Minerals seen in some of the moon’s craters may not belong to the moon at all but instead were likely delivered by asteroids or possibly even the Earth. Unusual minerals like spinel (ruby-like red gemstone) and olivine (olivine-green gemstone) have been found on the floors and especially in the central peaks of several larger lunar craters including the familiar Tycho, Copernicus and Theophilus using instruments like NASA’s Moon Mineralogy Mapper. Scientists assumed they were seeing material excavated from deep below the moon’s surface.
Maybe not. These very same minerals are also common in meteorites. A recently published study in the journal Nature Geoscience by a team of scientists from the U.S. and China used computer models to fling simulated asteroids and meteorites at the moon with speeds under 26,820 mph (43,160 kph). Some 30% of asteroidal debris striking the moon travels below that speed according to the study.
When these slower-moving space rocks slam into the moon, the researchers found that fragments survived the impact. If the newly formed crater was 12 miles (20 km) wide or larger, asteroid material sent flying outward toward the crater’s rim would later fall back through gravity into the crater’s central peak. Peaks form in big craters when material that’s crushed and compacted by the incoming asteroid rebounds or rises back up in the crater’s center after impact.
“This observation may explain recent observations of exotic Mg-rich spinels and olivine in the central peaks of craters too small to have excavated the deep crust or mantle of the Moon,” they wrote. By extension, the team suggests that crater peaks on Mars and Vesta may also preserve remnants of exotic minerals delivered by asteroids.
It’s generally assumed meteorites vaporize upon impact and leave only tiny fragments in crater floors, but if the impactor moves below a critical speed, the results of the study show it can leave bigger pieces. That means scientists must be cautious when deciding if the rock in the moon’s peaks really do represent samples excavated from deep down in the moon’s mantle or whether they’re alien rocks left by potshot asteroids.
More intriguing is the possibility that some of those olivines and other exotic minerals came from Earth. Our planet got whacked as much or more than the moon several billion years back. More than 170 named lunar meteorites have been found on Earth, and studies have shown that delivery of “Earth meteorites” to the moon via impact is easily accomplished. You never know – there may even be the hardened, glassy remains of stromatolites, one of the planet’s earliest life forms dating from as early as 3.5 billion years ago, sparkling atop some lunar mountain. In simulations, materials leaving Earth would have melted on the outside but remained intact within.