Water Production Rates of the Interstellar Object 3I/ATLAS

Abstract

We report the detection of water activity in the third confirmed interstellar object, 3I/ATLAS, based on ultraviolet imaging with the \emph{Neil Gehrels Swift Observatory}'s Ultraviolet/Optical Telescope (UVOT). Assuming a reddening of 29\% between 3325.7~\AA\ and 5437.8~\AA, measurements on 2025 July 31 -- August 1 yielded a first, marginal detection of OH (A$^2\Sigma$ -- X$^2\Pi$) emission near 3085~\AA, corresponding to a water production rate of $(0.74 \pm 0.50) \times 10^{27}$ molecules\,s$^{-1}$. The subsequent visit on 2025 August 18 -- 20 revealed a clear OH detection, implying a higher water production rate of $(1.36 \pm 0.35) \times 10^{27}$ molecules s$^{-1}$ (40 kg~$s^{-1}$) at a heliocentric distance of 2.90~au. This places 3I/ATLAS among the few comets with confirmed OH emission beyond 2.5~au, where water ice sublimation from the nucleus is typically inefficient. The inferred production rate at 2.9 au implies an active area of at least 7.8~km$^2$, assuming equilibrium sublimation. This requires that over 8\% of the surface is active, which is larger than activity levels observed in most solar system comets. Contemporaneous near-infrared spectroscopy indicated the presence of icy grains in the coma, which may serve as an extended source of water vapor.