Infrared spectra of dust aggregates in cometary comae: Calculation with olivine formed by exothermic chemical reactions

Abstract

Mineralogy of cometary dust plays an important role in understanding the formation and evolution of comets, the most primitive objects in the solar system. A correct interpretation of infrared spectra observed for cometary comae is the key to success in identifying mineral constituents of cometary dust. However, the composition, size, and structure of cometary dust might have been misinterpreted in previous studies, owing to a lack of a unique solution in their analyses of infrared spectra. We present a semianalytic method to compute infrared spectra for large aggregate particles consisting of submicrometer size grains inclusive of crystalline minerals. The method is applied to calculate the absorption cross section of primordial interstellar dust that is processed in a cometary coma. The processed interstellar dust is here modeled as clusters of concentrically stratified spheres consisting of an organic refractory outer mantle, an olivine inner mantle, and an amorphous silicate core. Spectral variations in the absorption cross sections for porous aggregates with a forsterite layer exhibit noticeable features at all the wavelengths where mineral features are observed in the infrared spectra of cometary comae. In contrast, the observed infrared spectra of cometary comae show no evidence for the presence of fayalite in cometary dust. Infrared observations of cometary comae are consistent with the picture that cometary nuclei contain primordial interstellar dust as well as interstellar ices. © 2009. The American Astronomical Society. All rights reserved.