Molecular Similarity and Water Diversity in Coeval Binary Disks: JWST/MIRI Observations of Sz 65 and Sz 66

We present JWST/MIRI Medium Resolution Spectrometer spectra of the wide-separation (projected separation = 980 au) binary protoplanetary disks Sz 65 (K7; 0.68 M ⊙ ) and Sz 66 (M3; 0.30 M ⊙ ), reduced using the uniform pipeline of the JWST Disk Infrared Spectral Chemistry Survey. Both disks show rich molecular emission, including H 2 O, CO 2 , HCN, C 2 H 2 , and OH. The scaled spectra of the two disks exhibit remarkably similar H 2 O, CO 2 , and HCN line emission in the 13–18 μ m region, with the only notable difference being stronger C 2 H 2 emission in the primary (Sz 65). Beyond 18 μ m, the difference in H 2 O line emission between the two disks increases. Both the flux ratios and the slab-model-derived mass ratios of cold to hot H 2 O (∼200 to ∼750 K) and warm to hot H 2 O (∼450 to ∼750 K) are significantly higher in the secondary (Sz 66). Because binary stars share nearly the same age and metallicity, and as both disks appear compact in millimeter emission (<30 au), we suggest that the excess cold H 2 O in the secondary is best explained by its unstructured dust disk, in contrast to the primary, which shows gaps at 6 and 20 au. The enhanced cold water in the secondary is consistent with efficient pebble drift across the water snow line and increased H 2 O vapor from the sublimation of icy mantles. Our results demonstrate that wide-separation binaries can serve as powerful control samples for isolating the impact of individual disk properties on inner-disk chemistry and evolution.