Analysis of hydrogen, carbon, oxygen, sulfur, chlorine, and mercury in samples returned from asteroid (101955) Bennu

Abstract NASA's OSIRIS‐REx mission delivered pristine regolith samples from the near‐Earth carbonaceous asteroid (101955) Bennu, enabling direct assessment of primitive asteroid material free from terrestrial alteration. We report comprehensive analyses of H, C, O, S, Cl, and Hg in Bennu samples using X‐ray fluorescence (XRF), thermogravimetric analysis coupled with mass spectrometry (TG‐MS), combination analyses of pyrolysis and combustion (EMIA‐Step), and direct Hg measurements. Quantification of O, Si, S, Cl, and Ge by XRF is reported for Bennu samples for the first time. TG‐MS reveals ~15 mass% total mass loss, with H 2 O and CO 2 release patterns attributed to phyllosilicates, carbonates, and organic matter. Bennu samples contain substantially higher interlayer H 2 O than those from asteroid Ryugu, consistent with subsurface deeper sampling by OSIRIS‐REx. Total carbon abundance is ~4.1 mass%, dominated by organic carbon (~2.9 mass%), comparable to Ryugu samples. Mercury abundances (82–141 ng g −1 ) are far lower than those reported for CI chondrites, indicating pervasive terrestrial contamination in meteorite samples. We recommend a Hg abundance of 98 ± 2 ng g −1 for Bennu samples and pristine CI‐like material, demonstrating that returned asteroid samples provide the most reliable record of primitive carbonaceous matter.