Scientists studying the moon have long puzzled over the low metal content of Earth’s satellite. After all, if the moon formed from fragments of Earth, shouldn’t it have similar metal content? NASA’s Lunar Reconnaissance Orbiter (LRO) might finally have found an explanation for this apparent discrepancy: the metal might be buried deep below the surface.
No one knows exactly how the moon formed, but most researchers accept the collision hypothesis as the most probable. According to this model, a massive planetoid the size of Mars collided with the primordial Earth several billion years ago. The impact blasted large portions of Earth’s crust into space, which would have formed a ring that slowly coalesced into the moon we know today.
The catch is that the moon’s chemical composition doesn’t seem to support an origin like that. At least, the part of the moon we can see doesn’t support it. The lunar highlands, visible as light regions on the surface, have lower metal content than Earth. Meanwhile, the darker maria planes have higher metal content, but the two features would have formed at the same time.
New data from the RO might finally help unravel this mystery thanks to an instrument called the Miniature Radio Frequency (Mini-RF). The Mini-RF measures the dielectric constant, a way to measure the conducting properties of a material compared with the vacuum of space. NASA designed this tool to scan craters for water ice, but it can also detect metals.
According to the new study, the dielectric constant on the moon increases with crater size. Craters between 1 and 3 miles (2 and 5 kilometers) in diameter showed higher metal content when viewed from the LRO, but the increase tapered off around 3-12 miles. The team speculated that the first few hundred meters of the surface were low in metal oxides, but the concentration was higher below that.
To confirm the speculation, the researchers compared their results to existing metal oxide maps of the moon from missions like Japan’s Selenological and Engineering Explorer (SELENE) and NASA’s Lunar Prospector spacecraft. Sure enough, the data showed larger craters have higher concentrations of metal. This data may also have some relevance to NASA’s Gravity Recovery and Interior Laboratory (GRAIL), which showed that there’s a lot of dense material deep below the moon’s surface.
The team is not calling this one solved quite yet. The next step is to conduct similar scans on the moon’s southern hemisphere to see if the craters have similar geology.
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