Scientists studying the moon have long puzzled over the low metal content of the Earth’s satellite. If the moon was made up of fragments of earth, shouldn’t it have a similar metal content? NASA’s Lunar Reconnaissance Orbiter (LRO) could finally have found an explanation for this obvious discrepancy: the metal could be buried deep below the surface.
No one knows exactly how the moon formed, but most researchers accept the collision hypothesis as the most likely. According to this model, a massive planetoid the size of Mars collided with the primeval earth a few billion years ago. The impact blasted large portions of the earth’s crust into space, which would have formed a ring that slowly melted into the moon we know today.
The catch is that the chemical composition of the moon does not appear to support such an origin. At least the part of the moon that we can see does not support it. The moon highlands, which are visible as light regions on the surface, have a lower metal content than the earth. Meanwhile, the darker Maria planes have a higher metal content, but the two features would have formed at the same time.
New data from the RO could finally help solve this puzzle thanks to an instrument called Miniature Radio Frequency (Mini-RF). The Mini-RF measures the dielectric constant to measure the conductivity of a material compared to the vacuum of space. NASA developed this tool to search craters for water ice, but it can also detect metals.
According to the new study, the dielectric constant on the moon increases with the crater size. Craters between 2 and 5 kilometers in diameter showed a higher metal content from the LRO, but the increase decreased by 3 to 12 miles. The team speculated that the first few hundred meters of the surface contained little metal oxide, but the concentration was higher below.
To confirm the speculation, the researchers compared their results with existing moon metal oxide maps from missions such as the Japanese Selenological and Engineering Explorer (SELENE) and NASA’s Lunar Prospector. Sure enough, the data showed that larger craters have higher metal concentrations. This data could also be important for NASA’s Gravity Recovery and Interior Laboratory (GRAIL), which showed that there is a lot of dense material deep below the surface of the moon.
The team does not yet call this fully resolved. The next step is to perform similar scans in the southern hemisphere to determine if the craters have a similar geology.