April 23, 2026
3I/ATLAS contains 30 times more semi-heavy water than comets in our solar system
by Jill Malusky, National Radio Astronomy Observatory
edited by Lisa Lock, reviewed by Robert Egan
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New observations of the interstellar comet 3I/ATLAS include the first measurement of the abundance of deuterated water relative to ordinary water in an interstellar object. Astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) discovered that the interstellar comet 3I/ATLAS is made of an astonishingly high ratio of semi-heavy water relative to water, indicating that its system of origin likely formed under conditions far colder than our own. The findings are published in Nature Astronomy.
Early ALMA observations and discovery
This team of researchers at the University of Michigan, led by Ph.D. student Luis E. Salazar Manzano, working with assistant professor Teresa Paneque-Carreño, made this discovery just six days after 3I/ATLAS was closest to our sun using ALMA's Atacama Compact Array. "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," said Salazar Manzano.
At this point in time, 3I/ATLAS has been observed by many telescopes and instruments; however, Paneque-Carreño emphasizes the unique contribution of ALMA to this discovery. "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."
What comet water can reveal
Comets are often nicknamed "dirty snowballs," in part because of their high water content. This water carries frozen information about the environment where they were formed. In addition to ordinary water (H2O: two hydrogen atoms and one oxygen), comets also contain a variation of water called deuterated water or "semi-heavy" water (HDO), which is composed of one hydrogen atom, one deuterated hydrogen—a hydrogen atom with an extra neutron—and one oxygen.
Deuterated water can be found on Earth and in comets within our own solar system as well, but the ratio of this deuterated water to ordinary water is set by the temperature and radiation conditions that existed as our solar system formed. In comets from our solar system, there is typically only about one molecule of semi-heavy water for every 10,000 molecules of ordinary water. This makes detecting deuterated water challenging to detect in comets, but ALMA's exquisite sensitivity makes these measurements possible.
An unusually high deuterium ratio
The team's observations of 3I/ATLAS's HDO/H2O ratio revealed them to be more than 30 times that which is found in comets that formed within our own solar system, and more than 40 times the ratio found in Earth's oceans. Salazar Manzano explained, "We now know that the cloud of gas that formed the star and other planets in the system where 3I/ATLAS came from was likely very cold and had very different conditions than the environment that created our solar system and local comets."
Furthermore, this discovery of the ratio of deuterated water to regular water offers a uniquely fundamental insight that is unmatched by the discoveries of other, more complex molecules in interstellar comets because the abundances of deuterium and hydrogen were set in the Big Bang itself.
Clues to a colder home system
Salazar Manzano explains, "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°F." The comet's HDO/H2O ratio was enhanced with respect to Big Bang values by 3I/ATLAS's home system as it formed and preserved through its interstellar journey. The interstellar comet must have formed in a system far colder than our own solar system's history, and under very specific radiation conditions, before it was ejected into interstellar space.
Added Paneque-Carreño, "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."
Publication details
Luis E. Salazar Manzano et al, Water D/H in 3I/ATLAS as a probe of formation conditions in another planetary system, Nature Astronomy (2026). DOI: 10.1038/s41550-026-02850-5
Journal information: Nature Astronomy
Key concepts
Provided by National Radio Astronomy Observatory
Citation: 3I/ATLAS contains 30 times more semi-heavy water than comets in our solar system (2026, April 23) retrieved 23 April 2026 from linkThis document is subject to copyright. Apart from any fair dealing for the purpose of private study or research, no part may be reproduced without the written permission. The content is provided for information purposes only.
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