Detection of H2O in the coma of interstellar comet 3I/ATLAS with MAJIS/JUICE
ATel #17726; Dominique Bockelee-Morvan (LIRA, Observatoire de Paris), Yves Langevin (IAS), Francois Poulet (IAS), Benoit Seignovert (LPG), Giuseppe Piccioni (INAF) and the MAJIS team
on 23 Mar 2026; 14:32 UT
Credential Certification: Dominique Bockelee-Morvan (dominique.bockelee@obspm.fr)
Subjects: Infra-Red, Comet
We report the detection of the 2.7-micron H2O emission band in the coma of interstellar comet 3I/ATLAS using spectroscopic measurements acquired with MAJIS (Moons And Jupiter Imaging Spectrometer) onboard the JUICE spacecraft (Poulet et al. 2024, SSR, 220, 3, 27) on 2025 November 2.26 UT (4 days after perihelion), when the comet was at a heliocentric distance rh = 1.363 au and at a distance of 0.444 au from the spacecraft.
Based on the H2O band radiance extracted in a squared aperture of 3.1 arcmin (i.e. 60000 km on the comet) we derived a water production rate of Q(H2O) = 6.58 +/- 0.13 E28 mol/s. A gas expansion velocity of 0.4 km/s was assumed, and the H2O photodissociation rate was taken equal to 1.45 x E-5 s-1 (rh = 1 au). The radial distribution of the H2O-band radiances up to 1.E5 km from the nucleus is consistent with H2O release from the nucleus.
H2O emission lines were also observed on November 12, 2025, and November 19, 2025. A similar Q(H2O) value is obtained for November 12.3 UT.
Acknowledgments: MAJIS observations of the comet were supported by the MAJIS science ground segment team, the ESA Mission Operations Centre (MOC), and funding from CNES, ASI, CNRS, and INAF.
