Considerations regarding the colors and low surface albedo of comets using the Hapke methodology

The single scattering albedos (SSA’s) determined for 9P/Tempel 1 are interpreted in terms of the Hapke model of irregular particle scattering efficiencies. Absorption coefficients versus wavelength from 0.31 to 2.5 μm are obtained. It is shown that the colors and exceedingly low reported SSA’s in the UV region of the spectrum below 0.4 μm cannot be reproduced with the geometric Hapke scattering model for irregular particles. However, by increasing the reported SSA’s by a small amount, absorption coefficients for particle radii of 10–100 μm versus wavelength from 0.31 to 2.5 μm can be fitted. Several reasons are given for slightly increasing the SSA’s, such as neglect of the effects of porosity, having a more complex phase function for the particles, uncertainties in the absolute calibration and the uncertainties associated with the complex treatment of surface roughness. The absorption coefficients determined show good agreement with potential surface constituents, Mg rich olivine and pyroxene with some amount of darkening iron or organic component.