Seasonal Changes in the Atmosphere of HD 80606b Observed with JWST's NIRSpec/G395H

High-eccentricity gas giant planets serve as unique laboratories for studying the thermal and chemical properties of H/He-dominated atmospheres. One of the most extreme cases is HD 80606b -- a hot Jupiter orbiting a sun-like star with an eccentricity of $0.93$ -- which experiences an increase in incident flux of nearly three orders of magnitude as the star-planet separation decreases from $0.88\,{\rm AU}$ at apoastron to $0.03\,{\rm AU}$ at periastron. We observed the planet's periastron passage using \emph{JWST}'s NIRSpec/G395H instrument ($2.8-5.2\,{\rm μm}$) during a $21\,{\rm hr}$ window centered on the eclipse. We find that, as the planet passes through periastron, its emission spectrum transitions from a featureless blackbody to one in which CO, CH$_4$, and H$_2$O absorption features are visible. We detect CH$_4$ during post-periapse phases at $4.1-10.7σ$ depending on the phase and on whether a flux offset is included to account for NRS1 detector systematics. Following periapse, H$_2$O and CO are also detected at $4.2-5.5σ$ and $3.7-4.4σ$, respectively. Furthermore, we rule out the presence of a strong temperature inversion near the IR photosphere based on the lack of obvious emission features throughout the observing window. General circulation models had predicted an inversion during periapse passage. Our study demonstrates the feasibility of studying hot Jupiter atmospheres using partial phase curves obtained with NIRSpec/G395H.