test their hypothesis, the team compared crater floor radar images from Mini-RF with metal oxide maps from the LRO Wide-Angle Camera, Japan’s Kaguya mission, and NASA’s Lunar Prospector spacecraft. In doing so, the team found exactly what they had expected: the larger craters, with their increased dielectric material, were also richer in metals.
"This exciting result from Mini-RF shows that [...] 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 in Greenbelt, Maryland.
"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!"
While the finding doesn't settle any hypotheses about the Moon's formation, it does bring us a step closer to knowing with any certainty how our lunar neighbor came into existence around Earth's orbit.
The team is already underway testing the same theory on the Moon's Southern hemisphere to see if the same trends exist.