The distribution of elements seen in the samples of Ryugu brought back by Hayabusa 2 hints at a wet past.
While most planetary geology is done from afar, it’s amazing what we can do when samples are brought back to Earth. In a recent study of pieces returned from the asteroid 162173 Ryugu, scientists report in Science Advances that the rubble-pile asteroid hailed from a parent body with a wet past.
Japan's Hayabusa 2 spacecraft returned the sample of more than 5 grams (about the mass of a U.S. nickel) to Earth on December 5, 2020. With the sample, researchers can study a pristine example of the primordial solar system that, unlike meteorites, is not contaminated and altered by a hot trip through Earth's atmosphere.
Previous studies have found that Ryugu is likely a carbonaceous chondrite, a type of asteroid that is rich in carbon, volatiles (including water), and organics.
However, while most elements are similarly distributed across Ryugu and its carbonaceous siblings, scientists have previously found the distribution of chromium doesn't follow the same pattern. This one element varies in concentration more widely over Ryugu's surface than in other studied examples of the same type of asteroid.
To understand why, a large team led by Tetsuya Yokoyama (Tokyo Institute of Technology) zeroed in on this element, measuring the ratio of chromium-54 to chromium-52 in the Ryugu sample. This isotopic ratio gives clues to the asteroid's origins; because chromium dissolves in water, water-based chemistry set chromium's distribution within the asteroid long ago. Now, however, that ratio remains stable under the dry conditions seen on modern-day Ryugu. The element's distribution thus preserves a record of past water exposure.
The researchers also looked at another isotopic ratio, of titanium-50 to titanium-47, which served as a point of comparison. Indeed, while the ratios of titanium seen in the study showed little variance, the ratios of chromium isotopes varied by nearly a factor of two.
“Both of the chromium and titanium isotope ratios for Ryugu are different from those of Earth and any type of meteorite, except for the Ivuna-type carbonaceous chondrite,” Yokoyama says. The result suggests that wherever Ryugu and other Ivuna-type chondrites come from, it's different from the origin of other meteorites — “possibly from the outer solar system,” he adds. “This finding reinforces our previous conclusion that Ryugu and carbonaceous chondrites share a common heritage.”
Carbonaceous chondrites only rarely survive a meteorite's journey to Earth's surface, so they are tough to study on Earth. Ryugu's sample return thus offers a pristine look at this rare rock type. It's even possible that carbonaceous chondrites, which are among the most water-rich asteroids, helped deliver water to early Earth.
Indeed, the way the chromium isotopes vary across Ryugu's surface suggests that water interacted with the element on millimeter scales within the first several million years of the solar system. That water-based chemistry would have happened when Ryugu was still part of a larger parent body. After the parent body broke up, the rubble that remained lost its water. While the impact itself might have caused water loss, space weathering might also have eroded water more gradually. Now the rubble pile appears to be completely dry, at least from the outside.
Ryugu samples are now open for scientists to request for study under the Japan Aerspace Exploration Agency’s Announcement of Opportunity, and additional studies on the samples' composition are forthcoming.
The researchers suggest that the OSIRIS-Rex team likewise take care to "obtain unbiased isotopic compositions," as samples from another rubble-pile asteroid, 101955 Bennu, also recently returned to Earth, are prepared for study.
Meanwhile, Hayabusa 2 is now in its extended mission, named Hayabusa# (read as "Hayabusa Sharp"). Watch for close flybys of asteroids 2001 CC21 and 1998 KY26 in 2026 and 2031, respectively.