Astronomers using the NASA/ESA/CSA James Webb Space Telescope have detected the most deuterium-rich methane ever measured in the Solar System, hinting that the interstellar visitor’s ices formed in a cold, distant, low-metallicity environment.
Hubble captured this image of 3I/ATLAS on July 21, 2025, when the comet was 446 million km (277 million miles) from Earth. Image credit: NASA / ESA / David Jewitt, UCLA / Joseph DePasquale, STScI.
“The recent discovery of comet-like objects passing through the Solar System on hyperbolic, gravitationally unbound trajectories, namely 2I/Borisov and 3I/ATLAS, has opened up a new window into the chemistry of planet formation around other stars by delivering their contents to our own planetary system for analysis,” said lead author Dr. Nathan Roth of NASA’s Goddard Space Flight Center and his colleagues.
“Dynamical and compositional studies of these enigmatic objects may provide clues to their origins.”
“Soon after its discovery, multiple studies leveraged models of Galactic dynamics and chemistry to suggest 3I/ATLAS formed in a low-metallicity star system over 3 billion years ago given its radiant direction and high heliocentric velocity, although discerning a thick- vs. thin-disk origin was difficult.”
The astronomers used Webb to measure the deuterium-to-hydrogen (D/H) ratio in the methane of 3I/ATLAS.
Deuterium is a heavier isotope of hydrogen, and the amount of it locked into molecules is a sensitive clue to the conditions in which those molecules formed.
The ratio is about 3.33%, roughly 14 times higher than the methane D/H ratio measured in comet 67P/Churyumov-Gerasimenko by ESA’s Rosetta spacecraft, the only other comet in which deuterated methane has been securely detected.
It also exceeds every other methane measurement in our Solar System and is at least 1.4 times higher than any other direct measurement of organic molecules in the Solar System.
The researchers observed the comet’s coma with Webb’s NIRSpec Integral Field Unit on December 22 and 23, 2025, when 3I/ATLAS was about 2.4 AU from the Sun.
Besides methane and its deuterated form, they detected methanol, ethane, formaldehyde, water and carbon monoxide.
The latter was the most abundant of these in the coma, with a production rate roughly 3.6 times that of water.
“Aside from comet 67P/Churyumov-Gerasimenko, the D/H ratio for methane in the Solar System has been measured for Venus, the giant planets, Titan, and the dwarf planets Eris and Makemake,” the scientists said.
“The D/H ratio for organics has also been measured in interplanetary dust particles, carbonaceous meteorites, and ultra-carbonaceous Antarctic micrometeorites.”
“The value in 3I/ATLAS is at least a factor of 1.4 higher than any other direct measure for organic molecules in the Solar System, with the closest being D/H = 1.5% for organic radicals in carbonaceous chondrite meteorite Orgueil.”
Methane D/H has essentially never been measured directly outside the Solar System, so there is no clean yardstick.
To get around this, the authors used deuteration in another hydrocarbon, cyclopropenylidene, as a proxy.
Their estimates suggest methane D/H could reach roughly 4 to 7% or more in some star-forming regions and cold cores, meaning the 3I/ATLAS value is plausible for interstellar material.
“It is uncertain whether theoretical models of either star formation or disk chemistry can reproduce the methane D/H ratio in 3I/ATLAS’s ices,” they said.
“This should ideally be confirmed by new chemical models.”
“On the other hand, there is tentative evidence that interstellar/protostellar D/H ratios could match that of 3I/ATLAS.”
The findings were published October 23 in the journal Nature Astronomy.
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N.X. Roth et al. An enriched methane D/H ratio in the interstellar object 3I/ATLAS. Nat Astron, published online October 23, 2026; doi: 10.1038/s41550-026-03002-5
