Preliminary Note: The nucleus of a water molecule has one proton. A stable isotope of water, deuterium, has an additional neutron in its nucleus. Comets show an elevated ratio similar to Earth’s oceans, 156 deuterium nuclei per 1 million hydrogen nuclei. This reinforces theories that much of Earth’s ocean water is of cometary origin.[5][6]

Comet water discovery supports theory on Earth’s origins

Researchers have found that water from Comet 12P/Pons-Brooks, also known as the Devil Comet, is nearly identical to Earth’s water, supporting the theory that comets delivered water to Earth. The discovery, published in Nature Astronomy, shows that the comet’s water has a deuterium-to-hydrogen ratio similar to that of Earth’s water.
Full Story: Space (8/19)

‘Devil Comet’ contains ‘strongest evidence yet’ that comets delivered water to Earth

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Water found on Comet 12P/Pons-Brooks is “virtually indistinguishable” from water found on Earth.

a bright dot leaves a hazy trail behind it as it flies in front of background stars

A photo of comet 12P/Pons-Brooks taken on April 9, 2024 from Payson, Arizona by astrophotographer Chris Schur. (Image credit: Chris Shur)

A “devil” comet’s water is strikingly similar to the water on Earth, researchers have discovered.

The finding supports the idea that water was brought to our planet through comet impacts, helping set the stage for life to evolve, the team reported Aug. 8 in the journal Nature Astronomy.

“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,” NASA molecular astrophysicist Martin Cordiner, who led the team, said in a statement.

The researchers made the discovery while observing a comet called 12P/Pons-Brooks — also dubbed the “Devil Comet” — with the Atacama Large Millimeter/submillimeter Array (ALMA) and NASA’s Infrared Telescope Facility (IRTF). Comet 12P/Pons-Brooks is a Halley-type comet, a class of comets with orbital periods between 20 and 200 years.

The researchers used ALMA and IRTF data to analyze the ratio of deuterium (“heavy” hydrogen, which has a neutron in its nucleus) to normal hydrogen (D/H) — a “chemical fingerprint” that can be used to trace the water’s origins — in the water on Comet 12P/Pons-Brooks. They found that the comet’s water is “virtually indistinguishable” from the water on Earth.

This is especially significant because previous measurements of the water on Halley-type comets revealed different D/H ratios, casting doubt on the theory that comets could have brought water to Earth. This new discovery, by contrast, strengthens the theory.

artisti's illustration showing multiple comets hitting earth, with stylized water molecules floating in the atmosphere around them

Earth’s water is thought to have been delivered several billion years ago, by a combination of cometary, asteroidal and meteoritic impacts. In contrast to previous findings, new work using the ALMA telescope shows that the isotopic (D/H) ratio in Earth’s water is consistent with delivery by Halley-type comets. (Image credit: NASA/Theophilus Britt Griswold)