Utilizing Thermal Infrared Spectra of Mars for Mission Planning

Reflectance and emission spectroscopy remain the most capable method for mineral identification from orbit. However, to best utilize spectra returned from Mars for mission planning, the current level of understanding of the spectral signature of weathered and rough materials should be considered. Materials measured in the laboratory tend to have band strengths that are stronger than those measured in the field. This difference results mainly from surface roughness, weathering effects, and the presence of a mixture of different materials in the field of view (mixed pixel effect). Thus it is critical to extend spectral measurements from the laboratory into the field in order to understand the spectral behavior of real-world materials. This is best done by utilizing a combination of airborne, field, and laboratory spectra of terrestrial materials, and then applying the lessons learned to spectra recorded of Mars. The ability of any spectral instrument to detect and identify minerals depends on a combination of (1) the mineral's band strength and (2) band width; and (3) the instrument's spectral resolution and (4) signal-to-noise ratio (SNR); (5) atmospheric interference. These points are not always considered in quotes of the detection limits for a given instrument and material. Here we discuss these effects, and explain their relevance for assessing the order in which spectral instruments should be flown to best implement a phased approach to "follow the water," and for landing site selection and sample return. Band depth. The minimum coverage by a material required for detection depends in part on the spectral contrast exhibited by the material. The Thermal Emission Spectrometer (TES) has been returning spectra of Mars covering the wavelength range from ~6 50 μm, and these spectra may be used to examine the spectral signature of the surface, and to set detection limits for materials. For example, in the wavelength range covered by TES, carbonate exhibits clear bands at ~6.5, 11.2, and 35 μm. Figure 1 shows the typical signature in the 11.2 μm region of large particles of carbonate. Strong, clear signatures such as these are expected from massive carbonates, and typical quotes of detection limits are based on these signatures. However, Figure 2 shows the signature measured in the laboratory and the field of a rough, indurated carbonate (calcrete) [1]. Although this material is also composed of carbonate, surface roughness decreases the spectral contrast as a result of cavity and volume scattering effects [1]. Thus the detection limit for a given material also depends on its physical properties, and not just its composition. When detection limits are quoted, it is important to state the assumed band depths of the material.