Craters Expose the Moon's Insides
This photo of craters at the moon's north pole was taken By Japan's Kaguya lunar orbiter on Oct. 31, 2007.
Credit: JAXA.

Rocks originating deep below the moon's surface that contain rich amounts of one of the most common minerals found on Earth were recently discovered around ancient impact craters on the lunar surface.

The rocks were chock full of the mineral olivine, which is thought to originate from within the lunar mantle, beneath the crust. Olivine is a silicate or rock-forming mineral that is commonly found on Earth. But, prior to this study, olivine findings on the moon were very limited.

"Before our research there were only two or three olivine exposure sites," Satoru Yamamoto, the study's lead author, told "The question was: why are the olivine exposures very limited on the moon?"

Yamamoto is a researcher at the Center for Global Environmental Research at the National Institute for Environmental Studies in Tsukuba, Japan. He and his team of researchers analyzed spectroscopic data from the Japanese Kaguya lunar probe, which surveyed the composition of the moon's surface by measuring the relative amounts of different wavelengths of light it emitted.

"Our team focused on the analysis of the spectra of the lunar surface," Yamamoto said. "Each data has many very fine high-resolution wavelengths. Therefore, we can distinguish the absorption band ? namely we can determine what type of silicate or what type of mineral it is from each spectrum."

Exposures of olivine were found in specific areas on the moon ? in concentric rings around big impact craters, in regions where the lunar crust is relatively thin and the mantle is closest to the surface.

The study's findings will be published online July 4 in the journal Nature Geoscience. In their paper, the researchers suggest that large lunar impacts many years ago penetrated the outer crust, which exposed the mantle olivine that is stored below the moon's surface.

Asteroid bombardment over billions of years has left the lunar surface pockmarked with craters of all sizes, and covered with solidified lava, rubble, and dust. But these large and ancient impacts, which were estimated to have occurred approximately 3.8 to 4 billion years ago, likely brought deposits of olivine material up to the surface.

There are four main silicate minerals found on the moon: olivine, plagioclase feldspar, and two types of pyroxene. Plagioclase fieldspar is mostly found in the lunar crust, said Yamamoto, while pyroxene and olivine are typically seen in the lunar mantle.

The study's findings provide greater insight into the history and development of the moon's mantle, and Yamamoto is keen to further examine the distribution of olivine, which he thinks has important implications.

This is because the mineral is not present in all large impact craters on the moon. Rather, only in select basins, and in areas of thin lunar crust. Additionally, impact craters with olivine exposures were more prevalent on the near side of the moon, where the lunar crust is not as thick.

There are currently only two known regions on the far side of the moon with olivine exposures ? in regions where the crust is thinner.

And while the study represents an important milestone in the field of lunar science, the researchers said, more research is still required.

"I think the distribution of olivine is very important, but unfortunately we need more data ? not just of olivine, but of other minerals too," Yamamoto said.