A Direct View of the Chemical Properties of Water from Another Planetary System

All detected water reservoirs in the solar system exhibit a deuterium enrichment that links back to the physical environment at the time of stellar birth [1–3]. Gas-phase and ice-grain D-enrichments occur through chemical processes that operate at low temperatures (< 30 K) pointing towards an origin in the pre-stellar molecular cloud [4–7]. However, not all stars are born in similar environment as our Sun. Stars in the Milky Way may form under different prestellar conditions resulting in environmental differences that could be traced by the water D/H ratio [8]. Using ALMA observations of the interstellar comet 3I/ATLAS, this work reports the first constraint of the water D/H ratio from an interstellar object. With a water D/H value of [D/H]H2 O > 1.3×10−3, 3I/ATLAS presents a higher D-enrichment than Earth’s oceans and solar system cometary bodies. The elevated deuterium enrichment points to water that formed under colder, less irradiated conditions, consistent with an origin in a planetary system that formed under different physical and chemical conditions than our own.