  The exoplanet GJ1214b presents an interesting example of compositional
degeneracy for low-mass planets. Its atmosphere may be composed of water,
super-solar or solar metallicity material. We present atmospheric circulation
models of GJ1214b for these three compositions, with explicit grey radiative
transfer and an optional treatment of MHD bottom drag. All models develop
strong, superrotating zonal winds (~ 1-2 km/s). The degree of eastward heat
advection, which can be inferred from secondary eclipse and thermal phase curve
measurements, varies greatly between the models. These differences are
understood as resulting from variations in the radiative times at the thermal
photosphere, caused by separate molecular weight and opacity effects. Our
GJ1214b models illustrate how atmospheric circulation can be used as a probe of
composition for similar tidally-locked exoplanets in the
mini-Neptune/waterworld class.
