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New research has uncovered compelling evidence that water from a comet is strikingly similar to that found in Earth's oceans, offering fresh support for the idea that comets may have played a crucial role in delivering water—and possibly some of the molecular ingredients for life—to our planet.

Using the powerful Atacama Large Millimeter/submillimeter Array (ALMA), an international team of scientists, led by Martin Cordiner of NASA's Goddard Space Flight Center, mapped the distribution of both ordinary water (H₂O) and "heavy" water (HDO, which contains the heavier isotope, deuterium) in the coma (the cloud of gas surrounding the nucleus) of the Halley-type comet 12P/Pons-Brooks during its approach to the sun. This marks the first time such detailed spatial mapping of these two forms 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 picture 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 chemical 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 research is in Nature Astronomy.

"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 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 signature as that found on Earth, supporting the idea that comets could have helped make our planet habitable."

Halley-type comets are a class of comet with intermediate orbital periods (between 20 to 200 years), and visit the inner solar system only rarely. The study's findings are significant because previous measurements of 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, providing a more accurate picture of the comet's true composition. "By mapping both H₂O and HDO in the comet's coma, we can tell if these gases are coming 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 capabilities, 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.