Albedo and reflection spectra of extrasolar giant planets

We generate theoretical albedo and reflection spectra for a full range of e xtrasolar giant planet (EGP) models, from Jovian to 51 Pegasi class objects . Our albedo modeling utilizes the latest atomic and molecular cross sectio ns, Mie theory treatment of scattering and absorption by condensates, a var iety of particle size distributions, and an extension of the Feautrier tech nique, which allows for a general treatment of the scattering phase functio n. We find that, because of qualitative similarities in the compositions an d spectra of objects within each of five broad effective temperature ranges , it is natural to establish five representative EGP albedo classes. At low effective temperatures (T-eff less than or similar to 150 K) is a class of "Jovian" objects (class I) with tropospheric ammonia clouds. Somewhat warm er class II, or "water cloud," EGPs are primarily affected by condensed H2O . Gaseous methane absorption features are prevalent in both classes. In the absence of nonequilibrium condensates in the upper atmosphere, and with su fficient H2O condensation, class II objects are expected to have the highes t visible albedos of any class. When the upper atmosphere of an EGP is too hot for H2O to condense, radiation generally penetrates more deeply. In the se objects, designated class III or "clear" because of a lack of condensati on in the upper atmosphere, absorption lines of the alkali metals, sodium a nd potassium, lower the albedo significantly throughout the visible. Furthe rmore, the near-infrared albedo is negligible, primarily because of strong CH4 and H2O molecular absorption and collision-induced absorption (CIA) by H-2 molecules. In those EGPs with exceedingly small orbital distance ("roas ters") and 900 K less than or similar to T-eff less than or similar to 1500 K (class IV), a tropospheric silicate layer is expected to exist. In all b ut the hottest (T-eff greater than or similar to 1500 K) or lowest gravity roasters, the effect of this silicate layer is likely to be insignificant b ecause of the very strong absorption by sodium and potassium atoms above th e layer. The resonance lines of sodium and potassium are expected to be sal ient features in the reflection spectra of these EGPs. In the absence of no nequilibrium condensates, we find, in contrast to previous studies, that th ese class IV roasters likely have the lowest visible and Bond albedos of an y class, rivaling the lowest albedos of our solar system. For the small fra ction of roasters with T-eff greater than or similar to 1500 K and/or low s urface gravity (less than or similar to 10(3) cm s(-2); class V), the silic ate layer is located very high in the atmosphere, reflecting much of the in cident radiation before it can reach the absorbing alkali metals and molecu lar species. Hence, the class V roasters have much higher albedos than thos e of class IV. In addition, for class V objects, UV irradiation may result in significant alkali metal ionization, thereby further weakening the alkal i metal absorption lines. We derive Bond albedos (A(B)) and T-eff estimates for the full set of known EGPs. A broad range in both values is found, wit h T-eff ranging from similar to 150 to nearly 1600 K, and A(B) from -0.02 t o 0.8. We find that variations in particle size distributions and condensat ion fraction can have large quantitative, or even qualitative, effects on a lbedo spectra. In general, less condensation, larger particle sizes, and wi der size distributions result in lower albedos. We explore the effects of nonequilibrium condensed products of photolysis a bove or within principal cloud decks. As in Jupiter, such species can lower the UV/blue albedo substantially, even if present in relatively small mixi ng ratios.