Pyrrhotite-driven early-stage terrestrial alteration in Ryugu grains.

Samples returned from the carbonaceous asteroid Ryugu preserve a rare record of water- and organic-rich materials from the early Solar System, but this record can be modified rapidly after arrival on Earth. Here we show through controlled atmospheric exposure experiments that Ryugu grains undergo measurable terrestrial alteration within weeks to months. Pyrrhotite develops Fe-O-rich amorphous rims through oxidation and sulfur loss, while adjacent phyllosilicates become amorphized and carbonaceous materials form nanoscale bubbles and carbon-oxygen-rich surface precipitates. X-ray absorption spectroscopy reveals partial oxidation of Fe and S and chemical modification of associated organic matter. From the thickness of pyrrhotite alteration rims, we estimate an early-stage alteration rate of ~0.1 nm per day under low-humidity laboratory conditions. These results identify pyrrhotite as a key trigger of coupled mineral and organic alteration and show that even moderate terrestrial exposure can compromise volatile-rich, fine-grained astromaterials. Our findings provide an empirical basis for cold curation of returned samples.