Aqueous alteration in icy planetesimals: The effect of outward transport of gaseous hydrogen

Parent bodies of carbonaceous chondrites that initially contained metallic iron potentially exert strong reduction power during aqueous alteration to generate molecular hydrogen in excess of hydrogen solubility in water-rich fluids. The surplus hydrogen escapes from the system, which is subsequently supplied to overlying regions in planetesimals. Based on this concept, we conducted chemical equilibrium modeling of the aqueous alteration and simulated gaseous H₂ migration within the icy planetesimal that has a melted mantle and an icy shell during the early stages of radiogenic heating. In the chemical equilibrium modeling, we simulated the aqueous alteration of chondritic rocks at 0–350 °C and a water/rock mass ratio of 0.2–10 with initial CO₂ contents of 0–10 mol% in the fluid. The results showed that the mineral assemblage and solution composition change with the temperature, water/rock mass ratio, and initial fluid composition. The reproduced mineral paragenesis and abundance well explain those of carbonaceous chondrites. Furthermore, it was revealed that the initial H₂ fugacity of the system influences not only the stability of minerals and solution compositions, but also the preservation potential of organic molecules. Indeed, within these parameter spaces, the modeling results account for the organic/inorganic carbon-rich alterations reported for the Tagish Lake meteorite, Ceres, and Ryugu. Simulations of gaseous H₂ migration in a planetesimal revealed that gaseous H₂ in the deep interior can be transported to the interface with an icy shell even if the permeability is low. Moreover, it is highly possible that an H₂-rich layer would have been widely formed just below the icy shell. Therefore, it is expected that H₂-rich regions beneath the ice layer in planetesimals have substantial potential for the synthesis and preservation of organic molecules. These results imply that the alteration of carbonaceous chondrite parent bodies and C-complex asteroids is characterized by not only the type of parent bodies (e.g., formation age and distance from the Sun) but also the locations within their parent bodies.