New research led by Okayama University has analyzed fragments from the asteroid Ryugu to investigate the abundance of amino acids within them. The study aims to investigate whether amino acids necessary for the formation of life on Earth came from extraterrestrial sources such as meteorites.
The solar system was formed from a molecular cloud composed of gas and dust. When the cloud collapsed, the early sun was formed, and a large disk of gas and dust was formed orbiting it. The dusty material collided to produce rocky material that eventually gave rise to planetesimals. Some of the planetesimals contained large amounts of ice, including volatile compounds such as water and organic compounds, likely including some amino acids.
Over time, the ice melted due to the presence of radioactive material that heated up the bodies. This period of liquid water enabled many reactions to occur, including Strecker synthesis and Formose-like reactions, which resulted in the production of new organic material, including amino acids. The same process also changed the rocky materials from their original minerals to new secondary minerals.
After several million years, the planetesimals began to freeze, and later catastrophic collisions and interactions with the solar system planets broke up the large bodies, sending their asteroidal and cometary fragments close to Earth. Impact events have since delivered fragments of these asteroids and comets to the Earth’s surface, supplying the Earth with organic material, including amino acids, over the course of its history.
Amino acids are necessary for living organisms, as they are the building blocks of proteins, which are essential for many processes within organisms. Interestingly, most amino acids come in at least two forms that are mirror images of each other, similar to human hands, and life on Earth uses one particular type of amino acid in its proteins, the left-handed optical isomer.
Currently, only a certain class of meteorites (carbonaceous chondrites) is known to contain excesses of left-handed optical isomers, which has led to the idea that the amino acids used by life may have originated from these meteorites. Despite this, the amino acids in meteorites could have formed before their incorporation into the meteorites or after the meteorites had already formed.
The scientists analyzed several fragments of the asteroid Ryugu and calculated the abundance of amino acids within them. The abundance of mineral phases within the particles had been previously reported in another publication, which allowed for a comparison between the abundance of amino acids and minerals.
It was found that one particle (A0022) contained a high abundance of an uncommon amino acid in extraterrestrial materials, called dimethylglycine (DMG), whereas the other particle (C0008) did not contain this amino acid above the detection limit. Meanwhile, the abundance of glycine was found to be lower in A0022 compared to C0008, while the abundance of β-alanine showed the opposite trend. Accordingly, the ratio of β-alanine to glycine was higher for A0022 than for C0008.
The ratio was shown previously to be indicative of the extent of aqueous alteration operating on planetesimals. Therefore, it was hypothesized that some reaction related to higher levels of aqueous alteration in A0022 may explain the high abundance of DMG in this particle, compared to C0008.
The mineral phases were examined to see if any additional evidence for what reaction may be causing the different amino acids abundances between the Ryugu particles. It was found that the abundance of secondary minerals (formed after aqueous alteration), including carbonate, magnetite, and Fe-sulfides, was higher in A0022 than in C0008.
The higher abundance of carbonate in A0022 compared to C0008 suggested that there was a greater quantity of CO or CO2 in the region of the planetesimal where A0022 had been altered. This, coupled with the stronger evidence of aqueous alteration from the β-Alanine to glycine ratio, indicated that there may have been more ice in the precursor of A0022 than in C0008.
One method for producing DMG, an essential nutrient for humans, is the Eschweiler-Clarke reaction, which involves the reaction of glycine with formic acid and formaldehyde in water and generates CO2. Comets contain glycine, formaldehyde, and formic acid, and it is anticipated that these chemicals would be present in the planetesimal precursors of asteroids.
Thus, if the Eschweiler-Clarke reaction occurred during aqueous alteration in the precursor of A0022, it could explain why this particle had a higher level of DMG and lower abundance of glycine compared to C0008. Moreover, the CO2 produced may have contributed to the formation of carbonates in A0022.
Overall, the results of this study suggest that even minor variations in the conditions during aqueous alteration on planetesimals can have significant impacts on the ultimate abundance of amino acids. The destruction or creation of some amino acids can have consequences for the availability of amino acids at the genesis of life on Earth.
Astronomers have found the building blocks of life, such as amino acids, in various places in the universe. Some of these places include meteorites, comets, and interstellar dust particles.
These building blocks have also been detected in the atmospheres and on the surfaces of planets and moons within our own solar system, such as Mars, Saturn’s moon Enceladus, and Jupiter’s moon Europa.
Additionally, complex organic molecules have been detected in the gas and dust clouds between stars, known as interstellar clouds, and in the protoplanetary disks around young stars.
The research is published in the journal Nature Communications.
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