Researchers have a good idea of the minerals that make up the surface of the moon, thanks to thousands of lunar rocks brought back by Apollo astronauts and reams of remote-sensing information. But what lies under the crust? Japanese scientists think they have a partial answer: Data collected by the lunar orbiter Kaguya suggests that material from the moon's interior—a relatively heavy mineral, olivine—can be found on the rims of a number of its major craters.
Planetary scientists generally agree that the moon formed from debris thrown into space when a huge celestial object slammed into Earth about 4.5 billion years ago. The early moon was so hot it was covered by a magma ocean in which heavier elements and minerals sank and formed the mantle, while lighter material floated to the surface and eventually solidified into the crust. Olivine is one of the main components of Earth's mantle, and scientists speculated that it must also be an important part of the moon's mantle. But evidence was scarce.
Previous observations from Earth and from spacecraft spotted a few patches of olivine on the surface of the moon. Kaguya was configured to take a closer look while orbiting the moon from November 2007 to June 2009. The spacecraft's Spectral Profiler instrument recorded the visible and near-infrared light reflected off the lunar surface at 70 million points; analysis found the signature of olivine reflectance at 245 of those. Plotting those sites on a map of the moon produced a pattern hinting at rings of olivine at the edges of major craters punched into the surface where the crust is relatively thin. Planetary scientist Satoru Yamamoto of Japan's National Institute for Environmental Studies in Tsukuba and colleagues report online today in Nature Geoscience that the most likely explanation for this pattern is that impacting asteroids and comets broke through the crust and pushed material from the upper mantle or lower crust toward the rims of the craters.
"The finding is exciting [because] for the first time there appears to be evidence for rocks exposed on the surface of the moon that do not originate from the upper lunar crust," says Carsten Münker, a geochemist at the University of Cologne in Germany. The results "are very important for understanding how the early crust and mantle of [bodies like the moon] form and evolve in the first few hundred million years," adds Carle Pieters, a planetary scientist at Brown University.
Like many findings, this one raises new questions, particularly about whether this olivine came from the mantle or the lower crust. Yamamoto thinks the pattern of the reflected light more closely matches what would be expected from material from the mantle. "But we can't rule out the possibility of a crustal origin," he says. Future moon missions will probably try to tackle this question, possibly by retrieving some lunar olivine. "Geochemists would have a strong interest in having a look at [such] samples," says Münker.