In case you had been assuming, assume no more. Science is about data and evidence:
Astrophysics > Earth and Planetary Astrophysics arxiv.org/abs/2508.04675
[Submitted on 6 Aug 2025]
Water Detection in the Interstellar Object 3I/ATLAS
Xing, Z. Oset, S. Noonan, J. et al.
ATLAS displays a weak coma. Weak, if you assume it’s near the Sun like a typical comet apparition. But interstellar object 3I was discovered near Jupiter’s orbit, and even during the observations above, it was still just in the outer Main Belt. Considering that water is still cold at this radius from the Sun, ATLAS’ activity is surprisingly high.
And yes, that activity is largely water emission. Comets include supervolatiles- other chemicals which, by definition, activate and escape before water. Xing et al. used an ultraviolet telescope (Gehrels Swift–UVOT) to detect hydroxyl (OH) emission lines, as opposed to the general reflection by a coma of whatever (including dust).
Again (and again, and again) I’ll caution that this paper came from arXiv.org, not a peer-reviewed science journal. You can read my prior posts during this entire ATLAS apparition to see what I grasp (and leave) regarding less-than-rigorous sources of talk. But here, the telescope and its use are pretty conventional. If anything’s debatable, it’s the other parameters of the 3I nucleus, and their implications/derivations. That’s where the cards form a house of cards.