A Mid-Infrared Survey of M-type Asteroids with SOFIA+FORCAST

The M-type asteroids are a taxonomic class associated with the iron meteorites, although there appears to be compositional diversity in this taxon. Near-infrared spectroscopy shows the presence of both mafic silicates and phyllosilicates among the M-types, while radar studies indicate high bulk density associated with metal content for a subset of this class. One such high-metal M-type asteroid is (16) Psyche, the target of the NASA mission Psyche. Understanding the nature and potential diversity within this taxon constrains models of material formation and transport in the solar system and provides a framework to interpret results from Psyche. We are conducting a mid-infrared spectroscopic study of the M-type population, as these wavelengths are ideally suited to constrain both mineralogy and thermal properties of asteroids. Using SOFIA+FORCAST, we have obtained 8-μm to 13-μm and 18-μm to 28-μm spectra for five M-type asteroids: (69) Hesperia, (75) Eurydike, (83) Beatrix, (347) Pariana, and (413) Edburga. We are fitting a thermophysical model to these spectra, a process through which we will derive a thermal inertia for each asteroid. Thermal inertia is related to the thermal conductivity of the surface, which is a function of grain texture and composition (i.e., silicate or metallic). Dividing a mid-infrared asteroid spectrum by its modeled thermal flux can reveal an excess of emission due to the presence of silicate grains. We are also interpreting preliminary emissivity spectra derived by fitting a simple thermal model that does not include thermal inertia explicitly. Diameters and albedos derived from simple thermal modeling are consistent with results from NEOWISE¹.

¹Mainzer, A.K., Bauer, J.M., Cutri, R.M., et al. NEOWISE Diameters and Albedos V1.0. EAR-A-COMPIL-5-NEOWISEDIAM-V1.0. NASA PDS, 2016.