Spectral Irradiance Calibration in the Infrared. VIII. 5-14 Micron Spectroscopy of the Asteroids Ceres, Vesta, and Pallas

     We describe our efforts to seek "closure" in our infrared absolute calibration scheme by comparing spectra of asteroids, absolutely calibrated through reference stars, with "standard thermal models" and "thermophysical models" for these bodies. Our use of continuous 5–14 μm airborne spectra provides complete sampling of the rise to, and peak, of the infrared spectral energy distribution and constrains these models. Such models currently support the absolute calibration of the Infrared Space Observatory Imaging Photopolarimeter (ISOPHOT) at far-infrared wavelengths (as far as 300 μm) and contribute to that of the Mid-Infrared Spectrometer on the Infrared Telescope in Space in the 6–12 μm region. The best match to our observed spectra of Ceres and Vesta is a standard thermal model using a beaming factor of unity. We also report the presence of three emissivity features in Ceres that may complicate the traditional model extrapolation to the far-infrared from contemporaneous ground-based N-band photometry that is used to support calibration of, for example, ISOPHOT. While identification of specific materials that cause these features is not made, we discuss families of minerals that may be responsible.