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Non-rocky sub-jovian exoplanets in high irradiation environments are rare.LTT 9979b, also known as TESS Object of Interest (TOI) 193.01, is one of thefew such planets discovered to date, and the first example of an ultra-hotNeptune. The planet's bulk density indicates that it has a substantialatmosphere, so to investigate its atmospheric composition and shed furtherlight on its origin, we obtained {\it Spitzer} IRAC secondary eclipseobservations of LTT 9979b at 3.6 and 4.5 $\mu$m. We combined the {\it Spitzer}observations with a measurement of the secondary eclipse in the {\it TESS}bandpass. The resulting secondary eclipse spectrum strongly prefers a modelthat includes CO absorption over a blackbody spectrum, incidentally making LTT9979b the first {\it TESS} exoplanet (and the first ultra-hot Neptune) withevidence of a spectral feature in its atmosphere. We did not find evidence of athermal inversion, at odds with expectations based on the atmospheres ofsimilarly-irradiated hot Jupiters. We also report a nominal dayside brightnesstemperature of 2305 $\pm$ 141 K (based on the 3.6 $\mu$m secondary eclipsemeasurement), and we constrained the planet's orbital eccentricity to $e <0.01$ at the 99.7 \% confidence level. Together with our analysis of LTT9979b's thermal phase curves reported in a companion paper, our results set thestage for similar investigations of a larger sample of exoplanets discovered inthe hot Neptune desert, investigations which are key to uncovering the originof this population.

Spitzer Reveals Evidence of Molecular Absorption in the Atmosphere of the Hot Neptune LTT 9979b