3I/ATLAS created quite the buzz as it flew through our Solar System. As just the third interstellar object (ISO) ever detected, what our instruments observed as it approached our Sun and began heading back to deep space provided tantalizing clues about the star system in which it formed. In particular, new observations from the Atacama Large Millimeter/submillimeter Array (ALMA) have yielded the first-ever measurement of deuterated water (or “semi-heavy water”) in an ISO. The discovery provides a chemical window into the cold conditions that characterize its home star system.
The research was led by PhD student Luis E. Salazar Manzano, a PhD student at the University of Michigan, and assistant professor Teresa Paneque-Carreño, the Principal Investigator of the ALMA Director’s Discretionary Time program that made the discovery. They were joined by researchers from the National Radio Astronomy Observatory (NRAO), the Laboratory for Instrumentation and Research in Astrophysics (LIRA), the Leach Science Center, the Millennium Nucleus on Young Exoplanets and their Moons (YEMS), and NASA’s Goddard Space Flight Center (GSFC) and Jet Propulsion Laboratory (JPL).
The team’s observations were made in December 2025, six days after 3I/ATLAS reached its closest point to the Sun. This narrow observation window was made possible by two things. First, there’s the ALMA’s Atacama Compact Array (ACA), a series of four 12-meter (39.4 ft) and 7-meter (23 ft) telescopes grouped in a compact configuration to combine measurements (aka short-baseline interferometry), allowing them to see very faint objects in space. Second, there’s ALMA’s unique ability to point toward the Sun, which most optical telescopes cannot. As Paneque-Carreño noted in an NRAO press release
Most instruments can’t point toward the Sun, but radio telescopes like ALMA can. We were able to observe the comet within days after perihelion, just as it peeked out from its transit behind the Sun. This gave us a constraint on these molecules that’s not possible using other instruments.
*The ALMA Compact Array (ACA), part of the Atacama Millimeter-submillimeter Array (ALMA) in Chile. Credit: T. Burchell, NRAO/NSF/AUI*
Comets are often nicknamed “dirty snowballs” because of their high water and volatile elements content, as well as the dust and chemicals these ices contain. In addition to regular water (H2O), comets also contain deuterated water (HDO), where one hydrogen atom is replaced by the hydrogen isotope deuterium (which contains an additional neutron). Of the comets observed in the Solar System, comets contain roughly one HDO molecule for every ten thousand molecules of water.
While 3I/ATLAS’ water content fell below ALMA’s detection threshold during the observations, the team indirectly constrained the D/H ratio by detecting HDO via methanol line excitation. This sophisticated method showcased another unique aspect of ALMA: its analytical capabilities. This allowed the team to infer the rate of water outgassing during 3I/ATLAS’s closest approach to the Sun. Based on their analysis of the ALMA data, Manzano and her team estimated that the ratio of deuterated water to normal water (D/H) in 3I/ATLAS is at least 30 times higher.
What’s more, this ratio is 40 times higher than the abundance of HDO and water in Earth’s oceans. The elevated content of deuterated water suggests that the comet formed in a distinct chemical environment where objects are exposed to much less stellar radiation. As Manzano explained:
Our new observations show that the conditions that led to the formation of our Solar System are much different from how planetary systems evolved in different parts of our Galaxy. The chemical processes that lead to the enhancement of deuterated water are really sensitive to temperature and usually require environments colder than about 30 Kelvin, or about minus 406 degrees Fahrenheit.
*Artist’s impression of 3I/ATLAS’ gaseous envelope (coma), which formed as it approached the Sun. Credit: NSF/AUI/NSF NRAO/M.Weiss*
In essence, 3I/ATLAS originated in a solar system where temperatures were below -243 °C, which is only slightly higher than absolute zero (-273.15 °C; −459.67 °F). These results, which were preserved for billions of years as 3I/ATLAS traveled through interstellar space, offer a window into the history of the Milky Way and the abundances of elements in other star systems. Similarly, a team led by Caltech researchers analyzed observations of 3I/ATLAS made with the James Webb Space Telescope (JWST), also in December 2025.
In that study, the team (including several co-authors of this study) found that the comet’s interior composition contained significant amounts of methane. providing more clues about the conditions under which it formed. But what’s most impressive is the significance these findings have for our cosmological models. Since the abundances of deuterium and hydrogen were set during the Big Bang, this measurement also provides a unique probe of the conditions under which other worlds in our Universe are born. As Paneque-Carreño concluded:
Each interstellar comet brings a little bit of its history, its fossils, from elsewhere. We don’t know exactly where, but with instruments like ALMA we can begin to understand the conditions of that place and compare them to our own.
Further Reading: ALMA, Nature Astronomy