Pre-Accretion Formation of Sulfuric Acid in Interstellar Ice Analogs: Implications for Sulfate in Ryugu and Bennu

Abstract Sulfate-bearing phases and soluble sulfate species, including sodium sulfate (Na2SO4), in carbonaceous asteroids are commonly attributed to parent-body aqueous alteration, yet the formation of S(+VI) sulfate under the reducing, low-temperature conditions inferred for primitive bodies remains chemically challenging. Here, we show that sulfuric acid (H2SO4) can form before accretion through irradiation of sulfur dioxide/water (SO2/H2O) interstellar ice analogs at 5 K. Threshold photoionization, deuterium labeling, and sulfur isotope analysis unambiguously identify sulfuric acid, establishing a route from preaccretion ice chemistry to oxidized sulfur reservoirs. If incorporated into accreting planetesimals, such preaccretion H2SO4 could subsequently be mobilized, neutralized, and precipitated during aqueous alteration, providing a potential contribution to sulfate-bearing phases in primitive carbonaceous bodies.