Asteroid Ryugu and some carbonaceous meteorites, which are thought to have similar origins in the early Solar System, are known as chemically "primitive" materials because they preserve the chemical composition of the early Solar System remarkably well. Yet these rocks also reacted extensively with water after they formed, transforming many of their original minerals into new ones. When rocks react with water, elements can dissolve into the fluid or move from one place to another, potentially altering the original chemical composition. This seemingly contradictory combination — extensive reaction with water while preserving the chemical composition of the early Solar System — has long been one of the puzzles of cosmochemistry.
A research team led by Kanazawa University and The University of Tokyo used powerful X-rays at the large-scale synchrotron facilities KEK-PF and SPring-8 to examine these extraterrestrial materials in detail. They identified a previously overlooked material that may provide an important clue to this long-standing puzzle.
In the conventional mineralogical picture, aqueous alteration of primitive carbonaceous meteorites has been interpreted mainly in terms of the formation of serpentine and saponite. Using synchrotron-based soft X-ray absorption spectroscopy, however, the researchers found that, in addition to the well-recognized serpentine and saponite, a previously under-recognized component resembling poorly crystalline magnesium silicate hydrate (M-S-H) is widespread in Ryugu and carbonaceous meteorites.
"What makes M-S-H particularly interesting is that it can accommodate a wide range of magnesium-to-silicon ratios," says Keisuke Fukushi of Kanazawa University. "This flexibility may be key to understanding how water-rock interactions proceeded without erasing pre-existing chemical differences within their parent asteroids."
In general, materials with different compositions tend to be stable under different aqueous conditions. However, laboratory experiments combined with previously published data on the solubility of M-S-H showed that M-S-H with different magnesium-to-silicon ratios can exist under similar aqueous conditions. The serpentine and saponite observed in the samples may also have formed from precursor M-S-H with corresponding compositions before transforming into these more crystalline minerals.
Together, these results suggest a pathway by which different alteration minerals could form while preserving pre-existing compositional differences from place to place. M-S-H may therefore provide an important clue to understanding how these materials could be extensively altered yet remain chemically primitive.
Funding
This work was supported by Grants-in-Aid for Scientific Research from the Japan Society for the Promotion of Science (JSPS; grant numbers JP22H04900, JP24H00268, JP26K22374, JP26H00438, and JP26K21720) and the Program for Forming Japan's Peak Research Universities (J-PEAKS) Grant Number JPJS00420230006.
