Pebbles that were swiped from an ancient asteroid by a Japanese spacecraft are “the most uncontaminated and unfractionated extraterrestrial materials studied so far,” and contain important clues about the evolution of the solar system and the origin of life on Earth, reports a new study.
The pristine samples were scooped up from the surface of Ryugu, a half-mile-wide asteroid that is considered potentially hazardous to Earth, by Japan’s Hayabusa2 mission, during two maneuvers in 2019 that involved shooting projectiles into the space rock. The spacecraft captured about five grams of particles that broke off from both the asteroid’s surface and subsurface, and delivered them back to Earth in 2020.
Scientists led by Motoo Ito, a cosmochemist at the Japan Agency for Marine-Earth Science Technology, said the “precious samples” are “undoubtedly among the most uncontaminated Solar System materials available for laboratory study” and “provide the best proxy we have for the bulk composition of the Solar System,” according to a study published on Monday in Nature Astronomy.
Ito is the lead of the Phase2 curation Kochi team, a group tasked with analyzing the samples with sophisticated techniques while limiting exposure to Earth-based terrestrial substances that could contaminate the otherworldly particles. All these precautions have now paid off, as the new study found incredible details in the particles, including features that have never been seen in any other asteroid samples.
The team “was working very hard for the past several years” to handle the samples, which are “pristine and full of water,” Ito said in an email. Their tools and instruments are designed to study “these precious materials without and/or minimal terrestrial environments,” he added, because “the samples are ultra-sensitive to the terrestrial environment.”
“We were feeling (1) great honor, (2) thanks to all contributors who were involved in the Hayabusa2 mission including their family and friends, and (3) of course a sense of awe a bit,” he added.
Hayabusa2 is the second mission in history to return samples from an asteroid to Earth, following Japan’s original Hayabusa mission, which brought grains back from the asteroid Itokawa in 2010. NASA’s OSIRIS-REx spacecraft is on track to be the third mission to deliver these outer space gems; it is currently on its way back from asteroid Bennu, another potentially hazardous rock, with at least 60 grams of surface samples.
Prior to these sample-return missions, our only tangible source of asteroid chunks were meteorites that fortuitously fell to Earth. While falling space rocks have filled crucial gaps in our knowledge of asteroids, they become weathered by Earth’s atmosphere and contaminated by its surface. Plus, the meteorite record is biased toward strong rocks that can survive the trip to the ground in the first place.
Samples returned directly from asteroids, in contrast, offer an unfiltered look at these relics, many of which have remained virtually unchanged since the birth of the solar system 4.5 billion years ago. The Ryugu particles, as the most uncontaminated samples returned so far, are full of strange surprises and new insights.
For instance, Ito and his colleagues note a “puzzling” discrepancy between observations of Ryugu from orbit, which suggested the asteroid belonged to a dehydrated class of rocks called CY chondrites, and the actual samples, which strongly suggest that the rock contains water and is similar to a chemically pristine group called CI-chondrite.
“This discrepancy between onboard spectral observation and real material analysis could be due to space weather,” Ito said. “As you may know, spectral observation can detect only a few layers of the asteroid so that it may introduce some complication in terms of understanding their data.”
“I would say that data analysts of onboard spectral observation and space weathering scientists,” who use a technique called transmission electron microscopy, could help “to solve this discrepancy,” he continued. “Ryugu is one good showcase for doing this, and then other asteroidal samples from Bennu by NASA’s OSIRIS-REx would provide a better understanding of this problem.”
Future samples may also shed light on a specific class of carbon-rich organic molecules inside the Ryugu particles that have “not been found in any other meteorite studies so far,” Ito noted. The molecules are locked into curious sub-millimeter minerals called “phyllosilicates” that may have also been influenced by interactions with water.
Many scientists believe that early Earth was pelted by asteroids that enriched it with water and various organic molecules, which are key ingredients for life. As a result, the pristine Ryugu particles can open new windows into the origin of habitable conditions on our planet.
“I would say that coarse-grained phyllosilicate containing organics and water (OH) in primitive asteroids may have acted as ‘cradles’ for organic molecules and water and then a potential mechanism for the coupled delivery of water and organics to the early Earth,” Ito said.
For this reason, Ito plans to continue examining the organics until next year, with help of international colleagues from the Open University and IonToF company. These efforts, along with future sample-return missions to other comets and asteroids, can unlock the secrets of the early solar system and explain how Earth came to be so abundant with the stuff of life.
“The findings of this study clearly demonstrate the importance of direct sampling of primitive asteroids and the need to transport returned samples in totally inert and sterile conditions,” the team concluded in the study.