These asteroid particles may be our most ‘pristine’ sample of the outer solar system: ScienceAlert

Fossils ejected from an asteroid in near-Earth solar orbit may be the most ‘pristine’ sample of space rocks we’ve had our primate paws on.

According to a new, in-depth analysis of material delivered to Earth by asteroid Ryugu, the rock and dust samples are among the most uncontaminated Solar System material we’ve ever had the chance to study—and their composition suggests they include chemistry from the external extent of the system.

This not only gives us a unique tool for understanding the Solar System and its formation, but gives us a new context in which to interpret other space rocks that have been contaminated by contact with Earth.

“Ryugu particles,” wrote a team led by cosmochemist Motoo Ito of the Japan Agency for Marine-Earth Science Technology (JAMSTEC) in Japan, “are the most uncontaminated and unfractionated extraterrestrial materials studied so far and provide the closest match well available to the greater composition of the Solar System.”

About 4.6 billion years have passed since the formation of the Sun and the Solar System around it. Obviously that’s a long time ago and a lot has changed since then; but we have time capsules that allow us to study the chemistry of the early Solar System in order to understand how it all came together. These are pieces of rock, such as comets and asteroids, that have moved through space more or less unchanged since they were formed.

Visiting a rock far from Earth is not easy, and collecting and returning samples even less so. Historically, we’ve relied on space rocks coming to us to get our hands on these time capsules. Meteorites known as carbonaceous chondrites have been the best available tool for investigating the composition of asteroids that may have delivered water to Earth while the Solar System was still forming.

However, this record is biased by a kind of mineral version of survival of the fittest. Only the strongest pieces of space rock survive the explosive rigors of atmospheric entry, and even then they are altered and contaminated by the terrestrial environment.

In recent years, efforts to touch asteroids have fallen within our reach. In December 2020, a probe sent to Ryugu by the Japanese Space Agency (JAXA) dropped an invaluable cargo: samples of material collected from the asteroid’s surface and transported home in sterile containers.

Scientists have been diligently studying the contents since then, finding that the asteroid is very similar in composition to those carbonaceous chondrites, making it what we call a C-type asteroid. It also contains prebiotic molecules – precursors to biological compounds. – and may once have been a comet.

The new analysis goes even deeper. Ito and his colleagues have found that the abundances of hydrogen and heavy nitrogen in the asteroid are consistent with an origin in the outer solar system; that is, Ryugu began his life much further away from the Sun. This would be consistent with the comet theory, since those icy bodies are visitors from the far reaches of the Solar System.

Ryugu, the researchers found, also has a distinct difference from carbonaceous chondrites. Absent from asteroid samples are ferrihydrite (compounds of iron and oxygen) and sulfate (sulfur and oxygen). Since these compounds are found in meteorites, they are thought to be a component of extraterrestrial materials. Their absence in Ryugu suggests that they may be the result of terrestrial weathering on meteorites.

This means that future meteorite studies should allow for this possibility… and that future asteroid sample return missions will be able to shed more light on the matter.

“In this study we prove this [carbonaceous] “Meteorites, despite their geochemical importance as representatives of the bulk composition of the Solar System, are contaminated samples from Earth,” the researchers wrote in their paper.

“The findings of this study clearly demonstrate the importance of direct sampling of primitive asteroids and the need to transport the returned samples under totally inert and sterile conditions. The evidence presented here shows that the Ryugu particles are undoubtedly among the most uncontaminated material in the Solar System available to the laboratory. Continued study and investigation of these precious samples will certainly expand our understanding of the early processes of the Solar System.”

The research was published in Astronomy of Nature.

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