A laboratory study of the effects of roughness on the thermal infrared spectra of rock surfaces

Abstract

Previous studies have shown that surface roughness can change the contrast or obscure diagnostic spectral signatures of geologic materials in the thermal infrared wavelength region potentially changing their detectability in remote sensing data of planetary surfaces. These studies attributed reduction in spectral contrast to blackbody cavity effects where multiple reflections reduce the number of photons measured at the sensor, resulting in shallowing of spectral features. However, not all rough surfaces have morphologies that create blackbody cavity effects. Here, we focus on investigating the effects of surface roughnesses on the infrared emission spectra of a variety of natural rock samples. Our results indicate that: (1) spectral contrast changes with roughness but complete obscuration of features is not observed; (2) the observed spectral contrast changes are not uniform across the spectrum; (3) spectral feature shapes do not change significantly and feature positions do not change significantly; (4) there is a threshold roughness for contrast changes (which varies across rock types), whereupon further reductions do not occur or in some cases, spectral contrast begins to increase; and (5) the spectral contrast reductions are best described by few surface reflections, not by a cavity effect.