Scientists have uncovered a unique mechanism behind the release of water vapor from 3I/ATLAS, an interstellar comet that briefly visited our solar system. The findings, published recently in The Astrophysical Journal Letters, shed light on the comet’s composition and formation, offering clues about planetary systems beyond our own. This research marks the first detection of an interstellar object by the Chinese Academy of Sciences’ (CAS) Tianma Radio Telescope, demonstrating its capabilities in observing these rare cosmic visitors.
3I/ATLAS, discovered in July 2025, is only the third interstellar object confirmed to have entered our solar system, following 1I/’Oumuamua and 2I/Borisov. Unlike typical comets that orbit the Sun, 3I/ATLAS is on a one-way trajectory, originating from outside our cosmic neighborhood. Researchers have been particularly interested in 3I/ATLAS due to its unusual activity, displaying both dust and gas emissions at considerable distances from the Sun. Understanding the composition of these interstellar objects is crucial for learning about the building blocks of planetary systems elsewhere in the galaxy.
Unusual Water Vapor Release
The research team from the Shanghai Astronomical Observatory (SHAO) of the CAS used the Tianma Telescope to monitor 3I/ATLAS, focusing on changes in water vapor release as the comet approached the Sun. They tracked these changes by observing the 1.6-GHz OH spectral line. The team detected OH signals at heliocentric distances of approximately 2.27 and 1.96 astronomical units (au), finding that water vapor release increased by more than 40% as the comet drew closer to the Sun – from 0.43 tons per second at 2.27 au to 0.62 tons per second at 1.96 au.
However, the study revealed a surprising detail: a significant portion of the water vapor wasn’t originating directly from the comet’s nucleus. Instead, it was being released from icy grains within the comet’s coma – the cloud of gas and dust surrounding the nucleus. This phenomenon, known as secondary sublimation, suggests that the comet’s coma plays a more substantial role in its observed activity than previously understood.
The ‘Extended Source’ Effect
Researchers identified what they call an “extended source” effect, where ice particles ejected from the nucleus continue to sublimate – transition from solid to gas – within the coma, releasing additional water vapor. This means that telescopes with smaller fields of view might underestimate the total water production rate, as they only capture vapor from the coma’s central region. Analysis suggests that before 3I/ATLAS reached its closest approach to the Sun (perihelion), the extended source could account for up to 80% of the total water production. Even near perihelion, it still contributed roughly half of the water output.
This isn’t the first time scientists have observed extended-source sublimation. The solar system comet 103P/Hartley has also exhibited this behavior. However, other comets, like 67P/Churyumov-Gerasimenko, tend to eject predominantly “dry” grains, meaning the extended source effect is not universal. Researchers plan further studies to understand why the grains released by 3I/ATLAS are particularly rich in water ice.
A Cold Origin?
In addition to water, the team also measured the production rate of carbon monoxide (CO) using the CAS Purple Mountain Observatory’s Delingha 13.7-meter millimeter-wave telescope. They found an average CO production rate of about 0.27 tons per second, resulting in a CO/H2O ratio of roughly 28%. This ratio is higher than typically observed in solar system comets at similar distances from the Sun, but lower than that of the earlier interstellar comet 2I/Borisov. The higher CO abundance suggests that 3I/ATLAS possesses a substantial reservoir of carbon monoxide volatiles.
3I/ATLAS exhibits an enhanced CO abundance relative to hydrogen cyanide, supporting the idea that it formed in a colder planetary system environment. Colder environments allow for greater preservation of highly volatile materials like CO. This finding adds to the growing understanding of the diverse compositions of interstellar comets and the varied conditions in which planetary systems can form.
The research, supported by the National Natural Science Foundation of China, provides valuable insights into the formation and composition of interstellar objects. As 3I/ATLAS continues its journey through our solar system, scientists will continue to monitor its activity, hoping to unlock further secrets about its origins and the planetary systems it once called home. The team plans to incorporate the comet’s full activity evolution around perihelion into future studies to refine their understanding of the processes driving its unique behavior.
Researchers will continue to analyze data collected by the Tianma Telescope and other observatories to build a more complete picture of 3I/ATLAS and other interstellar objects as they pass through our solar system. The next major step will be to analyze the full dataset of observations around the comet’s perihelion, expected in the coming months.
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