Lunar craters may hold ancient remnants of earth
Minerals found in craters on the moon may be remnants of asteroids that slammed into it and not, as long believed, the moon's innards exposed by such impacts, a study has found.
The findings, published in the journal Nature Geoscience on Sunday, cast doubt on the little we knew of what the moon is actually composed of.
It had long been thought that meteoroids vaporise on impact with large celestial bodies.
Unusual minerals like spinel and olivine found in many lunar craters, but rarely on the moon's surface, were therefore attributed to the excavation of sub-surface lunar layers by asteroid hits.
Olivine and spinel are common components of asteroids and meteorites, and have been found on the floors and around the central peaks of such lunar craters as Copernicus, Theophilus and Tycho that are about 100 kilometres in diameter.
A team from China and the United States simulated the formation of moon craters and found that at velocities under 12 kilometres per second a projectile may survive impact, though fragmented and deformed.
"We conclude that some unusual minerals observed in the central peaks of many lunar impact craters could be exogenic (external) in origin and may not be indigenous to the moon," they wrote.
Co-author Jay Melosh, from Purdue University in Indiana, said the finding answers the conundrum exposed by earlier studies that said craters the size of Copernicus were not big enough to have dredged up the contents of the moon's deep, interior mantle.
In an article commenting on the study, Erik Asphaug of the School of Earth and Space Exploration at Arizona State University, said the theory meant material excavated from earth by large impacts during its early days may still be found on the moon.
This suggested yet another explanation for the existence of spinels on the moon, said Asphaug: They came from earth. "Even more provocative is the suggestion that we might someday find earth's protobiological materials, no longer available on our geologically active and repeatedly recycled planet, in dry storage up in the lunar 'attic'.
"Certainly, the potential of finding early earth material is emerging as one of the primary motivations for a return to the moon by human astronauts in our ongoing search for the origin of life."