Mid-infrared spectroscopy of zodiacal emission with AKARI/IRC

Interplanetary dust (IPD) is thought to be recently supplied from asteroids and comets. Grain properties of the IPD can give us information about the environment in the protosolar system, and can be traced from the shapes of silicate features around 10 mu m seen in the zodiacal emission spectra. We analyzed mid-infrared slit-spectroscopic data of the zodiacal emission in various sky directions obtained with the Infrared Camera on board the Japanese AKARI satellite. After we subtracted the contamination due to instrumental artifacts, we successfully obtained high signal-to-noise spectra and have determined detailed shapes of excess emission features in the 9-12 mu m range in all sky directions. According to a comparison between the feature shapes averaged over all directions and the absorption coefficients of candidate minerals, the IPD was found to typically include small silicate crystals, especially enstatite grains. We also found variations in the feature shapes and the related grain properties among the different sky directions. From investigations of the correlation between feature shapes and the brightness contributions from dust bands, the IPD in dust bands seems to have a size frequency distribution biased toward large grains and shows indications of hydrated minerals. The spectra at higher ecliptic latitudes showed a stronger excess, which indicates an increase in the fraction of small grains included in the line of sight at higher ecliptic latitudes. If we focus on the dependence of detailed feature shapes on ecliptic latitudes, the IPD at higher ecliptic latitudes was found to have a lower olivine/(olivine + pyroxene) ratio for small amorphous grains. The variation of the mineral composition of the IPD in different sky directions may imply different properties of the IPD from different types of parent bodies, because the spatial distribution of the IPD depends on the type of the parent body.