Astronomers have detected hydroxyl (OH) gas, a chemical indicator of water, from the interstellar object 3I/ATLAS using an ultraviolet/optical telescope on NASA’s Neil Gehrels Swift Observatory.
Stacked images of the interstellar comet 3I/ATLAS obtained with NASA’s Neil Gehrels Swift Observatory: the first was captured on July 31 and August 1, 2025 (visit 1, upper half), and the second was on August 19, 2025 (visit 2, lower half). Image credit: Xing et al., others, doi: 10.3847/2041-8213/ae08ab.
The identification of the third interstellar object, 3I/ATLAS, on July 1, 2025, initiated a comprehensive characterization effort globally.
Learning from prior discoveries of interstellar objects 1I/Oumuamua and 2I/Borisov, an observation campaign was implemented to swiftly measure its initial brightness, morphology, light curve, color, and optical and near-infrared spectra.
Given the apparent brightness and early extension of the coma, there was suspicion of a gas outburst, yet none was detected.
Investigating the early activity of interstellar objects is crucial for understanding their chemical and physical evolution as they approach the Sun, as this may signify the first notable heating during their extensive dynamic lifetimes.
“The discovery of water marks a significant step in our grasp of how interstellar comets evolve,” stated Dennis Bordewitz, an astronomer from Auburn University.
“For solar system comets, water serves as a baseline for scientists to gauge their total activity and track how sunlight stimulates the release of other gases.”
“This is the chemical standard against which all assessments of volatile ice in cometary cores are made.”
“Detecting the same signal in an interstellar object means we can for the first time position 3I/ATLAS on the same scale employed to study comets indigenous to our Solar System. This is a progress toward juxtaposing the chemistry of planetary systems throughout our Milky Way galaxy.”
“What’s fascinating about 3I/ATLAS is the location of this water activity.”
Swift noticed the hydroxyl groups when the comet was nearly three times further from the Sun than Earth (well beyond the area where water ice on the surface could easily sublimate), recording a water loss rate of approximately 40 kg per second. At such distances, most solar system comets remain inactive.
The robust ultraviolet signal from 3I/ATLAS implies there might be additional mechanisms at play. Possibly, sunlight is warming small ice particles expelled from the core, causing them to vaporize and contribute to the surrounding gas cloud.
Such extensive water sources have only been detected on a limited number of far-off comets, suggesting intricate layered ice that holds clues regarding their formation.
Every interstellar comet discovered to date has unveiled a distinct aspect of planetary chemistry beyond our Sun.
Collectively, these observations illustrate that the composition of comets and the volatile ice that constitutes them can vary considerably from one system to another.
These variations imply the diversity of planet-forming environments and how factors like temperature, radiation, and composition ultimately influence planetary formation and, in some instances, the materials that lead to life.
Capturing the ultraviolet signals from 3I/ATLAS was a technological achievement in itself.
Swift employs a compact 30 cm telescope, yet from its orbit above Earth’s atmosphere, it can detect wavelengths of ultraviolet light that are largely absorbed by the atmosphere.
Free from sky glare or air interference, Swift’s ultraviolet/optical telescope achieves the sensitivity comparable to that of ground-based telescopes with 4-meter apertures for these wavelengths.
Its rapid targeting abilities allowed astronomers to analyze comets just weeks after their discovery, well before they become too faint or too close to the Sun for space study.
“When we observe water from an interstellar comet or its subtle ultraviolet signature (OH), we are interpreting notes from another planetary system,” Bordewitz notes.
“This indicates that the components essential for life’s chemical processes are not exclusive to us.”
“All interstellar comets we’ve observed thus far have been unexpectedly intriguing,” remarked Dr. Zexy Shin, a postdoctoral fellow at Auburn University.
“‘Oumuamua was dry, Borisov was rich in carbon monoxide, and now Atlas is revealing water at a distance we didn’t anticipate.”
“Each of these cases is transforming our understanding of how planets and comets form around stars.”
A study detailing the survey findings was published on September 30th in Astrophysics Journal Letter.
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Zexy Shin et al. 2025. Water production rate of interstellar object 3I/ATLAS. APJL 991, L50; doi: 10.3847/2041-8213/ae08ab
Source: www.sci.news
