The interstellar comet 3I/ATLAS may be showing signs of active eruptions from “ice volcanoes” on its surface, according to new analysis of the unusual space object.
The discoveries, which build on observations that have revealed the comet’s surprisingly metal-rich interior, add to a growing number of factors that have prompted revised thinking on how comets form, and what future discoveries may await with the ongoing detection of similar interstellar objects in the years ahead.
3I/ATLAS, first discovered in July of this year, has been continuously tracked by astronomers throughout its visit through our solar system. These observations offered scientists a rare glimpse of a truly alien object, whose mysterious origins far beyond the gravitational or radiative influence of our Sun led it to exhibit several behaviors unique among comets.
Of key significance to observations of 3I/ATLAS is that its unusual behaviors offered clues to astronomers about how objects might exist in their natural state under cosmic conditions in which they formed billions of years ago.
The fact that the comet’s behaviors align with our expectations for how such pristine celestial objects would react once subjected to heat from a star like our Sun makes 3I/ATLAS a unique learning opportunity for studying not only interstellar comets, but also the chemistry and physics of distant planetary systems.
A Massive Surge in Ice Activity
Now, according to a new study by researchers Josep M. Trigo-Rodríguez, Maria Gritsevich, and Jürgen Blum, which recently appeared on the arXiv preprint server, photometric observations of the comet have revealed a significant sustained increase in brightness that occurred as the object approached approximately 2.5 astronomical units from the Sun.
This unique behavior was sudden, but it was hardly fleeting—the explosive outburst appeared to be long-lasting, and seemingly points to the activation of water ice as the comet was warmed during its approach toward the Sun.
However, the recent observations reveal more than just sudden evaporation due to ice melt, suggesting something even more fascinating may have been occurring: cryovolcanism.
When Icy Volcanoes Erupt
Unlike typical volcanoes, whose eruptions are driven by pressure building beneath the Earth’s surface and extreme heat from magma deep below ground, cryovolcanism is a similar volcanic process driven by explosive pressure associated with erupting ice.
According to new research, in the absence of a dust mantle encasing 3I/ATLAS, which most comets in our solar system develop over time, the interstellar visitor likely had a bare icy surface instead. The direct exposure of its icy shield as it approached the Sun, the researchers argue, likely triggered widespread cryovolcanic eruptions across its surface.
The Mystery of 3I/ATLAS’s Metal
Additional observations by the research team also added to the accumulation of data about the comet’s unusual spectral profile. Specifically, comparisons between light reflected from 3I/ATLAS and terrestrial meteorite samples revealed a compelling match with carbonaceous chondrite (CR) meteorites. These are rare, very ancient meteorites rich in certain metals, particularly iron and nickel.
“The spectral similarities indicate that 3I/ATLAS may be a primitive carbonaceous object,” the researchers write in their paper, adding that it is “likely enriched in native metal and undergoing significant aqueous alteration during its approach to the Sun, experiencing cryovolcanism as we could expect for a pristine Trans-Neptunian Object.”
Based on such similarities, if 3I/ATLAS shares a similar composition with this metal-rich class of meteorites, it would remove it even further from typical comets observed in our solar system, which, at best, appear to contain modest metallic concentrations.
This is a potentially significant finding, as the study suggests that the metal-rich interior of 3I/ATLAS may be directly related to its activity. This is because, as its surface warmed and ice melted into liquid water, the researchers propose that the water may have begun corroding fine metal grains within the comet’s interior. The energy released by gases, such as carbon dioxide, and by other means could further sustain the ongoing cryovolcanism.
Revising Our Views on the Formation of Comets
The data being amassed about 3I/ATLAS indicate that metal-fueled ice volcanoes are very different from the processes normally associated with our current models of comet formation, which assume they involve mainly ice, dust, rock, and low metal content.
By contrast, the behavior exhibited by 3I/ATLAS reveals a far more complex tapestry of factors, in which comets formed in distant star systems may follow rules that differ surprisingly from those observed in our solar system.
Presently, 3I/ATLAS is only the third confirmed interstellar object, just two of which exhibited cometary behavior. In the coming decades, additional observations of interstellar objects will no doubt increase our understanding of the rich chemical compositions of these ancient cosmic treasures and the processes that shaped them, as well as those behind the formation of planetary systems across the galaxy.
The new study, “Spectrophotometric evidence for a metal-bearing, carbonaceous, and pristine interstellar comet 3I/ATLAS,” appeared at the preprint server arXiv.org on November 24, 2025.
Micah Hanks is the Editor-in-Chief and Co-Founder of The Debrief. A longtime reporter on science, defense, and technology with a focus on space and astronomy, he can be reached at micah@thedebrief.org. Follow him on X @MicahHanks, and at micahhanks.com.