Analogue Materials Measured Under Simulated Lunar and Asteroid Environments: Application to Thermal Infrared Measurements of Airless Bodies

Remote sensing observations provide key insights into the composition and evolution of planetary surfaces. A fundamentally important component to any remote sensing study of planetary surfaces is laboratory measurements of well-characterized samples measured under the appropriate environmental conditions. The near-surface vacuum environment of airless bodies like the Moon and asteroids creates a thermal gradient in the upper hundred microns of regolith. Lab studies of particulate rocks and minerals as well as selected lunar soils under vacuum and lunar-like conditions have identified significant effects of this thermal gradient on thermal infrared (TIR) spectral measurements [e.g. Logan et al. 1973, Salisbury and Walter 1989, Thomas et al. 2010, Donaldson Hanna et al. 2012]. Compared to ambient conditions, these effects include: (1) the Christiansen feature (CF), an emissivity maximum diagnostic of mineralogy and average composition, shifts to higher wavenumbers and (2) an increase in spectral contrast of the CF relative to the Reststrahlen bands (RB), the fundamental molecular vibration bands due to Si-O stretching and bending. Such lab studies demonstrate the high sensitivity of TIR emissivity spectra to environmental conditions under which they are measured. The Asteroid and Lunar Environment Chamber (ALEC) is the newest addition to the RELAB at Brown University. The vacuum chamber simulates the space environment experienced by the near-surface soils of the Moon and asteroids. The internal rotation stage allows for six samples and two blackbodies to be measured without breaking vacuum (<10-4 mbar). Liquid nitrogen is used to cool the interior of the chamber, creating a cold, low emission environment (mimicking the space environment) for heated samples to radiate into. Sample cups can be heated in one of three configurations: (1) from below using heaters embedded in the base of the sample cup, (2) from above using a solar-like radiant heat source, and (3) from below and above to allow the magnitude of the thermal gradient to be examined. ALEC is connected to RELAB's Thermo Nicolet FTIR spectrometer which allows laboratory emissivity spectra to be collected at a resolution of 4 cm-1 over a nominal ~400 - 7400 cm-1 spectral range. An initial set of experiments have been run to understand how variations in the internal chamber pressure, power from the solar-like halogen lamp, and sample cup temperature affect spectral measurements of fine particulate (< 25 μm) mineral separates. These early results corroborate previous lab measurements showing the sensitivity of TIR spectra to the conditions under which they are measured and for the first time illustrates how the pressure and the thermal gradient each contribute to the changes in TIR spectral measurements. Spectral measurements of lunar soils under varying controlled conditions will be compared with Diviner data to understand how to accurately simulate conditions of the real near-surface environment of the Moon. Once conditions are constrained future spectral measurements will focus on building a spectral library of well-characterized minerals, rocks, soils, and meteorites measured under lunar environmental conditions. Such measurements are essential to interpret current TIR datasets like Diviner and future missions like OSIRIS-REx.