2026ApJ...997..136O

We present a new analysis of the spectroscopic variability of WISE J104915.57-531906.1AB (WISE 1049AB, L7.5+T0.5), observed using the NIRSpec instrument on board the James Webb Space Telescope (GO 2965; PI: Biller). We explore the variability of the dominant molecular bands present in their 0.6-5.3 μm spectra (H₂O, CH₄, and CO), finding that the B component exhibits a higher maximum deviation than the A component in all the wavelength ranges tested. The light curves reveal wavelength-dependent (atmospheric depth) and possibly chemistry-dependent variability. In particular, for the A component, the variability in the light curves at the wavelengths traced by the CH₄ and CO molecular absorption features is higher than that for of H₂O, even when both trace similar pressure levels. We conclude that clouds alone are unlikely to explain the increased variability of CO and CH₄ with respect to H₂O, suggesting that an additional physical mechanism is needed to explain the observed variability. This mechanism is probably due to thermochemical instabilities. Finally, we provide visual representations of the 3D atmospheric maps reconstructed for both components using the molecular band contributions at different pressure levels and the fits of planetary-scale waves.