Water Production Rates of the Interstellar Object 3I/ATLAS

2025/08/06 by Zexi Xing, Shawn Oset, Xing, Zexi +6 · 7 voices · 14 citations
Engineering · Physics and Astronomy · #Astro and Planetary Science #Astronomy and Astrophysical Research #Spacecraft and Cryogenic Technologies #astro-ph.EP #astro-ph.GA

paper · pdf · doi:10.48550/arxiv.2508.04675

openalex publication_date 2025/08/06 · openalex created_date 2025/10/10 · openalex updated_date 2026/07/28

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 (UVOT). Assuming a reddening of 29% between 3325.7~Å and 5437.8~Å, measurements on 2025 July 31 -- August 1 yielded a first, marginal detection of OH (A²Σ -- X²Π) emission near 3085~Å, corresponding to a water production rate of (0.74 ± 0.50) × 10²⁷ 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) × 10²⁷ 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~km², 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.

Citations

• Hypothèse de Régénération Continue d'une Coque en Nickel sur 3I/ATLAS : Analyse Technique d'une Option Possible
• SPHEREx Discovery of Strong Water Ice Absorption and an Extended Carbon Dioxide Coma in 3I/ATLAS
• Hubble Space Telescope Observations of the Interstellar Interloper 3I/ATLAS
• From a Different Star: 3I/ATLAS in the Context of the Ōtautahi–Oxford Interstellar Object Population Model
• Snapshot of a new interstellar comet: 3I/ATLAS has a red and featureless spectrum
• The Astropy Project: Building an Open-science Project and Status of the v2.0 Core Package^*

Cited by

• The Volatile Inventory of 3I/ATLAS as Seen with JWST/MIRI
• Nucleus and Postperihelion Activity of Interstellar Object 3I/ATLAS Observed by Hubble Space Telescope
• The size of 3I/ATLAS from non-gravitational acceleration
• An Overview of Exocomets
• Very Large Telescope Observations of Interstellar Comet 3I/ATLAS. II. From Quiescence to Glow: Dramatic Rise of Ni <scp>i</scp> Emission and Incipient CN Outgassing at Large Heliocentric Distances*
• The Feasibility of a Spacecraft Flyby with the Third Interstellar Object 3I/ATLAS from Earth or Mars
• Interstellar Object 3I/ATLAS Observed from Mars by China's Tianwen-1 Spacecraft
• Water Production of Interstellar Comet 3I/ATLAS from SOHO/SWAN Observations after Perihelion
• CH₃OH and HCN in Interstellar Comet 3I/ATLAS Mapped with the ALMA Atacama Compact Array: Distinct Outgassing Behaviors and a Remarkably High CH₃OH/HCN Production Rate Ratio
• Interstellar Comet 3I/ATLAS: Evidence for Galactic Cosmic Ray Processing
• Water D/H in 3I/ATLAS as a Probe of Formation Conditions in Another Planetary System
• Prospects for the Crossing of Comet 3I/ATLAS's Ion Tail
• 3I/ATLAS: In Search of the Witnesses to Its Voyage
• Onset of CN Emission in 3I/ATLAS: Evidence for Strong Carbon-Chain Depletion
• 3I/ATLAS [wikipedia]

Discussions

• There’s an exciting new paper out this morning (not yet reviewed) which uses observations from the Neil Gehrels-Swift observatory to find water in our interstellar comet, 3I/ATLAS (1/n) 🔭 🧪 arxiv.or [bsky, 78 points, 3 comments]
• Volatile time! As anticipated for a comet ☄️, interstellar object 3I/ATLAS is now showing water - thanks to OH emission seen by Swift in the UV. A little distant at 3.51 au, but that suggests it could [bsky, 66 points, 3 comments]
• Reportan que el objeto interestelar 3I/ATLAS, el tercero descubierto que visita nuestro sistema solar, muestra señales claras de actividad acuosa. El telescopio ultravioleta UVOT del observatorio Swif [bsky, 4 points, 0 comments]
• Why do I like my job? Because sometimes you work with people who write and submit a paper *within a week of the observations*. Amazing stuff to confirm water vapor on this interstellar comet. arxiv.or [bsky, 4 points, 0 comments]
• Today on the #arXiv: Xing et al. 2025, "Water Detection in the Interstellar Object #3I/ATLAS" - arxiv.org/abs/2508.04675 [bsky, 2 points, 0 comments]
• Today on the # arXiv : Xing et al. 2025, "Water Detection in the Interstellar Object # 3I /ATLAS" - https:// arxiv.org/abs/2508.04675 [mastodon, 0 points, 0 comments]
• 3I/ATLAS looking more comet-ish w a new paper just submitted: water detected. It's still a bit strange, though, because at this distance seeing water is extremely rare & it also suggests the surface i [bsky, 0 points, 0 comments]

Related

• Water Production of Interstellar Comet 3I/ATLAS from SOHO/SWAN Observations after Perihelion
• Hypothèse de Régénération Continue d'une Coque en Nickel sur 3I/ATLAS : Analyse Technique d'une Option Possible
• SPHEREx Pre-Perihelion Mapping of H₂O, CO₂, and CO in Interstellar Object 3I/ATLAS
• Spectroscopic Characterization of Interstellar Object 3I/ATLAS: Water Ice in the Coma
• SPHEREx Discovery of Strong Water Ice Absorption and an Extended Carbon Dioxide Coma in 3I/ATLAS