  Observations of WASP-121 b have suggested an under-abundance of titanium and
titanium-oxide from its terminator region. In this study, we aim to determine
whether this depletion is global by investigating the day-side emission
spectrum. We analyse 8 epochs of high-resolution spectra obtained with
ESPRESSO, targeting orbital phases when the day-side is in view. We use a
cross-correlation method to search for various atoms, TiO and VO and compare to
models. We constrain the velocities and phase-function of the emission signal
using a Bayesian framework. We report significant detections of Ca I, V I, Cr
I, Mn I, Fe I, Co I and Ni I, but not T i or TiO. Models containing Ti are
unable to reproduce the data. The detected signals are consistent with the
known orbital and systemic velocities and with peak emission originating from
the sub-stellar point. We find that Ti is depleted from regions of the
atmosphere where transmission and emission spectroscopy are sensitive. We
interpret this as evidence for the night-side condensation of titanium,
preventing it from being mixed back into the upper layers of the atmosphere
elsewhere on the planet. Species with lower condensation temperatures are
unaffected, implying sharp chemical transitions exist between ultra-hot
Jupiters that have slight differences in temperature or dynamical properties.
As TiO can act as a strong source of stratospheric heating, cold-trapping
creates a coupling between the thermal structures on the day-side and
night-side, and thus condensation chemistry needs to be included in global
circulation models. Observed elemental abundances in hot Jupiters are not
reliably representative of bulk abundances unless night-side condensation is
accounted for or the planet is hot enough to avoid night-side cold-traps
entirely. Planetary rotation may significantly lower the apparent orbital
velocity of emission signals.
