What Five Grams of ‘Primordial’ Stardust From an Asteroid Tell Scientists About How the Early Solar System Formed

Ryugu is a carbonaceous, water-rich space rock with a unique, dark coloration and porous composition

An image of black rocky like material in a small metal dish. The sample is material from the surface of Asteroid Ryugu.
Two reseach teams analyzed a sample of the Ryugu's surface. Yada et al./Nature Astronomy 2021

In 2019, Japan Aerospace Exploration Agency’s (JAXA) Hayabusa2 asteroid explorer collected particles of rock from a diamond-shaped, near-Earth asteroid: 162172 Ryugu. A year after returning the sample to Earth on December 6, 2020, researchers have revealed that the space rock is a primitive asteroid rich in water and organic matter. Studying the material may provide more clues to the origin and evolution of the solar system and Earth’s habitable conditions, reports Nicoletta Lanese for Live Science.

Details on the asteroids results were published as two different studies in Nature Astronomy. The first study looked at the composition of the asteroid, and the second study focused on the chemical analysis of the sample from Ryugu.

Ryugu is of interest to researchers because the space rock has remained unchanged since the formation of the solar system 4.5 billion years ago, Michelle Star for Science Alert reports. The ancient celestial body measures one kilometer across and orbits the sun in between Earth and Mars. It is only the second asteroid that scientists have taken samples from for analysis.

A total of 5.4 grams of material, which resembles dark bits of fine pepper, was collected from the space rock. In the first study, Cédric Pilorget, an astronomer at the Université Paris-Saclay in France, and his team analyzed the composition of the sooty dust, per Science Alert. The team found that the sample has minerals and compounds in common with other meteorites found on Earth, but also contained organic and water-bearing molecules, reports Alex Wilkins for New Scientist. The volatiles such as hydroxyls, made of oxygen and hydrogen atoms, most likely originated in the outer solar system, reports Jon Kelvey for Inverse.

An image of the asteroid 162173 Ryugu  as it appears in space. The asteroid looks like a grey diamond shaped chunk.
This image of of the asteroid 162173 Ryugu was taken by the Optical Navigation Camera aboard Hayabusa2 Spacecraft in 2018. JAXA, via Wikimedia Commons under CC BY 4.0
In the second study, researchers led by Toru Yada, an astronomer at JAXA, found that Ryugu was a lot darker and more porous than expected. The team analyzed the rock while keeping it in a vacuumed sealed room filled with purified oxygen to avoid exposing it to Earth’s atmosphere, per Live Science. Using an optical microscope and other instruments, the scientists measured how the sample absorbed, emitted and reflected wavelengths of light in both infrared and visible spectrums. 

The dark asteroid only reflects about two to three percent of light that hits it, making it among the darkest samples ever examined, Live Science reports. “We have to understand why and what it implies regarding the formation and evolution of this material," says Pilorget to Live Science.

In the analysis, Yada’s team also found that Ryugu is about 50 percent more porous than other carbonaceous meteorites that have fallen to Earth, per Inverse. Determining whether space rock is porous is crucial to understanding if asteroids like Ryugu could threaten Earth or simply disintegrate into small pieces upon impact with our planet's atmosphere.

Based on the meteorite’s dark and porous properties, both studies agree that Ryugu is carbonaceous and should be classified as a CI chondrite, Science Alert reports. C- type celestial objects are suspected to be carbon-rich and are the most numerous in the Solar System. Carbon-rich C- type celestial objects are the most numerous in the solar system. While previous studies based on ground-based evidence have categorized the space rock as a carbonaceous asteroid, no direct evidence was obtained until now, a statement explains. 

Based on these preliminary results, there is much more to find out about Ryugu. Research is already underway to determine the asteroid's age to understand how and when it formed as well as when it encountered water. Details such as these could help astronomers understand early solar system formation. "Once again, we are only at the beginning of our investigations," Pilorget tells Live Science