First generation optical coronagraphic telescopes will obtain images of cool
gas and ice giant exoplanets around nearby stars. The albedo spectra of
exoplanets at planet-star separations larger than about 1 AU are dominated by
reflected light to beyond 1 {\mu}m and are punctuated by molecular absorption
features. We consider how exoplanet albedo spectra and colors vary as a
function of planet-star separation, metallicity, mass, and observed phase for
Jupiter and Neptune analogs from 0.35 to 1 {\mu}m. We model Jupiter analogs
with 1x and 3x the solar abundance of heavy elements, and Neptune analogs with
10x and 30x. Our model planets orbit a solar analog parent star at separations
of 0.8 AU, 2 AU, 5 AU, and 10 AU. We use a radiative-convective model to
compute temperature-pressure profiles. The giant exoplanets are cloud-free at
0.8 AU, have H2O clouds at 2 AU, and have both NH3 and H2O clouds at 5 AU and
10 AU. For each model planet we compute moderate resolution spectra as a
function of phase. The presence and structure of clouds strongly influence the
spectra. Since the planet images will be unresolved, their phase may not be
obvious, and multiple observations will be needed to discriminate between the
effects of planet-star separation, metallicity, and phase. We consider the
range of these combined effects on spectra and colors. For example, we find
that the spectral influence of clouds depends more on planet-star separation
and hence temperature than metallicity, and it is easier to discriminate
between cloudy 1x and 3x Jupiters than between 10x and 30x Neptunes. In
addition to alkalis and methane, our Jupiter models show H2O absorption
features near 0.94 {\mu}m. We also predict that giant exoplanets receiving
greater insolation than Jupiter will exhibit higher equator to pole temperature
gradients than are found on Jupiter and thus may have differing atmospheric
dynamics.Comment: 62 pages, 19 figures, 6 tables Accepted for publication in Ap
