Precise Determination of the Metallicity and C/O of WASP-39 b From a Single JWST Instrument Mode with Phase-Resolved Cross-Correlation Retrievals

Measuring atmospheric metallicities and C/O ratios is a key goal of JWST exoplanet science, given their proposed link to planet formation. Achieving this goal has previously been shown to require broad wavelength coverage (about 1--5 mu m), typically demanding multiple instrument modes to complete the molecular inventory. Here, we show that the multi-instrument requirement can be circumvented using phase-resolved cross-correlation retrievals at native pixel resolution -- an approach more typically applied to ground-based high-resolution spectroscopy. By applying this novel analysis technique to an archival single-mode transit of the hot Jupiter WASP-39 b obtained with NIRSpec/G395H, we detect and obtain bounded abundances for all of its major carbon- and oxygen-bearing molecules: H 2 O ((Z) = 67), CO ((Z) = 25), CO 2 ((Z) = 475), and SO 2 ((Z) = 10). Notably, while standard retrieval methods fail to detect CO in these same data ((Z) = 0.2), our approach detects it decisively, confirming it as the dominant carbon carrier in WASP-39 b's atmosphere. From these abundances, we robustly derive a metallicity of [(C + O) / H] = 1.2 plus or minus 0.2 and a C/O ratio of 0.68+0.10-0.14, generally consistent with previous multi-instrument analyses. In comparison, traditional retrievals performed on the G395H data alone produce biased and inaccurate values of both parameters, driven primarily by the non-detection of CO as well as incomplete water band coverage. Our results establish phase-resolved cross-correlation retrievals as a powerful tool for extracting maximum atmospheric information from existing and future JWST data sets.