Comet’s Water Mirrors Earth’s Oceans, Strengthening Life-Origin Theory

ALMA observations of Halley-type comet 12P/Pons-Brooks reveal water with the same isotopic signature as Earth’s oceans

New observations with the Atacama Large Millimeter/submillimeter Array (ALMA) have revealed that water in the Halley-type comet 12P/Pons-Brooks has an isotopic composition virtually identical to that of Earth’s oceans. This finding strengthens the theory that comets may have played a crucial role in delivering water, and possibly some of the molecular ingredients for life, to our young planet.

Earth’s water is thought to have arrived billions of years ago through impacts by comets, asteroids, and meteorites. While previous measurements in many comets showed significant differences from Earth’s water, the new results provide the strongest evidence yet that at least some Halley-type comets carried water with the same chemical “fingerprint” as that found on our planet.

Using ALMA’s exceptional sensitivity and imaging capabilities, an international team led by Martin Cordiner (NASA’s Goddard Space Flight Center) mapped, for the first time, the spatial distribution of both ordinary water (H₂O) and heavy water (HDO, containing deuterium) in a comet’s coma—the cloud of gas surrounding its nucleus. These observations, made as 12P/Pons-Brooks approached the Sun, were combined with infrared measurements from NASA’s Infrared Telescope Facility (IRTF) to determine the ratio of deuterium to hydrogen (D/H) with unprecedented precision for a comet of this class.

Remarkably, the D/H ratio—(1.71 ± 0.44) × 10⁻⁴—is the lowest ever measured in a Halley-type comet and falls at the lower end of all cometary values, matching Earth’s oceans. “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.”

“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, co-author of the study.

The detection of faint heavy water signals so close to the nucleus—never before mapped in a comet—was only possible thanks to ALMA’s unmatched imaging power.

Additional Information

The research paper appeared in Nature Astronomy as "A D/H ratio consistent with Earth’s water in Halley-type comet 12P from ALMA HDO mapping" by M. Coordiner et al.

This text is based on the original press release by the National Radio Astronomy Observatory (NRAO), an ALMA partner on behalf of North America.

The Atacama Large Millimeter/submillimeter Array (ALMA), an international astronomy facility, is a partnership of the European Southern Observatory (ESO), the U.S. National Science Foundation (NSF), and the National Institutes of Natural Sciences (NINS) of Japan in cooperation with the Republic of Chile. ALMA is funded by ESO on behalf of its Member States, by NSF in cooperation with the National Research Council of Canada (NRC) and the National Science and Technology Council (NSTC) in Taiwan, and by NINS in cooperation with the Academia Sinica (AS) in Taiwan and the Korea Astronomy and Space Science Institute (KASI).

ALMA construction and operations are led by ESO on behalf of its Member States; by the National Radio Astronomy Observatory (NRAO), managed by Associated Universities, Inc. (AUI), on behalf of North America; and by the National Astronomical Observatory of Japan (NAOJ) on behalf of East Asia. The Joint ALMA Observatory (JAO) provides the unified leadership and management of ALMA's construction, commissioning, and operation.

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