THE EFFECTS OF FINE PARTICULATES ON THERMAL INFRARED EMISSIVITY SPECTRA: IMPLICATIONS FOR SOLAR SYSTEM AIRLESS BODIES

The surfaces of the Moon and other Solar System airless bodies show regions dominated by regolith, regions dominated by boulders, and other regions a combination of the two. The observed thermal infrared (TIR) spectral signature of a planetary surface and whether or not boulders will dominate the measured spectrum depends on the spatial footprint of the observation and the boulder to regolith ratio within that footprint. However, it is unclear how much fine-particulate regolith is needed to obscure the spectral signatures of boulders. While Diviner TIR compositional observations do not show spectral evidence for the boulders and outcrops we visibly see on the Moon, the OSIRIS-REx Thermal Emission Spectrometer (OTES) observations of Bennu suggest a combination of fine (< 125 μm) and coarse (< 125 μm) particulate materials and/or whole rocks are needed to explain the shape of the average global spectrum.

Here we present TIR emissivity laboratory measurements of (1) a suite of samples that have been ground and sieved to a range of particle size fractions (fine to coarse), (2) physical mixtures of the finest and coarsest particle size fraction of each sample measured and (3) physical mixtures of all of the particle size fractions combined in proportions expected in the regolith of airless bodies. Samples in this study include San Carlos olivine, Allende (CV3), and Murchison (CM2). We made thermal infrared emissivity measurements under simulated airless body environment (SAE) conditions using the Planetary Analogue Surface Chamber for Asteroid and Lunar Environments (PASCALE) at the University of Oxford.

Initial results for the physical mixtures of San Carlos olivine and Allende suggests that only 5 vol.% fine particulates are needed to start obscuring the spectral signatures of the coarse particulates. As increasing amounts of fines are added to the physical mixtures, we observe a reduction in the contrast of the fundamental vibration bands and an increase in contrast in transparent regions of the spectra. These laboratory measurements will better enable current observations of boulder-rich asteroids Bennu and Ryugu, and future observations of the Moon at higher spatial resolutions by L-CIRiS and Lunar Trailblazer.