  Hot Jupiters are among the best-studied exoplanets, but it is still poorly
understood how their chemical composition and cloud properties vary with
longitude. Theoretical models predict that clouds may condense on the nightside
and that molecular abundances can be driven out of equilibrium by zonal winds.
Here we report a phase-resolved emission spectrum of the hot Jupiter WASP-43b
measured from 5-12 $\mu$m with JWST's Mid-Infrared Instrument (MIRI). The
spectra reveal a large day-night temperature contrast (with average brightness
temperatures of 1524$\pm$35 and 863$\pm$23 Kelvin, respectively) and evidence
for water absorption at all orbital phases. Comparisons with three-dimensional
atmospheric models show that both the phase curve shape and emission spectra
strongly suggest the presence of nightside clouds which become optically thick
to thermal emission at pressures greater than ~100 mbar. The dayside is
consistent with a cloudless atmosphere above the mid-infrared photosphere.
Contrary to expectations from equilibrium chemistry but consistent with
disequilibrium kinetics models, methane is not detected on the nightside
(2$\sigma$ upper limit of 1-6 parts per million, depending on model
assumptions).
