A "time capsule" from outer space is right here on Earth

About 4.5 billion years ago, the solar system was born from a chaotic nebula. Although this process seems far away, fortunately, we can reveal many secrets of the formation of the solar system by studying the "time capsules" that have traveled through time and space to Earth - chondrites. These ancient rock fragments carry a wealth of information, especially the small, round particles contained in them - chondrules.

Chondrules are an important component of chondrites. They are usually about one millimeter in diameter and are mainly composed of two silicate minerals, olivine and pyroxene. The formation of chondrules went through a process of high-temperature melting to cooling and solidification. In the early space before the planets gathered, dust particles or asteroids may have melted into droplets instantly due to high-energy events such as high-speed collisions, and then quickly cooled and crystallized into the chondrule structure seen today.

Cross section of a typical chondrite: This meteorite has been cut open to reveal the chondrules inside, which are mainly composed of silicate minerals such as olivine. In addition to the chondrules, chondrites often contain unoxidized (rusted) iron particles because they have not been exposed to the Earth's oxygen-rich atmosphere for a long time. This makes them easily distinguishable from terrestrial rocks, which do not contain iron in metallic form (cited from information 7).

When a typical chondrite is cut open, it reveals not only the beautiful chondrules, but also the unoxidized iron particles, which make them easily distinguishable from terrestrial rocks on Earth. As the most common type of meteorite, chondrites account for more than 85% of the total meteorite falls, mainly originating from the asteroid belt. Among them, ordinary chondrites are especially believed to come from asteroids. These ancient meteorites were formed about 4.5 billion years ago, and their composition is very similar to that of the planets in the solar system. Chondrites have become an important carrier for recording the formation of the solar nebula and the early stages of planetary evolution due to their relatively unmodified ancient chemical composition. However, scientists are still working hard to decipher the deep-level information contained in it. However, the information chondrites bring us goes far beyond this. When conducting geochemical analysis of chondrites, scientists found unique clues of the noble gas helium. It is worth noting that chondrites also contain primitive noble gas helium, which is of great significance for studying the origin of the planets in the solar system.

Helium isotope ratios in different samples (Sano, Y. (2018). Helium Isotopes. In: White, WM (eds) Encyclopedia of Geochemistry. Encyclopedia of Earth Sciences Series. Springer, Cham. https://doi.org/10.1007/978-3-319-39312-4_205)

The ratio of helium isotopes (3He/4He) can help scientists trace the origin and distribution of early solar system materials, reveal the process of stellar evolution, and explore the physical environment conditions of the early solar system. 3He and 4He are two stable helium isotopes. Most of the natural helium on Earth is 4He, with only a very small amount of 3He. The 3He/4He ratio in the Earth's atmosphere is about 1.4 x 10*-6. In solar wind and lunar samples, the 3He/4He ratio is significantly higher, reaching 4 x 10*-4, which means that they contain more primitive solar system helium. The 3He/4He ratio of helium released from hydrothermal vents is also high, mainly because the primitive helium in the core is released. The measurement of the 3He/4He ratio of helium from different sources can study the evolution of the Earth and the solar system, trace the source of mantle helium, and estimate geothermal resources. In geological research, the 3He/4He ratio is an important quantitative parameter. The 4He on Earth mainly comes from the alpha decay of uranium and thorium. 3He is mainly inherited from the original helium of the solar nebula when the Earth was formed, and it may also come from artificial nuclear reactors. The 3He/4He ratio of helium in the Moon and undifferentiated gaseous planets such as Jupiter and Saturn is close to the original ratio of the Sun, about 2x10*-4. These heliums retain the signature of the solar nebula and can trace the origin of the solar system. Understanding the sources of helium in different geological structures and their 3He/4He ratios is of great significance to the study of spherical chemistry and planetary science.

Helium isotope geochemical characteristics of chondrites

The ratio of 3He to 4He has unique characteristics in chondrites, especially the 3He/4He ratio detected in the samples is as high as 10*-4, which is similar to carbonaceous chondrites and some celestial bodies believed to come from the outer regions of the solar system. This difference in ratio reflects different genetic backgrounds and source region characteristics, thus helping scientists track the distribution and evolution of early solar system materials. Further research has found that the differences in helium isotope composition in chondrites may also be related to large-scale cosmic events, such as impact events during planet formation.

A 700-gram NWA 869 meteorite. Chondrules and metal flakes are visible on the cut and polished surface of the sample. NWA 869 is a common chondrite (L4-6) (Wikipedia)

For example, a high ratio of 3He may come from supernova remnants produced by the explosion of ancient stars outside the solar system. When celestial fragments rich in such helium isotopes enter the Earth or other planetary systems through impact, they bring information about these ancient interstellar media into the inner solar system. Through the study of helium isotopes in chondrites, scientists can not only gain a deeper understanding of the initial conditions and material sources of the formation of the solar system, but also help to clarify major astronomical events that affect the components of the solar system and their evolutionary paths. These traces of helium that have traveled through time and space are like ancient codes, allowing us to peek into the past of the universe hidden deep in the long river of history. These precious "fossils" have opened a new window for us to explore the mystery of the origin of life in the universe.

References:

Busemann, H., Baur, H., & Wieler, R. (2000). Primordial noble gases in “phase Q” in carbonaceous and ordinary chondrites studied by closed-system stepped etching. Meteoritics & Planetary Science, 35(5), 949-973.

Huss, GR, Lewis, RS, & Hemkin, S. (1996). The “normal planetary” noble gas component in primitive chondrites: Compositions, carrier, and metamorphic history. Geochimica et Cosmochimica Acta, 60(17), 3311-3340.

Schultz, L., Weber, HW, & Franke, L. (2005). Rumuruti chondrites: Noble gases, exposure ages, pairing, and parent body history. Meteoritics & Planetary Science, 40(4), 557-571.

https://www.universetoday.com/tag/chondrites/

https://www.britannica.com/science/chondrite

https://www.britannica.com/science/chondrule

http://www.faithfulscience.com/astronomy-and-cosmology/planetary-systems.html

appendix:

Appendix: Mineralogical classification of meteorites

Meteorites, as precious recorders of early materials in the solar system, carry key clues to major scientific issues such as the origin of the universe, stellar evolution, and geochemical processes. They originate from various celestial bodies in interplanetary space. After a long journey of billions of years, they occasionally land on Earth and become an important window for scientists to explore the mysteries of deep space. The classification of meteorites is mainly based on their mineral composition, structural characteristics, isotopic composition and other factors. They can be roughly divided into three categories: chondrites, achondrites and iron meteorites. Among them, chondrites are the most common type, containing a large number of round particles as small as millimeters - chondrules. The formation process of these chondrules provides us with important information about the high temperature environment in the early solar system. Achondrites contain no or almost no chondrules, representing the crust or mantle materials of the parent celestial body that have undergone melting and geological transformation, such as Martian meteorites, lunar meteorites and Vesta meteorites. Iron meteorites are mainly composed of metallic iron and nickel, reflecting the composition of the material in the core of metal-rich asteroids. Through in-depth research on various types of meteorites, we can not only trace the birth and evolution of the solar system, but also reveal the unsolved mysteries in many fields such as the composition of the crust and core of different planets, surface processes, and even the origin of life. Therefore, the classification and research of meteorites is not only the intersection of astronomy and earth science, but also an important way to promote the deepening of human understanding of the universe.

① Carbonaceous chondrites: Carbonaceous chondrites are a special type of chondrites, which are characterized by rich organic compounds and water, as well as a high carbon content. This type of meteorite is an important fossil record of early materials in the solar system and is considered to be one of the most primitive celestial remnants that has not undergone significant high-temperature melting and chemical differentiation. In addition to the common chondrite structure, carbonaceous chondrites also contain a variety of minerals such as silicates, iron oxides, sulfides, and a large amount of carbonaceous materials. These carbonaceous materials can be amorphous carbon, graphite, organic molecules, and even amino acids, which are complex organic compounds that form the basis of life. In addition, they often contain a variety of volatile elements, such as nitrogen, hydrogen, oxygen, sulfur, and rare gases. These components are of great significance for studying the sources of water and organic matter in the solar system, especially on Earth. By analyzing the composition and structure of carbonaceous chondrites, scientists can obtain valuable information about the conditions in the early solar nebula, the evolution of matter in the solar system's asteroid belt, and how possible life precursors were transported to Earth from interstellar space.

Carbonaceous chondrite Allende, carbonaceous chondrite is dark gray to black picture

Carbonaceous chondrite, Allende meteorite, 4.560-4.568 Ga, Carbonaceous chondrite - Allende meteorite, Small broken individuals. (1.4 cm in diameter) Carbonaceous chondrites are dark grey to black chondrites with a relatively carbon-rich matrix. Meteoritologists obtained the first large carbonaceous chondrite sample when the Allende meteorite fell in 1969. Prior to Allende, carbonaceous chondrite material was very rare. Allende has become the most intensively studied and best-known carbonaceous chondrite. It impacted Earth at 1:05 AM on February 8, 1969. It is known to be distributed in a southwest to northeast direction near the town of Allende in the southeastern state of Chihuahua in northern Mexico. The black material covering the rock shown above is the original molten crust. The fused crust represents the outer portion of the original rock fragments that were partially melted when the Allende fireball passed through the Earth's atmosphere. The light grey areas show the interior appearance of the rock (where the fused crust has broken apart).

② Ordinary chondrites: Ordinary chondrites are the most common type in the meteorite family, accounting for about 85% of the total known meteorites. This type of meteorite is characterized by containing a large number of tiny silicate mineral chondrules, which were formed in the early solar system and retain information about the original nebula material. Not only does it have the lowest iron-nickel metal content, but its overall chemical composition is poorer than that of the H and L groups. By studying ordinary chondrites, scientists can understand a lot of information about the material composition, element abundance, isotope ratios, and physical and chemical conditions during the formation of planets in the early solar system. In addition, because ordinary chondrites are widely distributed and easy to obtain, they have become an important window for studying the origin of the Earth and other planets.

Ordinary chondrite NWA 10114 L5 W1, S2 2380g

③ Enstatite chondrites: Enstatite chondrites are a rare type of chondrites, whose main mineral component is enstatite, a magnesium and iron silicate mineral. This type of meteorite is characterized by being rich in enstatite and related low-calcium pyroxenes, and relatively lacking in other common chondrite minerals such as olivine. Their chemical composition reflects that they were formed in an extremely reducing environment, that is, a part of the primitive solar nebula that was rich in metallic iron and almost free of water or volatile substances. In the early history of the solar system, the parent body of enstatite chondrites may be the remains of celestial bodies in very dry areas inside the asteroid belt. They are of great significance for studying the early conditions of the solar system, especially for understanding the material composition of areas far away from the source of water and other volatile substances.

Enstatite chondrite impact melt breccia NWA 6258
④R-type chondrite Rumuruti (R): It is a relatively rare type of chondrite, which is distinguished from other chondrites by its unique mineralogy and chemical characteristics. This type of meteorite is characterized by rich calcium aluminum inclusions (CAIs) and low iron content, and contains a large amount of silicate minerals such as feldspar and plagioclase, as well as rich chondrite structures. Rumurutiites are named after the Rumuruti area in Kenya, where such meteorite samples were first discovered. Their chemical composition and petrological properties indicate that they were formed in a relatively oxidizing environment in the early solar system. In contrast, for example, enstatite chondrites represent products in a reducing environment. Because R-type chondrites have a high SiO2 content, a low metal and sulfide ratio, and a special combination of chondrules and melt inclusions, they provide valuable information for studying the evolution of early solar system materials, the differentiation history of asteroid parent bodies, and the conditions for planetary formation.

Photomicrographs of unkernized (a) and kernized (b and c) R chondrites: (a) Hammadah al Hamra 119 (R4) is unkernized on a thin section scale. Chondrules and chondrule fragments are embedded in a fine-grained matrix; (b) R chondrite weathering breccia Rumuruti (R3-6). The hand specimen consists of light and dark fragments embedded in a clastic matrix. The abundance of large clasts is about 50% (Schulze et al., 1994); (c) Dar al Gani 013 (R3-6) consists of unequilibrated lithologies (type 3), various types of equilibrium clasts, and impact melt fragments (compare with Figure 3) (cited from: https://www.researchgate.net/figure/Photomicrographs-of-unbrecciated-a-and-brecciated-b-and-cR-chondrites-a-Hammadah_fig1_313549596)

⑤Achondrite is a type of silicate meteorite that does not contain chondrites. Chondrites are small round particles contained in most other silicate meteorites. Achondrites come from asteroids that have been heated enough to melt and differentiate into cores, mantles, and crusts. The largest group of achondrites are Howardite, Eucrite, and Diogenite meteorites, which may come from Vesta, the second largest asteroid in the solar system. Achondrites contain minerals such as olivine, pyroxene, and feldspar that were formed when the parent asteroid cooled. This provides insights into the geological processes of asteroids. The chemical composition of achondrites shows a greater lack of volatile elements than chondrites. This indicates that the parent asteroid was heated to a high temperature enough to escape volatile elements. Achondrites account for about 8% of all meteorite falls. They are valuable samples for studying asteroid geology and the early planetary formation process.

Achondrite: May 2009 TKW: 2290 g Number of individuals in half: 463 g Very fresh end-cut with smooth cut surface and smooth black crust preserved in block form. Chromite crystals extend from the crust. Clearly paired with olivine pyroxene NWA 5480. Record of MB 101: History: Four meteorites were found in May 2009 by an anonymous finder in the eastern part of Agaraktem, Mali. Physical characteristics: Four nearly complete individuals, totaling 2290 g. Petrology: (R. Bartoschewitz, Bart) Clusters of euhedral to subhedral polycrystalline olivine (about 50 vol%) (about 1 mm, average about 0.1 mm) distributed in bands of xenolithic to subhedral pyroxene grains (about 1 mm) with rare intergranular feldspar. Chromite and metals are mainly present in pyroxene and olivine grains. Geochemistry: (R. Bartoschewitz, Bart; P. Appel, B. Mader, Kiel) Pyroxene Fs23.8-25.4 Wo2.0-4.4, olivine Fa29.3-30.3, feldspar An76-83 Ab1-5. Chromite Al2O3 = 14.4-15.4, TiO2 = 0.9-1.1, MgO = 4.1-4.8; Ni = 0.3-1.1, Co = 0.7-0.9 (all in wt%). Classification: Olivine diorite, S1, very fresh.

A more general classification diagram is as follows:

Chondrites: Chondrites

Carbonaceous: Carbonaceous chondrite

Ordinary: Ordinary chondrite

Enstatite: Enstatite chondrite

Rumuruti (R): R-type chondrite

Achondrites: Achondrites

Primitive Achondrites: Primitive Achondrites

Iron meteorites: Iron meteorites

Stony-iron meteorites: Stony-iron meteorites