Free Astronomy Magazine November-December 2025

31 ASTRO PUBLISHING of water has been achieved in a comet. The ALMA observations were then combined with data on water and other gases, observed using NASA’s Infrared Telescope Facility (IRTF), to form a more complete pic- ture of the comet. By combining the complementary capabilities of these two telescopes, the researchers were able to measure more accurately the ratio of deuterium to hydrogen (D/H) in the comet’s water, a chemi- cal fingerprint that helps scientists trace the origins and history of water throughout the Solar System. Remarkably, the D/H ratio of water in 12P/Pons-Brooks was found to be virtually indistinguishable from that of Earth’s oceans. The measurement, (1.71±0.44)×10 −4 , is the lowest such ratio ever measured in a Halley-type comet, and is at the lower end of values previously observed in other comets. [The value of terrestrial water is (1.5576 ± 0.0005)×10 −4 ] “Comets like this are frozen relics left over from the birth of our Solar System 4.5 billion years ago,” said Cordiner. “Since Earth is believed to have formed from materials lacking NOVEMBER-DECEMBER 2025 A . I. generated illustration showing a comet approach- ing Earth. [N. Lira − ALMA (ESO/NAOJ/NRAO)]