Free Astronomy Magazine November-December 2025

NOVEMBER-DECEMBER 2025 O rigins of Earth’s Water. Terrestrial H2O is thought to have been delivered several billion years ago, by a combination of cometary, asteroidal and me- teoritic impacts. In contrast to previous findings, new work using the ALMA tele- scope shows that the isotopic (D/H) ratio in Earth’s water is consistent with delivery by Halley-type comets. [NASA/Theophilus Britt Griswold] icant because previous measure- ments in other comets often showed water with a D/H ratio different from Earth’s, leaving the cometary origin of Earth’s water in doubt. This new measurement suggests that some comets—particularly those like 12P/Pons-Brooks—could indeed have delivered water, and possibly other life-essential elements, to a young Earth. The research also confirms the origin of the observed gases, provid- ing a more accurate picture of the comet’s true composition. “By mapping both H 2 O and HDO in the comet’s coma, we can tell if these gases are com- ing from the frozen ices within the solid body of the nucleus, rather than forming from chemistry or other processes in the gas coma,” said NASA’s Stefanie Milam, a co-author of the study. The observations were only possible thanks to ALMA’s exceptional sensitivity and unique imaging capabili- ties, which allowed the team to detect the faint signature of heavy water emanating from the innermost regions of the coma—something that has never before been mapped in a comet. A LMA maps showing the distribution of ordinary water (H ₂ O) and heavy water (HDO) in comet 12P/Pons-Brooks. The contours indicate how strong the signals are, with higher contours showing stronger detections. The small panels in the upper right display the strength of the water signals at the comet’s center. The lower left shows ALMA’s res- olution for these observations, while the lower right indicates the direction toward the Sun and the comet’s path through space. [M. Coordiner et al. − ALMA (ESO/NAOJ/NRAO)] water, comet impacts have long been suggested as a source of Earth’s water. Our new results provide the strongest evidence yet that at least some Halley-type comets carried water with the same isotopic signa- ture as that found on Earth, support- ing the idea that comets could have helped make our planet habitable.” Halley-type comets are a class of comet with intermediate orbital pe- riods (between 20 to 200 years), and visit the inner Solar System only rarely. The study’s findings are signif- !