  Transmission spectroscopy has been a workhorse technique over the past two
decades to constrain the physical and chemical properties of exoplanet
atmospheres. One of its classical key assumptions is that the portion of the
atmosphere it probes -- the terminator region -- is homogeneous. Several works
in the past decade, however, have put this into question for highly irradiated,
hot ($T_{eq}\gtrsim 1000$ K) gas giant exoplanets both empirically and via
3-dimensional modelling. While models predict clear differences between the
evening (day-to-night) and morning (night-to-day) terminators, direct
morning/evening transmission spectra in a wide wavelength range has not been
reported for an exoplanet to date. Under the assumption of precise and accurate
orbital parameters on WASP-39 b, here we report the detection of inhomogeneous
terminators on the exoplanet WASP-39 b, which allows us to retrieve its morning
and evening transmission spectra in the near-infrared ($2-5\ \mu$m) using JWST.
We observe larger transit depths in the evening which are, on average, $405 \pm
88$ ppm larger than the morning ones, also having qualitatively larger features
than the morning spectrum. The spectra are best explained by models in which
the evening terminator is hotter than the morning terminator by
$177^{+65}_{-57}$ K with both terminators having C/O ratios consistent with
solar. General circulation models (GCMs) predict temperature differences
broadly consistent with the above value and point towards a cloudy morning
terminator and a clearer evening terminator.
