Abstract We report the detection of water activity in the third confirmed interstellar object, 3I/ATLAS, based on ultraviolet imaging with the Neil Gehrels Swift Observatory’s Ultraviolet/Optical Telescope. Assuming a reddening of 29% between 3325.7 and 5437.8 Å, measurements on 2025 July 31–August 1 yielded a first, marginal detection of OH (A 2 Σ–X 2 Π) emission near 3085 Å, corresponding to a water production rate of (0.74 ± 0.50) × 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 ± 0.35) × 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.
Xing et al. (Tue,) studied this question.
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