THERMAL EMISSION FROM PARTICULATE SURFACES - A COMPARISON OF SCATTERING MODELS WITH MEASURED SPECTRA

Emissivity spectra of particulate mineral samples are highly dependent on particle size when that size is comparable to the wavelength of li ght emitted (5-50 mu m for the midinfrared). Proper geologic interpret ation of data from planetary infrared spectrometers will require that these particle size effects be well understood. To address this issue, samples of quartz powders were produced with narrow, well-characteriz ed particle size distributions. Mean particle diameters in these sampl es ranged from 15 to 277 mu m. Emission spectra of these powders allow the first detailed comparison of the complex spectral variations with particle size observed in laboratory data with the predictions of rad iative transfer models. Four such models are considered here. Hapke's reflectance theory (converted to emissivity via Kirchhoffs law) is the first model tested. Hapke's more recently published emission theory i s also employed. Both Hapke theories were originally formulated for su rfaces composed of closely packed particles, which unlike the situatio n of interest in this work, are large compared to the wavelength, For this case the particle extinction efficiency approaches unity, and thu s diffraction effects become unimportant. The third model, referred to as the ''Mie/Conel'' model, is a model resurrected from earlier work by others. It uses Mie single scattering with a two-stream approximati on for multiple scattering. This model, like the first, is a converted reflectance model, Mie scattering assumes particles are both spherica l and well separated, which is not true for the quartz powders, but in cludes diffraction effects. The fourth model uses the Mie solution for single scattering by spheres and inputs those results into the multip le scattering formalism of Hapke's emission theory. The results of the four models are considered in relation to the values of the optical c onstants it and h. We have grouped these as class 1 (Ii large), class 2 (k moderate, n similar to 2), class 3 (k small, n similar to 2), and class 4 (k small, n similar to 1). In general, the Mie/Hapke hybrid m odel does best at predicting variations with grain size. In particular , it predicts changes of the correct pattern, although incorrect magni tude, for class 1 bands, where large increases in emissivity with decr easing grain size are observed. This model also does an excellent job on moderate (class 2) and very weak and intraband (class 3) regions, a nd correctly predicts the emission maximum and its invariance with gra in size near the Christiansen frequency (class 3). The Mie/Hapke hybri d model also has the fewest free parameters of the four models examine d, while maintaining the most physical treatment of the radiative tran sfer. The two unmodified Hapke models fail to predict any spectral var iation in strong bands, predict a significant decrease in emissivity w ith grain size near the Christiansen frequency, and overpredict the va riations in moderate bands. The Mie/Conel model performs as well as th e Mie/Hapke Hapke hybrid model in strong bands (class 1) but does not accurately model the behavior of moderate (class 2) and very weak (cla ss 3) bands.