REFINING THE CHEMISTRY OF MARS: COMPARING VNIR AND TIR SPECTRA OF CLAY-BEARING ROCKS

Aqueous processes are inferred on Mars in numerous locations where clays have been identified using OMEGA and CRISM data [e.g. 1,2]. While most studies use lab spectra of pure minerals as a basis for identification of clays, we have measured visible/near-infrared (VNIR) reflectance spectra, thermal region infrared (TIR) emissivity spectra, and gathered x-ray diffraction (XRD) data of clay-bearing terrestrial rocks. This process will allow coordinated analysis of multiple data sets for comparison with Martian spectra and CheMin data. Stretching overtone bands of OH near 1.4 µm and of H₂0 near 1.9 µm, as well as OH stretching and bending combination bands near 2.20-2.21 µm for Al/Si, 2.29 µm for Fe3+, and 2.30-2.31 µm for Mg are VNIR signatures indicative of clay minerals. TIR signatures include a doublet near 450-600 cm^-1 for dioctahedral phyllosilicates and a single band near 475 cm^-1for trioctahedral phyllosilicates [e.g. 3].

For this study three Al-rich clay-bearing rocks, two Fe-rich clay-bearing rocks, and two altered quartz-bearing rocks were measured for VNIR, TIR, and XRD analyses. An example of the usefulness of these comparisons can be seen in the Delamar Mine beidellite-bearing rock. The Delamar mine rock exhibits bands near 1.4, 1.9 and 2.2 µm, correlating with the XRD analysis showing that the rock is primarily smectite. The XRD data also shows that the Delamar mine rock is ~5-10% quartz, which does not exhibit features in the VNIR region but can be seen in the TIR data as an Si-O stretching vibration doublet with endpoints in the 1078-1212 cm^-1 range. However, montmorillonite and opal (both typical components of smectite bearing rocks) exhibit endpoints near this region as well. Obtaining library data sets for rocks that contain key components of minerals that are found on Mars may enable more refined and accurate spectroscopy data analysis than modeling alone, allowing for a better interpretation of the history of the nearest habitable planet to Earth.

References: [1] Murchie et al. (2009) JGR 114, doi:10.1029/2009JE003342. [2] Carter et al. (2010) Science 328, 1682. [3] Michalski et al. (2005) Icarus 174, 161.