  The primordial internal structures of gas giant planets are unknown. Often
giant planets are modeled under the assumption that they are adiabatic,
convective, and homogeneously mixed, but this is not necessarily correct. In
this work, we present the first self-consistent calculation of convective
transport of both heat and material as the planets evolve. We examine how
planetary evolution depends on the initial composition and its distribution,
whether the internal structure changes with time, and if so, how it affects the
evolution. We consider various primordial distributions, different
compositions, and different mixing efficiencies and follow the distribution of
heavy elements in a Jupiter-mass planet as it evolves. We show that a
heavy-element core cannot be eroded by convection if there is a sharp
compositional change at the core-envelope boundary. If the heavy elements are
initially distributed within the planet according to some compositional
gradient, mixing occurs in the outer regions resulting in a compositionally
homogeneous outer envelope. Mixing of heavy materials that are injected in a
convective gaseous envelope are found to mix efficiently. Our work demonstrates
that the primordial internal structure of a giant planet plays a substantial
role in determining its long-term evolution and that giant planets can have
non-adiabatic interiors. These results emphasize the importance of coupling
formation, evolution, and internal structure models of giant planets
self-consistently.
