Asteroid Sample Brought to Earth Exposes Ryugu’s Hidden Interior

Asteroid Sample Brought to Earth Exposes Ryugu’s Hidden Interior
Asteroid Ryugu, as imaged by the Hayabusa2 probe. (Image: JAXA, University of Tokyo, Kochi University, Rikkyo University, Nagoya University, Chiba Institute of Technology, Meiji University, Aizu University, AIST)

In December 2020, Japan’s Hayabusa2 probe returned to Earth with samples taken from the asteroid Ryugu. New research finds that the asteroid’s subsurface materials are strikingly similar to the ones found up top, in what is an important finding.

A study published today in Science is the first to describe the subsurface materials retrieved from Ryugu, an asteroid located 280 million km from Earth. The scientists didn’t find anything spectacular — no alien biosignatures or exotic elements — but they did find that the subsurface materials bore a resemblance to materials found elsewhere on the asteroid. This likely means both samples sets are representative of the asteroid as a whole, including the parts hidden within.

In an email, planetary scientist Shogo Tachibana from the University of Tokyo and the lead author of the new paper said the samples will inform future studies of asteroid (162173) Ryugu, both in terms of its composition and history. His team “aimed to determine the representativeness of the samples because, if they represent the asteroid surface, detailed analysis on [Earth] will lead to an understanding of the entire asteroid even though they were gathered from limited areas on the asteroid,” he wrote.

Japan’s Hayabusa2 spacecraft visited Ryugu from June 2018 to November 2019, during which time it acquired both surface and subsurface samples. The probe gathered the subsurface materials by shooting a small projectile at the asteroid, forming a crater. The probe returned to Earth with the two sets of samples, each stored in separate containers, on December 6, 2020.

In total, Hayabusa2 managed to deliver nearly 5.5 grams of material — enough to fit onto a single teaspoon. Doesn’t sound like much, but that’s “about fifty times more than the mission minimum requirement of 0.1 grams,” according to the study. The successful return of surface materials meant that scientists could perform in situ observations of sand and tiny pebbles found on an actual asteroid, as opposed to studying a meteorite that crashed dramatically through our atmosphere.

In this case, it allowed for the study of Ryugu — a carbonaceous asteroid which, as its name suggests, is primarily made from carbon but also some water. Asteroids of this type, also known as C-type asteroids, are dark and rocky objects that likely formed in the outer reaches of the asteroid belt. They’re also survivors from the earliest days of the solar system. Scientists with the mission are hoping to investigate “questions regarding the origin of the Earth’s water and where the organic matter that forms life originally came from,” and to “examine how the planets formed through the collision, destruction, and combination of planetesimals, which are thought to have been formed early in the Solar System,” according to JAXA, Japan’s space agency.

In May 2020, scientists described the sample materials taken from the surface. That research found that the surface materials were not entirely representative of known meteorites, in a finding that suggested asteroids are more dynamic than we realised. With that done, it came time to investigate the second set of samples. While doing so, Tachibana and his colleagues referenced images of Ryugu’s surface as imaged by the MASCOT lander and the two MINERVA-II rovers, all three of which made observations at multiple locations across the asteroid.

“The sample examination was made inside the clean chamber dedicated to the sample, and little or no contamination was expected,” Tachibana explained. He said the safe recovery of the capsule and the rapid preparation of the container prior to its installation inside the clean room was the “most nerve-wracking” part of the process. Like the first container, the second container contained millimetre-sized sand, nearly centimeter-sized pebbles, and submillimeter-sized fine powder.

“We focused on comparisons between pebbles observed by the spacecraft and the returned samples to evaluate the representativeness of returned grains gathered from limited areas of the asteroid,” Tachibana told me. “We found that the returned samples well represent Ryugu surface particles from a morphological point of view and that there are characteristic flat and elongated particles on the asteroid, which are also present in the returned sample.”

It’s hardly a result that will take your breath away, but it’s a key finding nonetheless. The study’s concluding paragraph sums it up rather well:

The colour, shape, surface morphology, and structure of returned pebbles and sand match those of Ryugu’s surface material observed from the spacecraft. We therefore conclude that the pebbles and sand inside [the two chambers] are representative samples of Ryugu at two surface sites, without substantial alteration during the sample collection and return to Earth. The variations in physical properties among the pebbles and sand, which were not expected before spacecraft arrival at the asteroid, reflect the geological history of Ryugu.

Indeed, the attributes of the two returned sample sets were consistent with the materials observed elsewhere on Ryugu, offering a potential glimpse into the entire structure as a whole. Tachibana said his team’s findings will provide a “base” for future studies done on the asteroid and for investigations into its history.

Indeed, the work on these samples is only beginning. Future studies will undoubtedly involve chemical and compositional analyses, among other investigations into the rare samples. It still seems incredible to me that we’re capable of collecting clumps of dust from distant asteroids, but such is the state of modern science.