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

Rubble recovered from a near-Earth solar-orbiting asteroid may be the most “pristine” sample of cosmic rock we’ve had our primate paws on yet.

According to a new in-depth analysis of materials delivered to Earth by asteroid Ryugu, the rock and dust samples are some of the most uncontaminated Solar System material we’ve ever had a chance to study – and their composition suggests that they integrate chemistry from the confines of the system.

This not only gives us a unique tool to understand the solar system and its formation, but it gives us a new context in which to interpret other space rocks that have been contaminated by coming into contact with Earth.

“Ryugu particles,” wrote a team led by cosmochemist Motoo Ito of the Japan Agency for Marine and Earth Science Technology (JAMSTEC) in Japan, “are the most uncontaminated and unfractionated extraterrestrial material studied so far. ‘at present, and provide the best available match to the composition of the bulk solar system.”

It’s been about 4.6 billion years since the formation of the Sun and the solar system around it. Obviously, it is very long, and many things have 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. They are pieces of rock, such as comets and asteroids, that drift through space more or less unchanged since their formation.

Visiting a rock far from Earth is not easy, and collecting and returning samples even less so. Historically, we relied on space rocks coming our way to put our mitts on these time capsules. Meteorites known as carbonaceous chondrites have been the best tool available to probe the composition of asteroids that may have provided water to Earth while the solar system was still forming.

However, this record is skewed by a kind of mineral version of survival of the fittest. Only the strongest pieces of space rock persist through the explosive rigors of atmospheric entry, and even then they are weathered and contaminated by Earth’s environment.

In recent years, venturing to land on asteroids has fallen within our capabilities. In December 2020, a probe that had been sent to Ryugu by the Japanese Space Agency (JAXA) deposited an invaluable payload: samples of material taken from the asteroid’s surface and transported home in sterile containers.

Scientists have since avidly studied the contents, revealing that the asteroid’s composition is very similar to these carbonaceous chondrites, making it what we call a C-type asteroid. It also contains prebiotic molecules – the precursors of biological compounds – and may have once been a comet.

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

Ryugu, the researchers found, also has a stark difference from carbonaceous chondrites. Asteroid samples lack ferrihydrite (compounds of iron and oxygen) and sulfate (sulfur and oxygen). Since these compounds are found in meteorites, they were thought to be part of extraterrestrial material. The absence of them from Ryugu suggests that they may be the result of terrestrial weathering from meteorites.

This means that future studies of meteorites should account for this possibility… and that future asteroid sample return missions may shed more light on the matter.

“In this study, we show that [carbonaceous] meteorites, despite their geochemical importance as indicators of the overall composition of the solar system, are contaminated terrestrial samples,” the researchers wrote in their paper.

“The results of this study clearly demonstrate the importance of direct sampling of early asteroids and the need to transport returned samples under completely inert and sterile conditions. The evidence presented here shows that Ryugu particles are undoubtedly among the most uncontaminated solar system materials available to the laboratory. The ongoing study and investigation of these valuable samples will certainly expand our understanding of early solar system processes.”

The research has been published in natural astronomy.

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