Isotopic measurements of Solar System bodies provide a primary paradigm within which to understand the origins and histories of planetary materials. The 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 (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±0.44)×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 suggest a possible common heritage between a component of the Oort cloud's water ice reservoir, and the water that was delivered to the young Earth during the early history of the Solar System.
@article{arxiv.2508.05925,
title = {A D/H Ratio Consistent with Earth's Water in Halley-type Comet 12P from ALMA HDO Mapping},
author = {M. A. Cordiner and E. L. Gibb and Z. Kisiel and N. X. Roth and N. Biver and D. Bockelée-Morvan and J. Boissier and B. P. Bonev and S. B. Charnley and I. M. Coulson and J. Crovisier and M. N. Drozdovskaya and K. Furuya and M. Jin and Y. -J. Kuan and M. Lippi and D. C. Lis and S. N. Milam and C. Opitom and C. Qi and A. J. Remijan},
journal= {arXiv preprint arXiv:2508.05925},
year = {2025}
}