ALMA HDO Mapping of Halley-type Comet 12P/Pons-Brooks Reveals a D/H Ratio Consistent with Earth’s Water

Isotopic measurements of Solar System bodies provide a primary paradigm within which to understand the origins and histories of planetary materials. Due to their sensitivity to specific molecular formation pathways, as well as the local thermal and radiation environment, stable isotopic abundance ratios in solid or gas-phase molecules provide insight into the physical and chemical properties of the environment from which our Sun and planets emerged 1–3. The D/H ratio in particular, helps reveal the relationship between (and heritage of) different H2 O reservoirs within the Solar System 4, 5 . Here we present the first interferometric maps of H2O and HDO in the coma of a comet (Halley-type comet 12P/Pons-Brooks), obtained using the Atacama Large Millimeter/submillimeter Array (ALMA). The maps are consistent with outgassing of both H2O and HDO directly from the nucleus, and imply a coma D/H ratio (for water) of (1.71 ± 0.28) × 10−4. This value is consistent with the D/H in Earth’s oceans, implying a possible common heritage between water ice in the Oort cloud, and the water that was delivered to the young Earth during the formation of the Solar System.