The Cosmic Sulfur Depletion Problem Extends to Interstellar Object 3I/ATLAS

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Abstract Constraining the sulfur reservoirs of interstellar objects provides essential clues to the nature of sulfur species in protoplanetary disks and interstellar molecular clouds and a potential solution to the cosmic sulfur depletion problem. Observations of interstellar objects, chemical messengers from these systems, allow us sensitive probes of their volatile and refractory content and, when placed in context with recent developments in laboratory astrophysics, allow us to propose hypotheses for their formation and initial conditions. We present Hubble Space Telescope Observations of the interstellar object 3I/ATLAS, taken in 2025 December and 2026 January, that show fluorescence emissions from the CO Fourth Positive Group that are well fit by production rates of 3.8 × 10 27 and 1.1 × 10 27 molec. s −1 , respectively. We also place stringent upper limits on the atomic sulfur column and show that 3I/ATLAS’s comae is significantly depleted in sulfur, with S/O 3 σ upper limits of 0.4% and 0.1% for the 2025 December and 2026 January epochs, respectively. We hypothesize that this sulfur depletion is the result of a physical and chemical formation environment that was extremely cold, UV-shielded, and/or sulfur-depleted and therefore not conducive to the formation of H 2 S or SO/SO 2 while still effectively forming OCS and depleting the H 2 S reservoir, with substantial material inherited from the dense interstellar cloud. That same process would also sequester sulfur into FeS and S 6 –S 8 allotropes, which would be refractory at the observed heliocentric distances and would not contribute to the atomic sulfur coma. These species, which are often highlighted as potential solutions to the cosmic sulfur depletion problem, are also theorized to be the source of excess atomic sulfur seen in solar system comets.

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