  We present radiation transfer models that demonstrate that reflected light
levels from three dimensional (3D) exoplanetary atmospheres can be more than
50% lower than those predicted by models of homogeneous or smooth atmospheres.
Compared to smooth models, 3D atmospheres enable starlight to penetrate to
larger depths resulting in a decreased probability for the photons to scatter
back out of the atmosphere before being absorbed. The increased depth of
penetration of starlight in a 3D medium is a well known result from theoretical
studies of molecular clouds and planetary atmospheres. For the first time we
study the reflectivity of 3D atmospheres as a possible explanation for the
apparent low geometric albedos inferred for extrasolar planetary atmospheres.
Our models indicate that 3D atmospheric structure may be an important
contributing factor to the non-detections of scattered light from exoplanetary
atmospheres. We investigate the self-shadowing radiation transfer effects of
patchy cloud cover in 3D scattered light simulations of the atmosphere of
HD209458b. We find that, for a generic planet, geometric albedos can be as high
as 0.45 in some limited situations, but that in general the geometric albedo is
much lower. We conclude with some explanations on why extrasolar planets are
likely dark at optical wavelengths.
