  Multi-band phase variations in principle allow us to infer the longitudinal
temperature distributions of planets as a function of height in their
atmospheres. For example, 3.6 micron emission originates from deeper layers of
the atmosphere than 4.5 micron due to greater water vapor absorption at the
longer wavelength. Since heat transport efficiency increases with pressure, we
expect thermal phase curves at 3.6 micron to exhibit smaller amplitudes and
greater phase offsets than at 4.5 micron; this trend is not observed. Of the
seven hot Jupiters with full-orbit phase curves at 3.6 and 4.5 micron, all have
greater phase amplitude at 3.6 micron than at 4.5 micron, while four of seven
exhibit a greater phase offset at 3.6 micron. We use a 3D
radiative-hydrodynamic model to calculate theoretical phase curves of HD
189733b, assuming thermo-chemical equilibrium. The model exhibits temperature,
pressure, and wavelength dependent opacity, primarily driven by carbon
chemistry: CO is energetically favored on the dayside, while CH4 is favored on
the cooler nightside. Infrared opacity therefore changes by orders of magnitude
between day and night, producing dramatic vertical shifts in the
wavelength-specific photospheres, which would complicate eclipse or phase
mapping with spectral data. The model predicts greater relative phase amplitude
and greater phase offset at 3.6 micron than at 4.5 micron, in agreement with
the data. Our model qualitatively explains the observed phase curves, but is in
tension with current thermo-chemical kinetics models that predict zonally
uniform atmospheric composition due to transport of CO from the hot regions of
the atmosphere.
