For millions of years, a fragment of ice and dust drifted between the stars—like a sealed bottle cast into the cosmic ocean. This summer, that bottle finally washed ashore in our solar system and was designated 3I/ATLAS, only the third known interstellar comet ever observed.
When scientists at Auburn University aimed NASA’s Neil Gehrels Swift Observatory at the object, they detected something never seen before in this kind of visitor: hydroxyl (OH) gas, which is a clear chemical marker of water. Swift’s space-based telescope was able to capture a faint ultraviolet signal that cannot be detected from the ground, because Earth’s atmosphere blocks most ultraviolet light before it reaches the surface.
Detecting water, through its ultraviolet byproduct hydroxyl, represents a major advance in the study of interstellar comets. In comets from our own solar system, water serves as the main reference point for measuring overall activity and understanding how sunlight drives the release of other gases. It is the chemical standard used to compare the mix of volatile ices inside comet nuclei.
Finding the same indicator in an interstellar object allows astronomers, for the first time, to evaluate 3I/ATLAS using the same framework applied to familiar solar system comets, opening a new path for comparing the chemistry of planetary systems across the galaxy.
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