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Asteroid samples give insights into early solar system and life on Earth

A new report adds weight to the hypothesis life on Earth was seeded by asteroids that formed in the outer regions of the disk of gas and dust that developed into our solar system.

(CN) — A Japan Aerospace Exploration Agency spacecraft brought back 5.4 grams of soil in December 2020 from the most primitive asteroid ever sampled by mankind, and a new study reveals the samples contain amino acids, a building block for life on Earth.

Ryugu asteroid is named after sea emperor Ryūjin’s ocean-floor palace in a Japanese fairy tale.

A fisherman named Urashima Tarō is given gills by a huge turtle, who takes him to the Palace of the Dragon God (Ryūgū-jō in Japanese) after he rescues a small turtle who, unbeknownst to him, is Ryūjin’s daughter, Otohime.

Tarō stays with Otohime, now a beautiful princess, for three days, then asks if he can return to his village to see his elderly mother. She agrees and gives him a box she says will protect him as long as he never opens it.

At his village, Tarō finds his home, mother and everyone he knew are gone and 300 years have passed, 100 years for each day he was at the palace. Numb with grief, he opens the box and instantly turns into an old, bowed-back man with a long, white beard.

Like Tarō in the fairy tale, Hayabusa2—a spacecraft launched in 2014 by the Japan Aerospace Exploration Agency—returned to terra firma years later with a box.

The scientists who opened it did not become elderly and frail, but its contents—5.4 grams of soil samples Hayabusa2 collected from Ryugu asteroid—have given them a glimpse of the infancy of our 4.5-billion-year-old solar system, and added weight to the hypothesis that such asteroids provided building blocks for life on Earth, according to a new report by researchers at Okayama University, published Thursday in the journal Proceedings of the Japan Academy.

“Asteroids and comets represent the material that was left over after the formation of the planets that orbit the Sun,” the report states.

“Such bodies would have initially formed in a vast disk of gas and dust (protosolar nebular) around what would eventually become the Sun (protosun) and thus can preserve clues about the processes that operated during this period of the solar system," it continues. (Parantheses in original.)

While an earlier mission of the Japanese space agency retrieved samples from an S-type asteroid, which come from material within the early inner solar system, Hayabusa2 targeted Ryugu because it is a C-type asteroid, a kind that was much less affected by heating from the protosun, so they preserve much more material from the primitive outer solar system, according to the report.

Astronomers using deep-space telescopes discovered Ryugu in 1999.

Telescope observations and remote sensing by Hayabusa2 suggested Ryugu contained organic matter and small amounts of water.

“However, C-type asteroids are incredibly hard to study using such methods, because they are very dark and the resulting data has very little information that can be used to identify specific materials,” the report states.

So the samples Hayabusa2 returned to Earth with have given scientists a goldmine of information.

Researchers cut into the samples with a machine equipped with a diamond knife and found the majority of minerals were phyllosilicates, the most common type of soil minerals (clays are phyllosilicates).

They believe the minerals formed through a process called aqueous alteration in which radioactive elements heated ice as they broke down, because particles in the samples had textures indicating they had experienced both liquid and frozen water.

“Ryugu also contains chromium, calcium and oxygen isotopes that indicate it preserved the most primitive source of materials from the protosolar nebular,” according to the report, led by Professor Eizo Nakamura of Okayama University’s Institute for Planetary Material.

Given evidence Ryugu had lots of water in it, along with the lack of any inner solar system material or isotopic signatures, the report’s authors think the matter within Ryugu stuck together and aqueously altered very early in the outer solar system.

They also posit that Ryugu—which has a diameter of 0.62 miles and resembles a spinning top—must have broken off from a much larger object called a planetesimal.

After Ryugu entered the inner solar system, its ice turned to gas—water vapor—as evidenced by the differences in soil Hayabusa2 collected from two sites on the asteroid.

When the sun vaporized Ryugu's ice, it became a low density and highly porous, rocky asteroid.

Then space weathering started, in which solar wind and cosmic rays from the sun and distant stars bombarded the asteroid and altered the chemical structure of organic matter on its surface, scientists say in their report.

Despite the effects of space weathering, the authors say, primitive organic materials were also found in Ryugu’s samples.

“Amino acids, such as those found within the proteins of every living organism on Earth, were detected in a Ryugu particle,” the report states. “The discovery of protein forming amino acids is important, because Ryugu has not been exposed to the Earth’s biosphere, like meteorites, and as such their detection proves that at least some of the building blocks of life on Earth could have been formed in space environments.”

Amino acids are key to hypotheses about the origins of life on Earth, and Okayama University scientists believe asteroids like Ryugu are strong candidates for having delivered these compounds to the early Earth.

The isotopic characteristics of the Ryugu samples also suggest such C-type asteroids could have supplied the Earth with its water, the scientists add.

They are optimistic more study of the samples will give further insights about our solar system, providing a time capsule to its infancy without the fairy tale curse from which Ryugu gets its name.

“The findings reported by the study provide invaluable insights into the processes that have affected the most primitive asteroid sampled by humankind,” the report notes.

“Such insights have already begun to change our understanding of the events that occurred from before the solar system and up until the current day. Future work on the Ryugu samples will no doubt continue to advance our knowledge of the solar system and beyond," it adds in conclusion.

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