In December 2020, Chang'e 5, China's first lunar sample return mission, successfully returned 1731 grams of samples in the northern part of the storm ocean on the front of the moon. This is the second return of lunar samples after the last Lunar sampling mission of the Soviet Union in 1976, 44 years, and the first time humans have sampled in the youngest volcanic rock area on the lunar surface. So what major scientific questions did the first lunar samples from Chang'e-5 reveal to us?
The Moon has the youngest recorded volcanic activity to date
If the magma produced by volcanic activity is likened to the "blood" of a planet, then the time when the "blood" last flowed represents the time when the geological life of the planet ended. The Earth is large and energetic, and there is still volcanic activity today. Mars (about half the diameter of Earth) stopped volcanic activity 200 million years ago and became a dead planet. When the Moon ( about half the diameter of Mars ) died has long been a concern for scientists.
Accurate radioisotope dating results show that samples returned by Apollo and Luna in the Ussuary were older than 3 billion years and lunar meteorites were older than 2.8 billion years. Chinese scientists analyzed and counted 47 basalt fragments of different structures of the first lunar samples of Chang'e 5, and used the self-developed ultra-high spatial resolution uranium (U)-lead (Pb) dating technology to analyze 51 zircon-containing minerals (oblique zircon, perovskite, jinghai stone, etc.) above 3 microns, and finally obtained an accurate age: 2.030 ±0.04 billion years. The discovery updates the age of the moon's youngest basalt sample to 2 billion years ago, meaning that the time it takes for the moon's "blood" to flow (i.e., the moon's geological lifespan) extends by 8-900 million years.
Comparing the diameter size of the Earth, Mars and moon (equal proportions) with the duration of volcanic activity, it can be seen that the smaller the planet, the shorter the geological life Source: Purple Mountain Observatory
Terrestrial planetary impact craters statistically yearn key anchor points
The Moon is a 4.5 billion-year-old history of planetary violence in the solar system, which records and preserves the asteroid impact events throughout its history. In general, the age relationship of geological units on the Moon simply follows the law of superposition: the upper rocks are younger, and the lower rocks are older. In addition, asteroid impacts on the lunar surface occur randomly, so theoretically the probability of any piece of lunar surface being hit is equal. More impact craters accumulate in older regions, resulting in a method of estimating the surface age of planets using the distribution density of impact craters – the impact crater statistical dating method.
Thanks to the Six Apollo Programs of the United States (381.69 kg of Returned Samples) and the Three Lunar Programs of the Soviet Union (321 g), scientists calibrated the corresponding geological units with the absolute age of the returned samples and established a statistical dating curve for impact craters. However, due to the lack of young lunar samples, the dating curve has no punctuation point between 1 billion and 3 billion. The Chang'e-5 sample fills this gap, providing a key anchor for this dating curve at 2 billion years, greatly improving the accuracy of the impact crater statistical dating method. This could not only help scientists better determine the geological age of other parts of the lunar surface, but also the geological age of the surfaces of Earth-like planets such as Mercury, Venus, Mars and asteroids.
Chang'e-5 sample provides key anchor points on the statistical dating curve of lunar impact craters Image source: Modified from the document[2]
The moon curtain is really "dry"
A large number of observations currently believe that the Moon originated from a major collision (hey, do you know what the Relationship between the Earth and the Moon is?). ), made from the condensation of high-temperature magma and gas ejected by the collision. After this process, a large number of volatile components such as water are lost, so theoretically the moon should be a very "dry" planet. However, the water content estimated by different scientific teams from anhydrous ("Bone Dry") to water-rich (up to 200 micrograms/g) varies by two orders of magnitude, causing the "dry" and "wet" battle between the moon to continue for decades.
Chang'e-5 returned samples as the youngest basalt to date, with a clear geological background and minimal post-modifications (e.g., asteroid and comet impacts, solar wind particle injections, etc.), making it an excellent object to answer this question. Scientists used high spatial resolution nanoion probes to analyze the water content and hydrogen isotope composition of mineral and melt inclusions, and estimated that the water content in the source area of the lunar mantle of the Chang'e-5 sample was only 1-5 μg/g, indicating that the lunar mantle was very "dry".
The water content of the Lunar Interior is distributed over time. Chang'e-5 sample results show that the water content of the lunar mantle is significantly lower than the previous estimates Source: Modified from the literature[1]
The Kelp component is not seen in the magma source area of Chang'e-5
When the Moon first formed, it was covered by a sea of magma hundreds of kilometers deep. As the temperature decreases, the magma begins to solidify, forming rocks. When the degree of magma ocean crystallization reaches about 98%, incompatible elements (elements that do not like to enter solids, but prefer to enter melt) are highly concentrated in the residual melt, eventually forming a thin Klipu rock (abbreviated KREEEP transliteration, named after the enrichment of potassium K, rare earth elements REE and phosphorus P) between the lunar crust and the lunar mantle. Cripp rocks are rich in radioactive thermogenic elements such as uranium (U), thorium (Th) and potassium (K).
Remote sensing data show that the youngest volcanic activity on the lunar surface is mainly distributed in a place on the front of the moon called the Storm Ocean Crippe (that is, the location of the "osmanthus tree" on the front of the moon), which is also the most abundant thorium on the lunar surface. As a result, heat from the decay of radioactive elements in the Cripp component has long been thought to be the main source of energy for sustaining the Moon's long-term volcanic activity. However, high-precision strontium (Sr), neodymium (Nd), and lead (Pb) isotope results show that the lunar mantle source area of Chang'e 5 is far from the characteristics of Klipu Rock.
Distribution of basalt and thorium (Th) content on the lunar front. A stands for the sampling point of the American Apollo mission, and L stands for the sampling point of the Soviet lunar (Luna) mission Image source: Modified from the document[6]
The heat source of the Moon's youngest volcanic activity is a mystery
The first scientific research results of the samples returned by Chang'e 5 have greatly refreshed our previous understanding of lunar volcanic activity, and at the same time, we have also questioned the history of lunar thermal evolution. The Moon is less than 1/3 the diameter of Earth, and for a planet with such a large surface area/volume ratio, it should have cooled down quickly, ending geological life early and stopping magmatic activity. Why could the moon's volcanic activity continue so late 2 billion years ago?
Previously, there were two possible speculations in the scientific community: the mantle rock was rich in radioactive thermogenic elements to provide a heat source, or the mantle was rich in water to reduce the melting point of the rock, so that no large amount of heat could be used to produce magma. However, the Chang'e-5 source region has neither a Cripp component nor water, which means that we may need a completely new theoretical framework or thermal evolution model to uncover the secret of the moon's geological "longevity."
There may be multi-stage mountain activity in the Chang'e-5 landing area
Lunar sea basalt is mainly distributed in the lunar basin, and mostly on the front of the moon, which may have been formed by the melting of the lunar mantle at a depth of 100 to 400 km. Compared to similar rocks on Earth, it has a higher and variable TiO content, ranging from 0.2 to 16.5 wt.%, a difference of about 80 times. Such a large range of compositional variations reflects not only the heterogeneity of the distribution of the crystal piles deep in the lunar mantle, but also the high complexity of the lunar magmatic processes. Therefore, through different types of lunar sea basalts, the evolution of the deep material composition and magmatic processes of the moon with time and space can be studied.
A relatively rare high titanium lunar sea basalt (numberED CE5C0000YJYX065) Source: Purple Mountain Observatory[7]
On July 12, 2021, the Purple Mountain Observatory of the Chinese Academy of Sciences applied for two chang'e-5 lunar basalt samples. Detailed mineral chemistry and 3D tomography studies were carried out on one of the samples (CE5C0000YJYX065) using high-resolution micro-CT, scanning electron microscopy, electron probe and other technologies. Multiple evidence suggests that, unlike the currently reported types of titanium and low titanium lunar sea basalts, CE5C0000YJYX065 is a relatively rare high titanium lunar sea basalt, suggesting that the Chang'e 5 landing area may have had multiple volcanic eruptions in history, and will hopefully interpret the fine spatio-temporal distribution of different material compositions in the lunar mantle source area and the late lunar volcanic activity.
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[4] Li et al., National Science Review, 2021. https://doi.org/10.1093/nsr/nwab188.
[5] Che et al., Science, 2021. https://doi.org/10.1126/science.abl7957.
[6] Yang and Lin, The Innovation, 2021. https://doi.org/10.1016/j.xinn.2020.100070.
[7] Jiang et al., Science Bulletin, 2021. https://doi.org/10.1016/j.scib.2021.12.006.
Source Purple Mountain Observatory, Chinese Academy of Sciences | Author Jiang Yun
Edited by Xie Yongli
Process Editor Wu Yue