A D/H ratio consistent with Earth’s water in Halley-type comet 12P from ALMA HDO mapping

Isotopic measurements of Solar System bodies provide a primary paradigm within which to understand the origins and histories of planetary materials. The deuterium-to-hydrogen (D/H) ratio, in particular, helps reveal the relationship between (and heritage of) different H₂O reservoirs within the Solar System. Here we present interferometric maps of water (H₂O) and semiheavy water (HDO) in the gas-phase coma of a comet (Halley-type comet 12P/Pons–Brooks), obtained using the Atacama Large Millimeter/submillimeter Array. The maps are consistent with outgassing of both H₂O and HDO directly from the nucleus, and they imply a coma D/H ratio (for water) of (1.71 ± 0.44) × 10⁻⁴. This is at the lower end of the range of previously observed values in comets and is consistent with D/H in Earth’s ocean water. Our results indicate a possible common heritage between a component of the water ice reservoir in the Oort cloud and the water that was delivered to the young Earth during the early history of the Solar System.