Aqueous Alteration on Ryugu’s Parent Asteroid May Have Occurred Earlier Than Previously Thought

Ca-carbonate in Ryugu particle C0009 isolated in the matrix and surrounded by an iron sulfide rim. The dotted white oval, red oval, and dashed yellow squares represent oxygen, carbon, and Mn-Cr analysis pits. Credit: McCain et al., 2023.

JAXA’s Hayabusa2 mission returned ~5.4 grams of material from the C-type asteroid Ryugu. Studies have shown that this material resembles the CI (Ivuna-type) carbonaceous chondrites, which are primitive meteorites that experienced aqueous alteration on their parent asteroids in the early solar system. Minerals such as carbonates and magnetite in the Ryugu samples record critical information about the timing and conditions of such alteration.

A new study led by Kaitlyn McCain at UCLA (now at Jacobs-NASA Johnson Space Center) and an international team used secondary ion mass spectrometry (SIMS) to investigate the oxygen and carbon isotopes of carbonates and magnetite, and the 53Mn-53Cr systematics of carbonates, in two Ryugu particles. Results showed that the earliest alteration took place between 0 °C and 20°C, involved a fluid that was enriched in 13C, 17O, and 18O (consistent with outer solar system ices), and resulted in the formation of calcite (Ca-carbonate) and magnetite. Phyllosilicates and dolomite (Mg-rich carbonate) were formed as the fluid evolved.

The study of 53Mn-53Cr systematics yielded crystallization ages of ~4566.6 Myr (million years), implying that aqueous alteration occurred <1.8 Myr after the formation of CAI, the oldest solar system solids. This is significantly older than previous ages for Ryugu carbonates and CI (4-6 Myr after CAI), with the difference being due to different analytical standards. If this methodological change proves valid, it will require substantially new formation scenarios for Ryugu’s (and CI) parent asteroids, principally because at <1.8 Myr after CAI, the abundance of 26Al, a major heat-producing element, would have been very high. McCain and colleagues calculate that to ensure that a buildup of heat did not lead to total loss of water and differentiation, CC asteroids must have been <20 kilometers in diameter, as opposed to previous estimates of >50-100 kilometers. These results could also have substantial implications for the relative timing of accretion and differentiation of materials in the inner versus the outer solar system. READ MORE