  WFIRST-CGI is a NASA technology demonstration mission that is charged with
demonstrating key technologies for future exo-Earth imaging missions in space.
In the process, it will obtain images and low-resolution spectra of a handful
to a dozen extrasolar planets and possibly protoplanetary disks. Its
unprecedented contrast levels in the optical will provide astronomers with
their first direct look at mature, Jupiter sized planets at moderate
separations. This paper addresses the question: what science can be done with
such data? An analytic noise model, which is informed by the ongoing
engineering developments, is used to compute maximum achievable signal-to-noise
ratios and scientifically viable integration times for hypothetical star planet
systems, as well as to investigate the constraining power of various
combinations of WFIRST-CGI photometric and spectral observations. This work
introduces two simple models for planetary geometric albedos, which are
inspired largely by the solar system's gas giants. The first planet model is a
hybrid Jupiter-Neptune model, which separately treats the short and long
wavelengths where chromophores and methane dominate absorption, respectively.
The second planet model fixes cloud and haze properties in CoolTLusty to match
Jupiter's albedo spectrum, it then perturbs only the metallicity. MCMC
retrievals performed on simulated observations are used to assess the precision
with which planet model parameters can be measured subject to different
exposure times and observing cases. Fit results for both models'
parameterizations of geometric albedo spectra demonstrate that a rough
indication of the metallicity or methane content should be possible for some
WFIRST-CGI targets. We conclude that real observations will likely be able to
differentiate between extreme cases using these models, but will lack the
precision necessary to uncover subtle trends.
