Lunar soil glass microspheres reveal the asteroid impact event of the Earth-Moon system

Chang’e-5 sampling of the lunar surface, the study of glass chondrites in the lunar soil in the landing area revealed that the Earth-Moon system has experienced a surge in impact frequency over the past 2 billion years. Image source: Lunar Exploration Center

The asteroid impact is one of the potential disasters facing human civilization in the future. About 200 large impact events have been recorded on Earth, and asteroid impacts thousands of meters in diameter are enough to cause mass extinctions.

Impact fluxes have declined exponentially since the Earth was formed 4.5 billion years ago. So, has the Earth-Moon system asteroid impact frequency been steadily decaying? Could the large number of near-Earth asteroids lead to a sudden increase in future impact events?

The lunar soil samples returned by Chang’e-5 are the best subjects for study looking for relevant answers. An international research team composed of Long Tao, a researcher at the Beijing Ion Probe Center of the Institute of Geology of the Chinese Academy of Geological Sciences, and Qian Yuqi, Ph.D. of China University of Geosciences (Wuhan), and other scholars inside and outside the United Nations have confirmed the change of the impact frequency of the Moon over time over 2 billion years by associating the glass chondries in the lunar soil with nearby impact craters. The results were recently published in Science Advances.

It is worth noting that the age of the impact glass chondrites in the Chang’e-5 lunar soil is the same as that of multiple sets of impact events in the asteroid belt, and one of the ages coincides with the dinosaur extinction event, representing the sudden increase in the frequency of impact at that time, containing the mystery of dinosaur extinction. “This means that the Earth’s history has experienced periods with above-average impact frequencies, and similar situations may occur in the future.” Long Tao, the first and corresponding author of the paper, told China Science News.

On December 17, 2020, Chang’e 5 successfully returned 1,731 grams of lunar soil samples. Among them, the uranium-lead dating results of basalt samples show that the moon still had magmatic activity 2 billion years ago, which “extended” the geological life of the moon by nearly 1 billion years and provided a key anchor for the impact crater dating method in planetary geology.

The key to obtaining changes in the frequency of asteroid impacts is to accurately analyze the age of the glass chondrites in the lunar soil and find the corresponding impact craters. “The lunar soil contains a large number of glass pellets, and the cause of the impact is an important research object to understand the impact history of the inner solar system, which can reflect the composition of the lunar crust and the impact dynamics of the inner solar system.” Long Tao said that the previous lack of understanding of the physical mechanism of the source and distribution of the impact glass chondries and their geological association with the landing area limited the use of glass chondrites for accurate reconstruction of the internal solar system impact history.

According to reports, the age measurement of glass pellets is extremely difficult, and its diameter is only “the thickness of the hair”, which must be bombarded by an ion beam and the composition of uranium and lead with extremely low content can be detected. It is also difficult to determine which crater the glass chondries in lunar soil come from, such as finding a needle in a haystack.

In order to fully understand the source and distribution of impact glass chondrites and accurately reconstruct the history of lunar impacts, the research team selected 215 glass pellets with a diameter of 50 to 200 microns (1 micron is 1 thousandth of a millimeter) in Chang’e-5 lunar soil CE5C0400YJFM00402. Through geochemical composition analysis, it was found that about 80% of the glass chondrites (176) were close to the Chang’e-5 lunar soil and basalt composition, which were “native” glass chondrites; The composition of the 39 glass chondrites differs from the composition of Chang’e-5 lunar soil and basalt, which is classified as “foreign” glass pellets. Among them, the age of formation of “local” impact glass chondrites ranged from several million years ago to 2 billion years ago.

In order to correlate the glass carticles of the Chang’e-5 impact with the impact crater, the research team established a numerical model of the impact sputter, and the results showed that the melting of the sputters to form carticles was the formation mechanism of the impact glass carticles. The average ejection velocity of the formation of glass chondrial melts is 300-500 m/s, and the transmission distance reaches tens of kilometers. The study also found that impact craters with a diameter of more than 1 km were able to provide a large number of glass pellets. They combined numerical models with the geology of the Chang’e-5 sampling area and eventually screened out possible source craters among more than 100,000 impact craters around the landing zone.

The study also showed that the age of impact glass chondrites is the same as that of multiple sets of impact events within the asteroid belt. For example, the age of cosmic ray exposure (CRE) of the Vesta meteorite (HED), discovered in Antarctica in 2015, is similar to the age of the Chang’e-5 impact glass chondrites, meaning that the HED meteorite parent disintegrates at the same time, producing a large number of larger impactors and documenting impact events on Earth and the Moon. At the same time, 460 to 480 million years ago, L-type asteroid fragments arrived on Earth, and this major cracking event caused the Earth’s impact flux in the Ordovician to surge. The Chang’e-5 glass chondrites are 377 ± 1 million and 577 ± the 12 million years have the largest number of glass crusts, presumably by L-type chondrite meteorites or other asteroids providing medium-sized impactors.

The researchers said that the Chang’e-5 impact glass chondrial study showed that its age distribution may represent short-term changes in impact frequency and asteroid driving mechanical process relationships, so it is necessary to conduct more lunar soil analysis and cross-comparison with other regional impact craters to distinguish significant all-month events from the impact flux background. (Source: China Science Daily, Feng Lifei)

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