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The classical way to treat absorption of solar light in thermophysical modeling of cometary nuclei (and other ice-rich bodies such as jovian satellites) has been to assume complete opaqueness of the surface material. However, as shown by Davidsson and Skorov (2002, Icarus156, 223–248), substantial light penetration can occur in porous ice even if it is very dusty, implying that gradual absorption of energy in a surface layer should be accounted for.We present a thorough comparison between a surface energy absorption model and a layer energy absorption model, for various combinations of heliocentric distances, conductivities, opacities, pore sizes, and rotational periods relevant for cometary nuclei, by fully solving the coupled differential equations of heat transfer and gas diffusion. We find substantial differences between the models in terms of gas production rate, thermal lag angle, surface temperature, and the origin of coma molecules. For example, the surface energy absorption model overestimates the total gas production by a factor of 2–7, underestimates the lag angle by a factor of 2–3, and places the origin of coma molecules at the surface, instead of the near-surface interior. [Copyright &y& Elsevier]

A small but increasing volume of observations of cometary nuclei has accumulated during the past two decades. This development is accelerating with upcoming space missions such as Stardust, Contour, and Rosetta. In response to the growing need for a theoretical understanding of optical properties of cometary nuclei, we have calculated synthetic reflectance spectra in the wavelength region 0.2–2.0 μm, photometric colors in the Johnson–Kron–Cousins UBVRI system, and visual geometric albedos for a large number of porous ice-dust mixtures with differing composition, regolith grain sizes, and grain morphologies, such as core-mantle grains, dense clusters of such grains, and large irregular particles with internal scatterers. The calculations are based on Mie theory, the discrete dipole approximation, Hapke theory, and a numerical solution to the equation of radiative transfer in particulate media. In addition, wavelength-integrated directional-hemispherical albedos and flux attenuation profiles in the regolith as functions of depth have been calculated in order to improve the energy budget and treatment of energy boundary conditions in thermal models of cometary nuclei.Our results are compared with spectra and colors of observed cometary nuclei. Our main conclusions are that only regolith consisting of relatively large core-mantle grains, or clusters of smaller core-mantle grains, is capable of reproducing the red colors seen in comets; that ice-dust mixtures actually can be darker than ice-free regolith in certain circumstances; and that solar radiation sometimes penetrates to a depth that is comparable to the region in which diurnal temperature variations occur. [Copyright &y& Elsevier]