Validation and comparison of two models based on the Mie theory to predict 8-14 mu m emissivity spectra of mineral surfaces

摘要

Remote sensing is a powerful tool for studying the planetary regolith surfaces through emission in the thermal infrared region (TIR, 8-14 mu m). Theoretical emissivity models are needed to interpret the measured data and eventually to get surface characteristics (such as the refraction index) through model inversion. A new era of orbiting satellites carrying Hyperspectral TIR sensors is coming, and the necessity of understanding the thermal emission of Earth and other planet surfaces at all wavelengths of the electromagnetic spectrum is of prime interest. In this paper we review most of the existing analytical models for predicting the emissivity spectra of minerals for different viewing angles, which are based on the Mie theory, and validated and compared two of them: the Hapke model with two compactness correction methods not tested yet and a model based on the -Eddington approximation, which has not been validated for mineral surfaces. The validation was performed using measurements over two samples rich in quartz and gypsum, respectively. The Hapke model showed the best results when compared with the Warren-Wiscombe-Dozier (WWD) model with respect to measured data, showing a RMSE of 0.04 in emissivity for particle diameter size of a quartz sample greater than 75 mu m. This model also showed improvements with regard to results of past published works, after applying to Mie solutions the compactness correction proposed for the WWD model. These results were confirmed for a gypsum sample, a mineral different to the widely used quartz. Finally, the results showed the deficiencies of both models in simulating mineral emissivity around 8.7 mu m, probably due to the underestimation of multiple scattering for large values of the imaginary part of the refractive index.