A newly published study in Nature Astronomy reveals that Comet 12P/Pons-Brooks, nicknamed the “Devil Comet,” contains water with an isotopic signature strikingly similar to that of Earth’s oceans – a finding heralded as the strongest evidence yet that comets may have helped deliver water to our planet.
Using the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, researchers mapped both regular water (H₂O) and heavy water (HDO, which contains deuterium) in the cloud known as the coma surrounding the comet’s icy core. These observations, combined with NASA’s infrared measurements, allowed scientists to calculate the comet’s deuterium-to-hydrogen (D/H) ratio with unprecedented precision.
The measured ratio is the lowest ever recorded for a Halley-type comet and matches the ratio found in Earth’s oceans.
Why the D/H Ratio Matters
The D/H ratio is often treated as a “fingerprint” for water sources. Most comets studied so far had ratios very different from Earth’s oceans, casting doubt on their role in delivering water to our planet. By contrast, Comet 12P/Pons-Brooks offers a near-perfect match.
“This finding provides the strongest evidence yet that at least some Halley-type comets carried water with the same isotopic signature as Earth’s, supporting the idea that comets could have helped make our planet habitable,” said NASA astrophysicist Martin Cordiner, who led the study.
Meet the “Devil Comet”
Comet 12P/Pons-Brooks is a Halley-type comet, meaning it takes between 20 and 200 years to orbit the Sun. Its most recent approach occurred in April 2024, the first time it was visible from Earth in 71 years. During that visit, the comet brightened dramatically and captured global attention, earning its nickname for its horn-like outbursts.
A Mixed Delivery Model
While this discovery strengthens the comet-delivery theory, most scientists still believe Earth’s water came primarily from asteroids. Asteroids have long been seen as the likeliest culprits because their water signatures align more consistently with Earth’s and their slower collisions made them more efficient deliverers.
The new results, however, show that comets may have played a meaningful supporting role. The “Devil Comet” suggests that icy visitors from the outer solar system could have added to the mix, broadening the picture of how Earth became a water-rich world.
A Technological First
Beyond its scientific implications, the study marks a technical milestone. This is the first time astronomers have mapped both H₂O and HDO in such detail within a comet’s coma. ALMA’s sensitivity, combined with NASA’s infrared instruments, allowed researchers to trace the water’s origin back to the comet’s frozen nucleus, proving it was not produced by chemical reactions in the outer layers.
Expanding the Search
Scientists caution that one comet is not enough to rewrite Earth’s history. More Halley-type comets will need to be studied to determine whether 12P/Pons-Brooks is unique or representative of a larger population. Future isotopic measurements—including oxygen and nitrogen ratios—may further clarify how water and other life-essential molecules spread through the early solar system. As researchers continue probing icy relics like the “Devil Comet,” they are inching closer to solving one of science’s oldest mysteries: how our once-dry planet became blue.