  We present an analysis of the bulk composition of the MEarth transiting super
Earth exoplanet GJ 1214b using planet interior structure models. We consider
three possible origins for the gas layer on GJ 1214b: direct accretion of gas
from the protoplanetary nebula, sublimation of ices, and outgassing from rocky
material. Armed only with measurements of the planet mass (M_p=6.55+/-0.98
M_{earth}), radius (R_p=2.678+/-0.13 R_{earth}), and stellar irradiation level,
our main conclusion is that we cannot infer a unique composition. A diverse
range of planet interiors fits the measured planet properties. Nonetheless, GJ
1214b's relatively low average density (rho_p=1870+/-400 kg m^{-3}) means that
it almost certainly has a significant gas component. Our second major
conclusion is that under most conditions we consider GJ 1214b would not have
liquid water. Even if the outer envelope is predominantly sublimated water ice,
the envelope will likely consist of a super-fluid layer sandwiched between
vapor above and plasma (electrically conductive fluid) below at greater depths.
In our models, a low intrinsic planet luminosity (<~2TW) is needed for a water
envelope on GJ 1214b to pass through the liquid phase.
