Near-perihelion activity and composition of 3I/ATLAS from JUICE/MAJIS observations

We present visible-to-infrared (0.5-5.56 microns) observations of the interstellar comet 3I/ATLAS obtained with the Moons and Jupiter Imaging Spectrometer (MAJIS) aboard the Jupiter Icy Moons Explorer (JUICE) spacecraft between 2025 November 2 and 25, shortly after perihelion. The fluorescence emission from H2O at 2.7 microns and CO2 at 4.3 microns is detected at heliocentric distances of 1.36-1.68 au. A weak dust-scattered continuum is identified from which we determine the spectral slope, with values ranging from 15% per 100 nm in the 0.65-0.9 microns region to 1-3% per 100 nm from 0.9 to 2.6 microns. Spatially resolved measurements show that the radial distributions of H2O and CO2 species are consistent with release in the near-nucleus environment. We derive H2O production rates that decreased from 8 x 1028 s-1 to 4 x 1028 s-1 over the period November 2-25, while the CO2/H2O ratio remained nearly constant at 10%. The heliocentric evolution of the CO2 production rate indicates activity controlled by solar heating. Combined with published CO measurements and the low gas expansion velocities, our analyses support a scenario in which CO2 plays a major role in driving the activity of 3I/ATLAS near perihelion. In addition, broad emission features are identified in the 3.2-3.6 microns region that cannot be explained by the fluorescence of common cometary CH-bearing volatiles. Their spectral characteristics are consistent with aliphatic C-H functional groups and provide tentative evidence for the release of complex organic material from dust grains in the coma.