How plesiosaurs swam underwater

What happened before, during and after the formation of the solar system? A recent study of the asteroid Ryugu contains the answers: A team of scientists conducts a comprehensive analysis of samples returned from Japan Aerospace Exploration Agency’s Hayabusa2 mission and provides invaluable insights into the formation and evolution of our solar system

Japan Aerospace Exploration Agency’s Hayabusa2 mission returned unpolluted primitive asteroid samples to Earth. A comprehensive analysis of 16 particles from the asteroid Ryugu revealed many insights into the processes that worked before, during and after the formation of the solar system, with some still forming the surface of the current asteroid. Elemental and isotopic data revealed that Ryugu contains the most primitive pre-solar nebula (an ancient disk of gas and dust that surrounds what would become the sun) materials hitherto identified and that some organic materials may have been inherited from before the solar system was formed.

Asteroids and comets represent the material that remained after the formation of the planets orbiting the sun. Such bodies would initially have formed in a large disk of gas and dust (protosol nebula) around what would eventually become the sun (protosun) and could thus preserve clues about the processes that functioned during this period of the solar system. The protozoan nebula would have spun most rapidly towards its center and this would have concentrated much of the material within this region. Some of the material then began to fall on the surface of the protosun, which increased its temperature. The higher temperature of the protozoan would have led to an increased output effect of radiation, which could have caused photo evaporation (evaporation due to energy from light) of the material in the inner solar system.

Later, as the inner solar system cooled, new material with distinct compositions condensed into what had previously existed. Eventually, such materials would stick together to produce large bodies (planetesimals) that would then break up from collisions, with some S-type asteroids forming. An S-type asteroid (Itokawa) was the target of the Hayabusa mission, the forerunner of Hayabusa2. The samples returned to Earth revealed much about such asteroids, including how their surfaces are affected by continuous small impacts and confirmatory identifications made by telescopes on Earth.

Haybusa2 was aimed at a completely different type of asteroid, C-type, which, unlike S-types, preserves much more of the material of the primitive outer solar system, which was much less affected by heating from the protosol. Initial ground-based telescope and remote sensing information from the Hayabusa2 spacecraft suggested that Ryugu may contain organic matter and small amounts of water (fixed on the surface of minerals or trapped within their structure). However, C-type asteroids are incredibly difficult to study using such methods, as they are very dark and the resulting data have very little information that can be used to identify specific materials. As such, the sample return represented a very important step in improving our understanding of C-type asteroids. Approximately 5.4 g of samples were returned to Earth in December 2020 and the samples were initially studied at the Japan Aerospace Exploration Agency (JAXA) Phase 1 treatment facility in Sagamihara, Japan. Extensive geochemical analysis began in June 2021 when the samples arrived at the Phase 2 cure facility at the Pheasant Memorial Laboratory (PML), Institute for Planetary Materials, Okayama University, Japan.

Initially, the external and physical information was obtained for the samples, but shortly after, the particles were cut open using a microtome equipped with a diamond knife. Inside, the particles revealed textures that indicate freezing-thawing and a fine-grained mass of various minerals, with some coarser-grained components spreading everywhere. The majority of the minerals were aqueous silicates called phyllosilicates (clays), which were formed by chemical reactions involving non-free silicate minerals and liquid water (aqueous change). Together with the freeze-thaw textures, the evidence indicated that the samples had experienced both liquid and frozen water before.

The aqueous change was found to have reached its peak before ~ ​​2.6 Myr after the formation of the solar system, by analysis of manganese and chromium in magnetite (iron oxide) and dolomite (calcium-magnesium carbonate) minerals. This means that the materials from Ryugu experienced liquid water very early in the history of the solar system and the heat that melted the ice would have been added from radioactive elements that only survive for a relatively short time (almost everything would be gone after 5 Myr). After many of the radioactive elements had decayed, the body would cool down and freeze again. Ryugu also contains isotopes of chromium, calcium and oxygen which indicate that it retained the most primitive source of protosol nebula material. In addition, organic material from Ryugu registers primitive isotopic signatures indicating their formation within the interstellar medium (the area in space between solar systems) or the outer protosol nebula. Together with the abundant water and the lack of materials or signatures in the inner solar system, the above findings suggest that the material in Ryugu was held together (accredited) and changed into water very early in the outer solar system.

But to form liquid water, from the heating of a rocky-icy body through radioactive decay, it is required that the body is at least several tens of km large. Consequently, Ryugu must have originally been part of a much larger body, called a planetesimal. Icy planetesimals are believed to be the source of comets, which can be formed by their collapse. If the planetesimal precursor to Ryugu were affected after freezing again, a comet that preserves many of the planetesimal’s original textures and physical and chemical properties could be produced. Like a comet, the fragment would have had to move from the outer to the inner solar system via some dynamic path, which involved the interactions of the planets. Once in the inner solar system, Ryugu would then have undergone significant sublimation (transition from solid ice to gas). Modeling in a previous study indicated that sublimation can increase the speed at which Ryugu spins and lead to its distinctive spinning top shape. The sublimation could also have led to the formation of water vapor jets (as seen on comet 67P) which would have re-deposited material below the surface of the surface and frozen it in place.

In addition, the rays may explain some interesting differences between the sampling sites where the Ryugu samples were taken. The Hayabusa2 mission sampled material from the actual surface at landing site 1 (TD1) and probably material below the surface from an artificial impact crater at landing site 2 (TD2). Some of the TD1 samples show elementary fractionation beyond the mm scale and scattered B and Be abundance. However, all TD2 samples detect elemental abundance similar to CI chondrites (a type of meteorite with elemental abundance similar to the sun) and show no evidence of elemental fractionation across the mm scale. One explanation is that the TD1 site registers the material enclosed in a beam, which is carried to the surface of the comet-like fragment from many distinct regions in the lower surface and thus represents a variety of compositions. Meanwhile, the TD2 samples can represent materials that come from a part of Ryugu and as such have a more uniform composition.

After complete sublimation of the ice at Ryugu’s surface, a rocky asteroid with low density and very porous was formed. While water-related processes ceased, space weathering began. Ryugu’s surface was eventually bombarded by large amounts of energy-rich particles from the solar wind and cosmic rays from the sun and distant stars. The particles modified the materials on Ryugu’s surface, which caused the organic material to change in its structure. The effects of such a process were more obvious in TD1 particles from Ryugu’s surface compared to those from TD2, which had probably been brought to the surface during the creation of an artificial impact crater. As such, space weathering is a process that still shapes the surfaces of asteroids today and will continue to do so in the future.

Despite the effects of space weathering, which acts to alter and destroy the information in organic matter, primitive organic materials were also discovered through the extensive geochemical analysis of the Ryugu samples. Amino acids, such as those found in the proteins of every living organism on earth, were discovered in a Ryugu particle. The discovery of protein-forming amino acids is important because Ryugu has not been exposed to the Earth’s biosphere, as meteorites, and as such, their discovery proves that at least some of the building blocks of life on Earth could have formed in space environments. Hypotheses about the origin of life, such as those involving hydrothermal activity, require sources of amino acids, where meteorites and asteroids such as Ryugu represent strong candidates because of their inventory of amino acids and because such material would have been easily delivered to the surface of early Earth. In addition, the isotopic properties of the Ryugu samples suggest that Ryugu-like materials could have supplied the earth with its water, another resource necessary for the emergence and maintenance of life on earth.

Together, the results reported by the study provide invaluable insights into the processes that have affected the most primitive asteroid taken by humans. Such insights have already begun to change our understanding of the events that occurred from before the solar system to the present day. Future work on the Ryugu samples will undoubtedly continue to advance our knowledge of the solar system and beyond.

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