The Anomalously High Abundance of Deuterium in 3I/ATLAS
Hydrogen is the most abundant element in the Universe, composed of an electron and a proton. Deuterium includes a neutron in addition to that proton in its nucleus. In the first twenty minutes after the hot Big Bang, a primordial abundance of one deuterium atom per 40,000 hydrogen was generated. This abundance ratio is similar to the value found in the Sun or Jupiter. Earth has a higher abundance, with about one for every 6,500 hydrogen atoms in seawater being deuterium. Deuterium can be extracted from seawater inexpensively, making it an abundant fusion fuel that could power human needs for millions of years.
In 1942, during early discussions for the Manhattan Project, Edward Teller asked whether the extreme temperatures of a fission atomic bomb explosion could cause deuterium in the oceans to undergo fusion and burn our planet. This hypothesized chain reaction was shown to be extremely unlikely by Hans Bethe who calculated that radiative energy losses would far exceed any energy gained from fusion, causing any such reaction to fizzle out.
Deuterium acts as a primary fuel source for nuclear fusion due to its high energy yield and relative ease of reaction. In fusion experiments, deuterium is commonly used in a mixture with tritium (with two neutrons in addition to the proton in its nucleus), a combination that ignites at the lowest possible temperature compared to other fusion fuels. The fusion of a deuterium nucleus with a tritium nucleus creates a helium-4 nucleus and a high-energy neutron.
What is the deuterium abundance in the interstellar object 3I/ATLAS?
Recently, two new papers used spectroscopic data from the Webb telescope to deduce exceedingly high fraction of deuterium in two molecules shed by 3I/ATLAS. They found one deuterium in 100 hydrogen atoms in water (H2O) and one deuterium in 30 hydrogen atoms in the organic molecule of methane (CH4) around 3I/ATLAS.
The first paper on March 6, 2026 (available here) analyzed spectroscopic data on water in the gas plume around 3I/ATLAS, and derived an enrichment at a level of D/H = (0.95 ± 0.06)%, which is more than an order of magnitude higher than all known comets. In addition, the 12C/13C ratios (141–191 for CO2 and 123–172 for CO) was reported to exceed typical values found in the Solar System and nearby interstellar clouds and protoplanetary disks.
Today, March 24, 2026, a new paper (available here) reported an unexpectedly high D/H = (3.31 ± 0.34)% for the organic molecule of methane (CH4) shed by 3I/ATLAS. This abundance is three orders of magnitude higher than found in methane on solar system planets and well above the values in comets or meteorites. In particular, it is a factor of 14 higher than the value measured in comet 67P/Churyumov-Gerasimenko by the Rosetta spacecraft.
The authors of both papers suggest that the extremely high D/H ratios of water and methane in 3I/ATLAS are a natural consequence of formation in a cold environment below 30 degrees Kelvin, within a protoplanetary disk about 10–12 billion years ago.
However, as I showed in a recent paper here, the association of 3I/ATLAS with the rare population of old metal-poor stars is untenable because they do not carry a large enough reservoir of heavy elements. It should also be kept in mind that ancient proto-planetary disks could not have been cooler than the cosmic microwave background at the time they formed, which at a redshift of ~10 had a temperature of 30 degrees Kelvin.
Hence, an important question arises: since deuterium is fusion fuel, might its over-abundance in 3I/ATLAS flag a technological signature?
ABOUT THE AUTHOR
Avi Loeb is the head of the Galileo Project, founding director of Harvard University’s — Black Hole Initiative, director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics, and the former chair of the astronomy department at Harvard University (2011–2020). He is a former member of the President’s Council of Advisors on Science and Technology and a former chair of the Board on Physics and Astronomy of the National Academies. He is the bestselling author of “Extraterrestrial: The First Sign of Intelligent Life Beyond Earth” and a co-author of the textbook “Life in the Cosmos”, both published in 2021. The paperback edition of his new book, titled “Interstellar”, was published in August 2024.
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