  We report near-infrared measurements of the terminator region transmission
spectrum and dayside emission spectrum of the exoplanet WASP-12b obtained using
the HST WFC3 instrument. The disk-average dayside brightness temperature
averages about 2900 K, peaking to 3200 K around 1.46 microns. We modeled a
range of atmospheric cases for both the emission and transmission spectrum and
confirm the recent finding by Crossfield et al. (2012b) that there is no
evidence for C/O >1 in the atmosphere of WASP-12b. Assuming a physically
plausible atmosphere, we find evidence that the presence of a number of
molecules is consistent with the data, but the justification for inclusion of
these opacity sources based on the Bayesian Information Criterion (BIC) is
marginal. We also find the near-infrared primary eclipse light curve is
consistent with small amounts of prolate distortion. As part of the calibration
effort for these data, we conducted a detailed study of instrument systematics
using 65 orbits of WFC3-IR grims observations. The instrument systematics are
dominated by detector-related affects, which vary significantly depending on
the detector readout mode. The 256x256 subarray observations of WASP 12
produced spectral measurements within 15% of the photon-noise limit using a
simple calibration approach. Residual systematics are estimated to be less than
70 parts per million.
