Meteorites are both the messengers and time capsules of the Solar System. As pieces of larger asteroids that broke apart, or debris thrown up by impacts on other bodies, these "space rocks" retain the composition of where they originated from. As a result, scientists can study other planets, moons, and objects by examining the abundance of chemical elements in meteorites. Unfortunately, such studies are limited when it comes to meteorites retrieved on Earth, due to erosion, atmospheric filtration, and geological processes (like volcanism and mantle convection).
However, meteor impacts are well-preserved in the lunar environment, as it has virtually no atmosphere, experiences no wind or water erosion, and is (for the most part) geologically inactive. Recently, a research team with the Chinese Academy of Sciences (CAS) examined samples returned by the Chang'e-6 mission from the far side of the Moon. They identified seven olivine-bearing minerals from the lunar regolith they examined, which they determined to have been deposited by Carbonaceous Ivuna-type (CI) chondrites, a type of fragile meteorite that rarely survives impact with Earth.
CAS Professors Xu Yigang and Lin Mang led the research team. It consisted of researchers from the CAS's Guangzhou Institute of Geochemistry (GIG), the College of Earth and Planetary Sciences at the CAS University, the Department of Earth, Environmental and Planetary Sciences at Brown University, the Research Organization of Science and Technology of Ritsumeikan University, and the Department of Archaeology, Environmental Changes and Geo-Chemistry at Vrije Universiteit Brussel. The paper describing their findings was published in Proceedings of the National Academy of Sciences (PNAS) on Oct. 20th.
*Meteorites bombard a molten landscape in this illustration of the Late Heavy Bombardment. Credit: NASA GSFC Conceptual Image Lab*
CI chondrites are a rare type of carbonaceous meteorite, which are defined by their relative abundance of carbon (up to 3%) in the form of graphite, carbonates, and organic compounds (including amino acids). The parent bodies originally formed in the outer Solar System, and many migrated into the inner Solar System when the planets were still forming. Due to their fragile nature, these meteorites account for less than 1% of meteorite samples examined by scientists. But on the Moon, chondrites are largely preserved, and their chemical makeup speaks volumes about the environment in which they formed.
"Systematic identification and classification of meteorites on the airless Moon thus provide additional critical constraints for reconstructing the primordial accretion history and impactor population of the inner Solar System," they state in their paper. However, this remains challenging since meteors will vaporize upon colliding at high velocities with the lunar surface. Upon examining the samples, the team confirmed that they were formed from molten droplets resulting from impact, which then underwent rapid cooling and crystallization due to exposure to the extreme cold and vacuum of space.
However, using textural characterization and an analysis of in-situ triple oxygen isotopes, the team confirmed that the samples are relics of CI-like chondrites that struck the Moon before the Nectarian Period (approximately 3.92 billion years ago). This coincides with the Late Heavy Bombardment, which took place 4.1 to 3.8 billion years ago. This period was characterized by a disproportionately high number of asteroids and comets striking the Earth-Moon system and other bodies in the inner Solar System.
These impacts are believed to have been the means through which water and organic molecules were introduced to the inner Solar System. Since CI chondrites are known to be rich in water and organic materials, as demonstrated by the samples returned from asteroid Bennu that showed traces of amino acids, these findings support the hypothesis that asteroids played a key role in delivering water and other volatiles to the inner Solar System. Additionally, the team suggests that previously-detected deposits of water ice on the Moon, which showed indications of certain positive oxygen isotopes, were likely delivered by CI chondrites in the past.
Based on these findings, the team conducted a preliminary statistical analysis of meteoritic materials, indicating that CI chondrites likely played a significantly greater role in shaping the Earth-Moon system than previously thought. Their study offers new insight into the evolution of our Solar System and the events that helped give rise to life. Furthermore, the integrated methodology they devised could be a valuable tool for assessing other returned samples of extraterrestrial materials, pointing the way towards future research opportunities.
