Excavation of lunar-mantle materials by huge meteoroid impacts
In past research, the Pluton hypothesis was proposed to explain how olivine-rich materials emerged on the lunar surface. The hypothesis posits that troctolite, produced by magma intrusion, was present in the lower crust of the Moon and exposed to the surface during the formation of medium-sized impact craters. Thus, previous research assumed that olivine on the surface originated from lower-crust magma not the mantle. Meanwhile, all the olivine-rich materials discovered by SP is located around the circumference of giant impact basins with a large diameter of several 100 km to 1,000 km where the crust is thin. In addition, SP found little olivine-rich material in the central peaks of medium-sized craters, where its existence had been forecast by previous research.
Based on these findings, we supposed that olivine-rich material distributed on the lunar surface must have come from very deep regions. In other words, the lunar mantle must have been excavated by the impacts of huge meteoroids. The olivine-rich areas identified by the SP are all areas of relatively thin crust. Further, the thickness of the original crust areas before impact was thinner than the maximum thickness (~100km) of the current crust on the far side of the Moon. Thus, we judge that, when giant impact basins exceeding several 100km to 1,000km in diameter were formed (excavation depth is around 100km at the maximum), even the mantle under the crust was excavated. One possible reason why olivine-rich material was not discovered in the central regions of giant impact basins is that the surface there was filled with basaltic lava due to mare formation process.
Detailed analysis of the reflection spectrum of the olivine-rich sites measured by the SP shows that the spectrum is very close to, among other olivine-rich rocks, dunite, whose origin is thought to be the upper lunar mantle. It is also verified that the spectrum is not consistent with troctolite believed to be present in the lunar lower crust. These findings show that olivine-rich material discovered on the lunar surface originated from the mantle. From these results, there is a high possibility that the material discovered by SP is from deep in the lunar mantle and was excavated by the impacts of huge meteoroids.
Significance of discovery by the SP
The discoveries this time by the SP provide very important information and/or constraints on the formation and evolution of the lunar mantle. For example, the lunar magma ocean model presumes that a layer of rock called KREEPE(potassium, rare-earth elements and phosphorus), the residue of the magma to solidify, exists between the crust and the upper mantle. Accordingly, it is thought that, when an impact large enough to excavate olivine from the upper mantle occurs, KREEP would emerge on the surface as well. According to past explorations, KREEP is concentrated in giant-basin terrains (e.g., Mare Imbrium and Oceanus Procellarum) located in the center of the near side of the Moon. However, KREEP concentrations have not been discovered in the Mare Moscoviense and the Mare Crisium, where much olivine was found. This suggests that KREEP concentrated in parts of the near side of the Moon or in the deeper mantle before the formation of giant-impact basins caused the excavation of olivine, and was not present in other regions. In this way, based on the olivine-rich site distribution, we have been able to obtain concrete information on the distribution of material in the lunar interior a long time ago. This discovery is thought to be very important because it could lead to a deeper understanding of not only the evolution of the lunar interior, but also the evolution of other celestial bodies such as the earth.
Satoru YAMAMOTO / Tsuneo MATSUNAGA