Enriched volatiles and refractories but deficient titanium on the day-side atmosphere of WASP-121b revealed by JWST/NIRISS

Context . With day-side temperatures elevated enough for all atmospheric constituents to be present in gas form, ultra-hot Jupiters offer a unique opportunity to probe the composition of giant planets. Aims . We aim to infer the composition and thermal structure of the day-side atmosphere of the ultra-hot Jupiter WASP-121b from two NIRISS/SOSS secondary eclipses observed as part of a full phase curve. Methods . We extracted the eclipse spectrum of WASP-121b with two independent data reduction pipelines and analysed it using different atmospheric retrieval prescriptions to explore the effects of thermal dissociation, reflected light, and titanium condensation on the inferred atmospheric properties. Results . We find that the observed day-side spectrum of WASP-121b is best fit by atmosphere models possessing a thermal inversion with temperatures reaching over 3000 K, with spectral contributions from H 2 O, CO, VO, and H − , along with either titanium oxide (TiO) or reflected light. We found the atmosphere of WASP-121b to be metal-enriched (~10× stellar), but comparatively titanium-poor (≲1× stellar), potentially due to partial cold-trapping. The inferred C/O depends on model assumptions, such as whether reflected light is included, ranging from being consistent with stellar, if a geometric albedo of zero is assumed, to being super-stellar for a freely fitted value of Ag = 0.16 −0.02 +0.02 . The volatile-to-refractory ratio was found to be consistent with the stellar value. Conclusions . From the NIRISS eclipse spectrum, we infer that WASP-121b has an atmosphere that is enriched in both volatile and refractory metals, but not in ultra-refractory titanium, suggesting the presence of a night-side cold-trap. Considering H 2 O dissociation is critical in free retrieval analyses, leading to order-of-magnitude differences in retrieved abundances for WASP-121b if neglected. Simple chemical equilibrium retrievals that assume all species are governed by a single metallicity parameter tend to drastically overpredict the TiO abundance, strongly biasing the inferred atmospheric composition.