A LABORATORY STUDY OF THE EFFECTS OF ROUGHNESS ON THE MID-INFRARED SPECTRA OF ROCK SURFACES

One of the fundamental goals of acquiring spectroscopic observations of planetary surfaces is to determine the composition of rocks and minerals, which can provide insight into a wide variety of geological processes that may have shaped the region. The mid-infrared region is able to provide such information because wavelength dependent molecular vibrations within the lattice structure cause absorption and emission of infrared radiation. Emission spectra from most mineral groups contain distinctive absorptions and it is these spectral signatures that can be exploited to determine the mineralogy of a surface. However, previous research has shown that surface roughness can change the contrast or possibly even obscure the diagnostic spectral signatures in the mid-infrared wavelength region. The roughness effect is similar to a blackbody cavity effect in which multiple reflections reduce the number of photons measured at the sensor, resulting in shallowing of spectral features. A solid laboratory basis for interpreting remotely acquired data is imperative to understanding the mineralogy, and thus geological evolution, of a surface. If the detectability of all minerals in the infrared may be dependent in part on their surface roughness, it is critical to establish which classes of rocks (e.g., silicates, carbonates, etc.) display this effect and to what degree. To this end, we will discuss results from our laboratory study, which investigates surface roughness effects on a diverse suite of rocks. We roughen our samples with abrasives and compare initial to roughened spectra. To characterize the surface created from our roughening process we use profilometer data to calculate the length scale dependent root mean square deviation (R_qε). Initial results indicate that 1) rock classes vary in the degree that they are susceptible to abrasion, 2) absorption features shallow with increasing roughness, and 3) we do not see complete obscuration of features in our roughened spectra.