"It is well documented that individual lava flows of differing age can be distinguished in infrared spectral imagery due to variations in oxidation coatings, depositional silica, glassy rinds and spallation, and texture. Aside from spectral variability due to geochemical processes, however, there are other factors, such as volcaniclastic sediment cover, microscale surface texture (vesicularity), and macroscale surface texture (flow morphology), which affect the spectral and thermophysical variability observed from orbit. Here we build upon previous work by investigating infrared spectral signatures from surface units in and around the December 1974 flow in the SW rift zone. This area allows us to sample relatively fresh flows, older flows, ash deposits and fumarole mineralogies, all which are distinguishable in remotely sensed infrared data sets. Our objective is to understand the dominant factors which might contribute to observed spatial variations in spectral and thermophysical properties on other planetary bodies at both the sample and unit (remotely sensed) scale. Thermal Infrared Multispectral Scanner (TIMS) data (8-12 µm, 2 m/pixel) acquired over the December 1974 flow were processed and spectral units were identified. During a field campaign conducted in April 2013, multiple samples were collected from units of differing age, textures and spectral properties, and photographs and thermal images were used to document the surface textures and temperatures at a macroscale.
A principal components transform was applied to the TIMS data. The first principal component, which accounts for ~82% of the total variance in the scene, is controlled by relative areal abundance of ash deposits, which exhibit relatively low ~11 µm emissivity, versus exposure of the pahoehoe subunit of the 1974 flow, which exhibits relatively low ~9 µm emissivity due to thin silica coatings. Nearly all of the spectral variability in the study region can be described as a mixture of these two components. The second principal component, accounting for ~14% of the total variance, is dominated by elevational differences, arising from imperfect atmospheric correction. The addition of temperature information allows for additional discrimination between spectrally similar units; for example, the 1974 a’a subunit is spectrally similar to relatively unmantled, older undivided flows. However, daytime temperatures between the two units differ by >4 K. Field-based thermal imaging shows that this temperature difference is likely due to increased area of shadowed surfaces associated with the clinker morphology of the a’a subunit.
Thermal emission spectra acquired from samples of each unit show that, despite varying geochemical processes affecting the surface colors and microscale textures, most samples are spectrally similar to opaline silica. The silica spectral signatures do not appear to vary with coating color or age of the flow, with essentially identical signatures observed for white, grey, and blue coatings, as well as for older undivided flows. The most significant spectral differences are between unconsolidated ash samples and lava flows. These results suggest that both geochemical and physical properties strongly affect the remote sensing-based interpretations, and that spectral and temperature information should be used in tandem to guide field traverses."