The lunar code hidden in the first batch of samples of Chang'e 5

In December 2020, my country's first lunar sample return mission, Chang'e 5, successfully returned 1,731 grams of samples in the northern part of the Ocean of Storms on the near side of the moon. This is the first time that lunar samples have been returned 44 years after the last lunar sampling mission of the Soviet Union's "Luna 24" in 1976. It is also the first time that humans have sampled the youngest volcanic rock area on the lunar surface. So what major scientific issues have the first batch of lunar samples from Chang'e 5 revealed to us?

Youngest volcanic activity on the moon ever recorded

If the magma produced by volcanic activity is likened to the "blood" of a planet, then the last time the "blood" flowed represents the end of the planet's geological life. The Earth is large and has enough energy, and it still has volcanic activity today. Mars (about half the diameter of the Earth) stopped volcanic activity 200 million years ago and became a dead planet. When the Moon (about half the diameter of Mars) died has always been a question of concern to scientists.

Accurate radioisotope dating results show that the age of samples returned by Apollo and Luna is more than 3 billion years, and the age of lunar meteorites is more than 2.8 billion years. Chinese scientists analyzed and counted 47 basalt fragments of different structures from the first batch of lunar samples from Chang'e 5. Using the independently developed ultra-high spatial resolution uranium (U)-lead (Pb) dating technology, 51 zircon-containing minerals (badgezircon, perovskite, and serene sea stone) larger than 3 microns were analyzed by ion probe, and finally an accurate age was obtained: 2.030±0.04 billion years. This discovery updates the age of the youngest basalt sample on the moon to 2 billion years ago, which means that the time of the flow of the moon's "blood" (i.e., the geological life of the moon) has been extended by 800-900 million years.

Comparison of the diameters of the Earth, Mars and the Moon (in equal proportions) and the duration of volcanic activity shows that the smaller the planet, the shorter its geological lifespan. Image source: Purple Mountain Observatory

Key anchor points for statistical dating of terrestrial planet impact craters

The moon is a 4.5 billion-year history of planetary violence in the solar system, which fully records and preserves asteroid impact events throughout the entire historical period. Generally speaking, the age relationship of geological units on the moon simply follows the law of superposition: the rocks on the upper layer are younger, and the rocks on the lower layer are older. In addition, asteroid impacts on the lunar surface occur randomly, so in theory, the probability of any piece of lunar surface being hit is equal. Older areas will accumulate more impact craters, thus forming a method of estimating the surface age of a planet using the distribution density of impact craters - impact crater statistical dating.

Thanks to the United States' six Apollo missions (returned samples of 381.69 kg) and the Soviet Union's three lunar missions (321 g), scientists used the absolute age of the returned samples to calibrate the corresponding geological units and established a statistical dating curve for impact craters. However, due to the lack of young lunar samples, the dating curve has no calibration points between 1 billion and 3 billion years. The Chang'e-5 samples just fill this gap, providing a key anchor point for this dating curve at 2 billion years, greatly improving the accuracy of the statistical dating method for impact craters. This can not only help scientists better determine the geological age of other areas on the lunar surface, but can also be used to determine the geological age of the surfaces of terrestrial planets such as Mercury, Venus, Mars and asteroids.

Chang'e 5 samples provide key anchor points in the statistical dating curve of lunar impact craters. Image source: modified from reference [2]

The moon mantle is really "dry"

At present, a large number of observational facts believe that the moon originated from a large collision and was formed by the condensation of high-temperature magma and gas ejected by the collision. During this process, a large amount of volatile components such as water were lost, so in theory the moon should be a very "dry" planet. However, the water content of the lunar mantle estimated by different scientific teams varies by two orders of magnitude, from no water ("Bone Dry") to rich water (up to 200 micrograms/gram), resulting in a "dry" or "wet" debate on the moon that has lasted for decades.

As the youngest basalts so far, the samples returned by Chang'e-5 have a clear geological background and have undergone the least late-stage transformation (such as asteroid and comet impacts, solar wind particle injection, etc.), making them an excellent candidate for answering this question. Scientists used high-spatial-resolution nano-ion probes to analyze the water content and hydrogen isotope composition of mineral and melt inclusions, and estimated that the water content of the lunar mantle source region of the Chang'e-5 samples is only 1-5 micrograms/gram, indicating that the lunar mantle is very "dry."

The distribution of water content in the lunar interior over time. The Chang'e 5 sample results show that the water content of the lunar mantle is significantly lower than previously estimated. Image source: modified from reference [1]

No KREEP component found in Chang'e-5 magma source

When the moon was first formed, it was covered by a magma ocean hundreds of kilometers deep. As the temperature dropped, the magma began to solidify and form rocks. When the crystallization degree of the magma ocean reached about 98%, incompatible elements (elements that do not like to enter solids but prefer to enter melts) were highly concentrated in the residual melt, and eventually formed a thin layer of KREEP (abbreviated as KREEP, named for its enrichment in potassium K, rare earth elements REE and phosphorus P) between the lunar crust and the lunar mantle. KREEP is rich in radioactive heat-generating 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 called Kreep Terrane on the front of the Moon (where the "Osmanthus Tree" on the front of the Moon is located), which is also the place with the highest concentration of thorium on the entire lunar surface. Therefore, for a long time, the heat generated by the decay of radioactive elements in the Kreep component has been considered to be the main source of energy to maintain the long-term volcanic activity on the Moon. However, the high-precision isotope results of strontium (Sr), neodymium (Nd), and lead (Pb) show that the lunar mantle source area of ​​Chang'e 5 is far from the characteristics of Kreep rock.

Distribution of basalt and thorium (Th) content on the near side of the moon. A represents the sampling point of the US Apollo mission, and L represents the sampling point of the Soviet Luna mission. Source: modified from reference [6]

Heat source of youngest moon volcano remains 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 also raised questions about the thermal evolution history of the moon. The diameter of the moon is less than 1/3 of the earth. For a planet with such a large surface area/volume ratio, it should have cooled quickly, ended its geological life early, and stopped magma activity. Why did the volcanic activity on the moon continue until 2 billion years ago?

Previously, there were two possible speculations in the scientific community: the lunar mantle rocks are rich in radioactive heat-generating elements to provide heat sources, or the lunar mantle is rich in water to lower the melting point of rocks, so that magma can be produced without a lot of heat. However, the Chang'e 5 source area has neither the addition of KREEP components 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 multiple periods of volcanic activity in the Chang'e 5 landing area

Mare basalts are mainly distributed in the basins of the moon, and are mostly found on the front side of the moon. They may be formed by partial melting of the lunar mantle at a depth of 100 to 400 kilometers. Compared with similar rocks on Earth, it has a higher and more variable TiO2 content, ranging from 0.2 to 16.5 wt.%, a difference of about 80 times. Such a large range of compositional variation not only reflects the heterogeneity of the distribution of cumulate rocks deep in the lunar mantle, but also reflects the high complexity of the lunar magmatic process. Therefore, through different types of mare basalts, we can study the evolution of the deep lunar material composition and magmatic processes over time and space.

A relatively rare high-titanium lunar mare basalt (code number CE5C0000YJYX065) ￨ Image source: Purple Mountain Observatory [7]

On July 12, 2021, the Purple Mountain Observatory of the Chinese Academy of Sciences applied for and obtained the first two Chang'e 5 lunar basalt samples. A detailed mineral chemistry and three-dimensional tomography study was carried out on one of the samples (numbered CE5C0000YJYX065) using high-resolution micro-CT, scanning electron microscopy, electron probe and other technologies. Multiple evidences show that, unlike the medium-titanium and low-titanium lunar mare basalt types reported for Chang'e 5, CE5C0000YJYX065 is a relatively rare high-titanium lunar mare basalt, which indicates that there may have been many volcanic eruptions in the Chang'e 5 landing area in history, which will hopefully interpret the fine spatiotemporal distribution of different material compositions in the lunar mantle source area and the late lunar volcanic activity.

CT video of lunar soil samples ￨ Source: Purple Mountain Observatory

References:

[1] Hu et al., Nature, 2021. https://doi.org/10.1038/s41586-021-04107-9.

[2] Li et al., Nature, 2021. https://doi.org/10.1038/s41586-021-04100-2.

[3] Tian et al., Nature, 2021. https://doi.org/10.1038/s41586-021-04119-5.

[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.

About the Author

Jiang Yun

Associate researcher at the Astrochemistry and Planetary Science Laboratory of Purple Mountain Observatory, Chinese Academy of Sciences. Research interests: petrology, mineralogy, isotope geochemistry and chronology of lunar, Martian and asteroid meteorites.

Rotating Editors-in-Chief: Ji Jianghui, Chen Xuepeng

Editor: Wang Kechao