The interstellar comet 3I/ATLAS, only the third such visitor ever detected, is challenging scientific expectations with the discovery of water on its surface. A study published in The Astrophysical Journal Letters details how researchers from Auburn University used NASA’s Neil Gehrels Swift Observatory to detect hydroxyl (OH) emissions, a clear indicator of water.
This observation was only possible using a space-based telescope, as the ultraviolet signatures produced by hydroxyl are blocked by Earth’s atmosphere. The findings are particularly surprising because the comet was releasing water vapor while still more than three times farther from the sun than Earth. At this distance, solar radiation is typically too weak to cause significant sublimation, the process where ice turns directly into gas.
Despite the cold environment, 3I/ATLAS was shedding water at a rate of approximately 40 kilograms per second, a flow the study’s authors compared to a “hydrant at maximum power.” This unusual activity suggests a more complex structure than is typically seen in comets from our solar system. One theory is that small ice fragments are detaching from the comet’s nucleus and then vaporizing, feeding a gaseous cloud that surrounds the object.
The presence of water allows scientists to study 3I/ATLAS using the same metrics applied to comets from our own solar system, providing valuable data on cometary processes in other star systems.
“When we detect water—or even its faint ultraviolet echo, OH—from an interstellar comet, we’re reading a note from another planetary system,” said Dennis Bodewits, an Auburn University physicist who collaborated on the research. “It tells us that the ingredients for life’s chemistry are not unique to our own.”
The discovery adds to the unique profile of each interstellar object observed to date. “Every interstellar comet so far has been a surprise,” said Zexi Xing, an Auburn University researcher and co-author of the study. “‘Oumuamua was dry, Borisov was rich in carbon monoxide, and now ATLAS is giving up water at a distance where we didn’t expect it. Each one is rewriting what we thought we knew about how planets and comets form around stars.”
