Water consists of two hydrogen atoms and one oxygen atom, represented as H2O. However, in a standard water molecule, those hydrogen atoms contain only a single proton. In contrast, cometary water features a significant proportion of deuterium (D), a form of hydrogen combining a standard proton with a neutron. This deuterated water, such as that found in the interstellar comet 3I/ATLAS, presents evidence of remarkably different environmental conditions from billions of years ago, akin to the half-heavy water (HDO) identified in comets within our solar system.
This image from the Subaru Telescope shows the interstellar comet 3I/ATLAS. Image provided by: National Astronomical Observatory of Japan
Water is an essential molecule pivotal to life and various astrophysical processes.
From an astrobiological standpoint, water serves as a crucial solvent facilitating the emergence of life on Earth, and it is tracked across the universe as a potential indicator of habitable environments beyond our solar system.
During the formation of stars and planets, water in its gaseous phase acts as an efficient coolant, assisting the collapse of molecular clouds and the birth of stars.
In frozen conditions, water coats dust particles, enhancing their adhesion and accelerating planetary core growth.
Water has been identified in both gaseous and icy forms throughout our galaxy and beyond, even in high-redshift galaxies.
These discoveries encompass the entire solar system, including molecular clouds, protostellar systems, prestellar nuclei, protoplanetary disks, comets, meteorites, active asteroids, planets, and moons.
Current research endeavors aim to link water pathways across these varied environments, aiming to unravel the origins and evolution of water in planetary system formation.
The deuterium-to-hydrogen (D/H) ratio in water acts as a potent chemical tracer, informing where the water was formed, the physical conditions during its creation, and its subsequent treatment.
“Recent observations from the Atacama Large Millimeter/Submillimeter Array (ALMA) indicate that the conditions that led to the formation of our Solar System differ significantly from those that shaped planetary systems in other regions of the Galaxy,” explained Dr. Luis E. Salazar Manzano, a student at the University of Michigan.
“Most instruments can’t survey the sun, but radio telescopes like ALMA can,” Dr. Teresa Paneque Carreño from the University of Michigan added. “We successfully observed the comet just as it passed behind the Sun, shortly after its perihelion.”
“This provides us with constraints on these molecules that other instruments cannot match.”
ALMA’s measurements of the D/H ratio in water from 3I/ATLAS indicate values over 30 times higher than those observed in comets formed within our solar system, and more than 40 times the levels found in Earth’s oceans.
“We’ve established that the gas cloud that birthed the star associated with 3I/ATLAS and the other planets in its system originated under distinct, frigid conditions, contrasting sharply with the environment that formed our solar system and local comets,” Salazar Manzano revealed.
This finding offers a unique fundamental insight, unlike other more complex molecular studies in interstellar comets, as the deuterium-to-hydrogen proportions were determined during the Big Bang.
“The chemical processes leading to deuterium water enrichment are highly temperature-sensitive, typically requiring environments below about 30 K (equivalent to -243 degrees Celsius or -406 degrees Fahrenheit),” Salazar Manzano stated.
The D/H ratio of water in this comet was enriched compared to the Big Bang baseline while it formed and was preserved during its interstellar journey.
This interstellar comet likely formed under very specific radiation conditions in a far colder environment than the history of our solar system before being ejected into interstellar space.
“Each interstellar comet carries fragments of its history and ‘fossils’ from diverse locations,” expressed Dr. Paneque Carreño.
“While the exact formation site remains elusive, instruments like ALMA enable us to begin comprehending the conditions there and drawing comparisons to our solar system.”
The research team’s results were published on April 23rd in the journal Nature Astronomy.
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LE Salazar Manzano et al. 3I/ATLAS water D/H as a probe of another planetary system’s formation status. Nat Astron published online on April 23, 2026. doi: 10.1038/s41550-026-02850-5
Source: www.sci.news
