Water D/H in 3I/ATLAS as a probe of formation conditions in another planetary system

Water reservoirs in the Solar System exhibit a deuterium enrichment that links back to the physical environment at the time of stellar birth. Gas-phase and ice-grain deuterium enrichments occur through chemical processes that operate at low temperatures ( 6.6\times 1{0}^{-3}$$ , 3I/ATLAS shows a deuterium enrichment exceeding Earth’s ocean value by more than a factor of about 40 and typical Solar System cometary values by more than a factor of about 30. The elevated deuterium enrichment points to water that formed under colder, less irradiated conditions and from less thermally processed material, consistent with an origin in a planetary system that formed under different physical and chemical conditions than our own. ALMA observations of the interstellar comet 3I/ATLAS provide a limit on the deuterium-to-hydrogen ratio in water. This ratio is a sensitive probe of temperature, suggesting that comet 3I formed in an ultracold environment with minimal thermal processing in its home system.