A research team from the Shanghai Astronomical Observatory (SHAO) of the Chinese Academy of Sciences (CAS) has conducted high-sensitivity radio observations of the interstellar comet 3I/ATLAS using the Tianma Telescope, marking the facility's first detection of an interstellar object.

The study reveals that this small body, originating from outside the solar system, released substantial amounts of water vapor and carbon monoxide (CO) as it approached the Sun. Notably, a significant fraction of this water vapor did not come directly from the comet's nucleus, but rather from the secondary sublimation of icy grains within its coma.

This discovery provides new clues for understanding the formation environments and volatile compositions of small bodies in other planetary systems. The findings were recently published in The Astrophysical Journal Letters.

Discovered in July 2025, 3I/ATLAS is the third interstellar object confirmed by humanity, following 1I/'Oumuamua and 2I/Borisov, and the second interstellar comet confirmed to exhibit clear cometary activity. Previous multi-wavelength observations had already indicated that 3I/ATLAS displayed dust and gas activity at large heliocentric distances, with characteristics distinct from those of typical solar system comets.

In this study, the team used the SHAO's 65-meter Tianma radio telescope to continuously monitor 3I/ATLAS, tracking changes in water vapor release with heliocentric distance via the 1.6-GHz OH spectral line. The team successfully detected OH signals when the comet was at heliocentric distances of approximately 2.27 astronomical units (au) and 1.96 au. The results show that at 2.27 au, 3I/ATLAS released about 0.43 tons of water vapor per second — a rate that rose to 0.62 tons per second at 1.96 au, an increase of more than 40%.

The researchers identified significant discrepancies in water production rates measured by different telescopes even at similar heliocentric distances. Through analysis, they attributed this to a pronounced "extended source" effect in 3I/ATLAS: ice-containing particles that had already left the nucleus continued to sublimate within the coma, releasing large amounts of additional water vapor. Telescopes with smaller fields of view can only capture water vapor from the coma's central region, producing values far lower than the total production rate.

To quantify this mechanism, the team developed a two-component parametric model to separate the contributions to water production from the nucleus and from the coma. Monte Carlo analysis suggests that before 3I/ATLAS reached perihelion, the extended source could account for up to 80% of its total water production — meaning most of the observed water was not ejected directly from the nucleus surface, but released continuously by icy grains within the coma. As 3I/ATLAS moved closer to the Sun, this proportion gradually decreased, yet even near perihelion, the extended source still contributed roughly half of the water output.

The solar system comet 103P/Hartley has also shown strong extended-source sublimation. However, this mechanism is not universal: the well-known 67P/Churyumov-Gerasimenko, for example, ejects predominantly "dry" grains. The researchers plan to conduct further studies incorporating the comet's full activity evolution around perihelion to understand why grains released by 3I/ATLAS are rich in water ice.

Beyond water vapor observations, the team used the CAS Purple Mountain Observatory's Delingha 13.7-meter millimeter-wave telescope to obtain the 115-GHz CO spectral line across a heliocentric distance range of approximately 2.33 to 1.75 au. They measured an average CO production rate of about 0.27 tons per second for 3I/ATLAS, yielding a CO/H₂O ratio of roughly 28%. This value is considerably higher than typical levels measured for most solar system comets at comparable heliocentric distances. While lower than that of the earlier interstellar comet 2I/Borisov, it still indicates that 3I/ATLAS has a relatively abundant reservoir of CO volatiles, highlighting strong compositional diversity among interstellar comets.

3I/ATLAS also shows an enhanced CO abundance relative to hydrogen cyanide, further supporting the idea that it formed in a planetary system environment colder than our own, allowing greater preservation of highly volatile materials.

This research was supported by the National Natural Science Foundation of China.

The Tianma Radio Telescope and some of the comets it has observed. (Image by SHAO)