A small but increasing volume of observations of cometary nuclei has
accumulated during the last 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 0.2-2.0 micrometer wavelength region, photometric
colors in the Johnson-Kron-Cousins UBVRI system, and visual geometric
albedos, for a large number of porous ice-dust mixtures with different
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
of 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, are capable
of reproducing the red colors seen in comets; that ice-dust mixtures
actually can be darker than ice-free regolith under certain circumstances;
and that solar radiation sometimes penetrates to a depth that is comparable
to the region in which diurnal temperature variations occur.
2002. Vol. 156, no 1, 223-248 p.