  Hot Jupiters exhibit atmospheric temperatures ranging from hundreds to
thousands of Kelvin. Because of their large day-night temperature differences,
condensable species that are stable in the gas phase on the dayside, such as
TiO and silicates, may condense and gravitationally settle on the nightside.
Atmospheric circulation may counterbalance this tendency to gravitationally
settle. This three dimensional (3D) mixing of chemical species has not
previously been studied for hot Jupiters, yet it is crucial to assess the
existence and distribution of TiO and silicates in the atmospheres of these
planets. We perform 3D global circulation models of HD209458b including passive
tracers that advect with the 3D flow, including a source/sink on the nightside
to represent condensation and gravitational settling of haze particles. We show
that global advection patterns produce strong vertical mixing that can keep
condensable species lofted as long as they are trapped in particles of sizes of
a few microns or less on the night side. We show that vertical mixing results
not from small-scale convection but from the large-scale circulation driven by
the day-night heating contrast. Although this vertical mixing is not diffusive
in any rigorous sense, a comparison of our results with idealized diffusion
models allows a rough estimate of the vertical diffusion coefficient.
Kzz=5x10**4/Sqrt(Pbar) m2/s can be used in 1D models of HD 209458b. Moreover,
our models exhibit strong spatial and temporal variability in the tracer
concentration that could result in observable variations during transit or
secondary eclipse measurements. Finally, we apply our model to the case of TiO
in HD209458b and show that the day-night cold trap would deplete TiO if it
condenses into particles bigger than a few microns on the planet's night side,
making it unable to create the observed stratosphere of the planet.
