Emission Measurements of Lunar Analogues Measured in a Simulated Lunar Environment for Interpretation of Data Returned from the Diviner Lunar Radiometer on NASA’s Lunar Reconnaissance Orbiter

A lunar thermal environment simulator has been constructed, in order to measure emission spectra of lunar analogue minerals in the same thermal environment as is present on the surface of the Moon. This data is directly comparable to measurements made by the Diviner instrument, currently in orbit around the Moon onboard the Lunar Reconnaissance Orbiter (LRO), allowing the composition of the Moon’s surface to be further determined, as part of the Diviner Compositional Investigation[1]. Diviner is a nine-channel infrared mapping radiometer, currently making high resolution (~160m per pixel) observations of the lunar surface from a ~50km lunar orbit[2]. The instrument’s filters are designed to map the temperature, mineralogy, albedo, rock abundance and bulk thermal properties of the surface regolith (soil)[2]. Three channels, located around 8µm, are capable of determining the spectral location of the Christiansen Feature (CF)[3], the primary spectral feature observed in mid-infrared measurements of the Moon[4,5]. Four other channels, from 13 to 400µm, are capable of mapping variations in emissivity of the lunar surface. The CF of a feldspathic mineral is located at a shorter wavelength than a mafic mineral, hence this emissivity maximum can be used as a compositional indicator[6,7]. It is observed as an emissivity maximum, and is enhanced by the lunar environment. In the top few hundreds of microns, at low to mid-latitudes during the daytime, large thermal gradients are induced due to very low heat transport within the lunar regolith[8,9,10,11]. The surface is cooled as it radiates to cold space, but soil transparency in the spectra around the CF region causes radiation to be emitted from the deeper, hotter layers, producing an emission maximum. Regolith grain size, mixing ratios, and the lack of atmosphere on the Moon also affect the shape and location of the CF[6,7,9,12]. The lunar thermal environment simulator creates an equivalent thermal gradient in lunar analogue minerals in the laboratory, by heating the minerals to ~400K while they are surrounded by a shroud cooled to ~120K by liquid nitrogen, all in a vacuum of <10^(-4)mbar. The simulator is attached to a Brüker IFS66v Fourier Transform Spectrometer at the University of Oxford. This setup is capable of measuring emission spectra in a simulated lunar environment from the mid to far-infrared of a wide range of minerals of various grain size distributions. This presentation includes the minerals albite, andesine, anorthite, augite, bytownite, diopside, enstatite, fayalite, forsterite, ilmenite, quartz etc, and several mineral mixtures. References: [1] Greenhagen B.T. & Paige D.A.(2009) LPS XL; [2] Paige, D.A. et al.(2009) Space Sci.Rev.; [3] Greenhagen B.T. & Paige D.A.(2006) LPS XXXVII; [4] Murcray F.H. et al.(1970) JGR, 75; [5] Lucey P.G.(1991) LPS XXI; [6] Salisbury J.W. & Walter L.S.(1989) JGR, 94; [7] Cooper B.L. et al.(2002) JGR, 107; [8] Logan L.M. & Hunt G.R.(1970) Science, 169; [9] Logan L.M. et al.(1973) JGR, 79; [10] Henderson B.G. et al.(1996) JGR, 101; [11]Henderson B.G. & Jakosky B.M.(1997) JGR, 102; [12]Nash D.B. et al.(1993) JGR, 98