Thermal infrared emissivity measurements in a simulated lunar environment of major silicate minerals on the Moon
Abstract
Thermal infrared spectra of planetary surfaces have diagnostic features indicative of rock and mineral compositions. These include: (1) the Christiansen Feature (CF), an emissivity maximum resulting from a rapid change in the refractive index just short ward of the fundamental molecular vibration bands, (2) the Reststrahlen bands (RB), the fundamental molecular vibration bands due to Si-O stretching vibrations, and (3) the Transparency Feature (TF), an emissivity minimum caused by volume scattering in a spectral region of relative transparency between the principal RB. Analyses of data returned from Mars orbiting spacecraft and rovers have mainly utilized laboratory thermal infrared spectral libraries of coarse-grained (> 100 μm) samples measured under ambient conditions as these can be added linearly to identify mixtures of minerals and rocks. However, determinations of surface compositions on airless bodies using thermal infrared data, including the Moon, Mercury and asteroids, are complicated by fine-grained particle sizes (< 63 μm) and thermal gradients. Analyses of thermal infrared spectra have demonstrated that as grain size decreases, spectral contrast in the RB decreases while transparency features form at wavelengths shorter than the CF as well as at wavelengths longer than the RB. Furthermore, analyses of thermal infrared spectra under a simulated lunar environment reveal near-surface thermal gradients are formed due to the absence of interstitial gases between regolith grains causing radiative heat transfer to dominate the upper microns of the planetary regolith. As pressure decreases and thermal gradients are introduced, the CF shifts to shorter wavelengths, spectral contrast of the CF increases, and the spectral contrast of the RB decreases. Determinations of surface compositions on airless bodies using thermal infrared spectra thus require accurate applications of lab measurements under realistic environmental conditions. In this study, we build upon previous work by making new measurements focused on the major silicate minerals identified on the Moon (plagioclase solid solution series: albite, oligoclase, andesine, labradorite, and anorthite; clinopyroxene and orthopyroxene; and olivine endmembers: forsterite and fayalite) as well as ilmenite. We characterize the thermal infrared spectral changes between ambient and lunar environmental conditions for the mineral suite and evaluate their application for lunar remote sensing. Specifically, the range of CF positions for the plagioclase solid solution in ambient conditions (7.62 - 8.10 μm) shifts to shorter wavelengths (7.41 - 7.82 μm) under simulated lunar environment conditions. Spectral contrast changes are also observed for all mineral spectra: the RB decrease, the TF short of the CF increases, and the TF at wavelengths longer than the RB decrease and sometimes disappear. Our new measurements demonstrate the high sensitivity of minerals to environmental conditions under which they are measured and provide important constraints on interpreting new thermal infrared datasets of the Moon, including the Diviner Lunar Radiometer Experiment.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2010
- Bibcode:
- 2010AGUFM.P53A1506D
- Keywords:
-
- 6250 PLANETARY SCIENCES: SOLAR SYSTEM OBJECTS / Moon