Emissivity spectra of anorthite-forsterite Mixtures in a Simulated … · Anorthite samples were...
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7.5 7.6 7.7 7.8 7.9 8 8.1 8.2 8.3 8.4 8.5 8.6 8.7 8.8 8.9 0 10 20 30 40 50 60 70 80 90 100 Diviner CF Wt% Anorthite SLE Div CF Linear Mixing SLE Diviner CF Ambient Div CF Linear Mixing Ambient Diviner CF 0.75 0.8 0.85 0.9 0.95 1 1.05 400 600 800 1000 1200 1400 Emissivity Wavenumber cm -1 An 100Wt% An 87.4Wt% An 75Wt% An 50Wt% An 25Wt% An 12.6Wt% Fo 100Wt% Sample Cup 120C + Solar Illumination, Vac, Cold Shield 0.75 0.8 0.85 0.9 0.95 1 1.05 400 900 1400 Emissivity Wavenumber cm -1 An 100Wt% An 25Wt% Fo 100Wt% Sample Cup 120C + Solar Illumination, Vac, Cold Shield 0.75 0.8 0.85 0.9 0.95 1 1.05 1150 1200 1250 1300 Anorthite 0.874An+0.126Fo 0.75An+0.25Fo 0.5An+0.5Fo 0.25An+0.75Fo 0.126An+0.874Fo Forsterite 0.75 0.8 0.85 0.9 0.95 1 1.05 1150 1200 1250 1300 Anorthite 0.874An+0.126Fo 0.75An+0.25Fo 0.5An+0.5Fo 0.25An+0.75Fo 0.126An+0.874Fo Forsterite Emissivity spectra of anorthite-forsterite Mixtures in a Simulated Lunar Environment Jessica Arnold 1 , Timothy Glotch 1 , Ian Thomas 2 , Neil Bowles 2 1 Stony Brook University, 2 Oxford University Introduction: We have taken laboratory emissivity spectra of fine-grained powder mixtures of anorthite and forsterite in varying weight percentages, to constrain changes in the Christiansen feature (CF) position with mixture composition. Emission spectra were measured in a chamber that approximates the thermal environment of the lunar surface produced by vacuum conditions [1]. Previously, simulated lunar environment (SLE) spectra had been acquired for pure end- members and compositions along solid solutions. However, the lunar regolith is composed of an intimate mixture of fine-grained materials. These measurements address two questions: 1) How does the CF position as estimated from Diviner data respond to mineral mixtures? 2) In regions where olivine has been detected with VNIR instruments [2,3,4], why olivine is less apparent some areas (Fig 1) than others (Fig 2) in the MIR? Comparison to linear mixing model: Full resolution lab spectra were convolved to Diviner spectral resolution (Fig 8). Then, the CF position was estimated as it would be from Diviner data. The estimated Diviner CF for each mixture under both ambient and SLE conditions is given in Fig 7. Not only are both mixing trends non-linear, but the deviation is different in each case. So while for ambient spectra the CF is depressed faster than expected, in a SLE linear mixing is a pretty good assumption down to about 50 wt% forsterite. Measurements and Results: The SLE emissivity spectra for all mixtures is shown in Fig 5. it is apparent that the mixture spectra are not linear combinations of the pure mineral spectra, especially between 700-1200 cm -1 .The spectral contrast of the mixtures are smaller than for the end- members. This is consistent with the subdued spectral contrast we see in the Diviner data. Fig 5: Emissivity spectra of all mixtures. Fig 6: Emissivity spectra of pure minerals and 25-75 mixture. Fig 7: Calculated Diviner CF of mixtures (symbols) compared with the assumption of linear mixing (lines). The Christiansen feature and composition: The Diviner Lunar Radiometer Experiment (Diviner) has three narrow band “8μm” channels located at 7.80 μm (Ch 3), 8.25 μm (Ch 4), and 8.55 μm (Ch 5). These were chosen to distinguish the Christiansen feature (CF) of silicate minerals [5]. The CF is a mid-IR emissivity maximum that moves to shorter wavelengths with increasing SiO 2 polymerization [6]. As a consequence, felsic minerals will have CF positions at shorter wavelengths than mafic minerals as shown in Fig 3. Conclusions: The difference in mixing behavior between ambient and SLE conditions shows SLE emissivity data is important for constraining the behavior of the CF for mixtures. Linear mixing is a reasonable assumption if the area is forsterite- rich. For the CF position of an anorthite-forsterite mixture to be elevated above average mare (~8.3) [8], there must be greater than about 90 weight percent forsterite present. This information can constrain olivine abundance in regions were it has been detected with VNIR instruments. In the future we would like to collect lunar environment data for a range of olivine compositions, as fayalite has a longer- wavelength CF, and hence a smaller amount would be needed for detection by Diviner. Samples and Methods: The olivine used for this study consisted of mm-sized forsteritic grains determined by electron microprobe to have an ~Fo 93 composition. Anorthite samples were ~1cm single crystals and originated from Miyake Isla, Japan. Based on the measured CF, the anorthite is ~An 80-90. The pure minerals were ground and sieved to less than 63μm, then seven samples were prepared with compositions ranging from 100 wt. % anorthite to 100 wt. % olivine, according to the percentages given in Fig 4. The arrows in Fig 3 point to this maximum, while Diviner channels 3,4 &5 are indicated by red markers. Fig 4 shows how the Ch 3,4&5 emissivities are fit to a parabola to estimate the CF position. References: [1] Thomas, I.R. et al. (2010) 41 st LPSC, 1364. [2] Issacson, P.J. et al. (2011) JGR, 116. [3] Mustard, J.F. et al. (2011) JGR, 116. [4] Yamamoto, S. et al. (2010) Nat. Geosci., 3, 8, 533- 536. [5] Paige, D.A. et al. (2010) SSR, 150, 125-160. [6] Glotch, T.D. et al. (2010) Science, 329, 5998, 1510-1513. [7] Song, E. et al. (2012) 43 rd LPSC, 2553. Greenhagen B.T. et al. (2010) Science, 329, 1507-1509. Fig 3: SLE spectra of various silicates (from Glotch et al. 2010 [6]). Fig 4: Estimating the CF position (from song et al. 2012 [7]). Fig 4. Fig 8: SLE emissivity spectra convolved to Diviner Ch 3,4&5 (left) compared with linear mixing (right). 8.55 μm 7.8 μm 40 km 30 km Fig 1a: Aristarchus crater Diviner CF map. Fig 1b: M 3 mafic index map . Red 1um band depth , green 2 um band depth, blue 1578 nm reflectance. Fig 2a: Pr. Agarum Diviner CF map. Fig 2b: M 3 mafic index map . Red 1um band depth, green 2 um band depth, blue 1578 nm reflectance.
Transcript of Emissivity spectra of anorthite-forsterite Mixtures in a Simulated … · Anorthite samples were...
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Emissivity spectra of anorthite-forsterite Mixtures in a Simulated Lunar Environment Jessica Arnold1, Timothy Glotch1, Ian Thomas2, Neil Bowles2 1Stony Brook University, 2Oxford University
Introduction: We have taken laboratory emissivity spectra of fine-grained powder mixtures of anorthite and forsterite in varying weight percentages, to constrain changes in the Christiansen feature (CF) position with mixture composition. Emission spectra were measured in a chamber that approximates the thermal environment of the lunar surface produced by vacuum conditions [1]. Previously, simulated lunar environment (SLE) spectra had been acquired for pure end-members and compositions along solid solutions. However, the lunar regolith is composed of an intimate mixture of fine-grained materials. These measurements address two questions: 1) How does the CF position as estimated from Diviner data respond to mineral mixtures? 2) In regions where olivine has been detected with VNIR instruments [2,3,4], why olivine is less apparent some areas (Fig 1) than others (Fig 2) in the MIR?
Comparison to linear mixing model: Full resolution lab spectra were convolved to Diviner spectral resolution (Fig 8). Then, the CF position was estimated as it would be from Diviner data. The estimated Diviner CF for each mixture under both ambient and SLE conditions is given in Fig 7. Not only are both mixing trends non-linear, but the deviation is different in each case. So while for ambient spectra the CF is depressed faster than expected, in a SLE linear mixing is a pretty good assumption down to about 50 wt% forsterite.
Measurements and Results: The SLE emissivity spectra for all mixtures is shown in Fig 5. it is apparent that the mixture spectra are not linear combinations of the pure mineral spectra, especially between 700-1200 cm-1.The spectral contrast of the mixtures are smaller than for the end-members. This is consistent with the subdued spectral contrast we see in the Diviner data.
Fig 5: Emissivity spectra of all mixtures.
Fig 6: Emissivity spectra of pure minerals and 25-75 mixture.
Fig 7: Calculated Diviner CF of mixtures (symbols) compared with the assumption of linear mixing (lines).
The Christiansen feature and composition: The Diviner Lunar Radiometer Experiment (Diviner) has three narrow band “8μm” channels located at 7.80 µm (Ch 3), 8.25 µm (Ch 4), and 8.55 µm (Ch 5). These were chosen to distinguish the Christiansen feature (CF) of silicate minerals [5]. The CF is a mid-IR emissivity maximum that moves to shorter wavelengths with increasing SiO2 polymerization [6]. As a consequence, felsic minerals will have CF positions at shorter wavelengths than mafic minerals as shown in Fig 3.
Conclusions: The difference in mixing behavior between ambient and SLE conditions shows SLE emissivity data is important for constraining the behavior of the CF for mixtures. Linear mixing is a reasonable assumption if the area is forsterite-rich. For the CF position of an anorthite-forsterite mixture to be elevated above average mare (~8.3) [8], there must be greater than about 90 weight percent forsterite present. This information can constrain olivine abundance in regions were it has been detected with VNIR instruments. In the future we would like to collect lunar environment data for a range of olivine compositions, as fayalite has a longer-wavelength CF, and hence a smaller amount would be needed for detection by Diviner.
Samples and Methods: The olivine used for this study consisted of mm-sized forsteritic grains determined by electron microprobe to have an ~Fo93 composition. Anorthite samples were ~1cm single crystals and originated from Miyake Isla, Japan. Based on the measured CF, the anorthite is ~An 80-90. The pure minerals were ground and sieved to less than 63μm, then seven samples were prepared with compositions ranging from 100 wt. % anorthite to 100 wt. % olivine, according to the percentages given in Fig 4.
The arrows in Fig 3 point to this maximum, while Diviner channels 3,4 &5 are indicated by red markers. Fig 4 shows how the Ch 3,4&5 emissivities are fit to a parabola to estimate the CF position.
References: [1] Thomas, I.R. et al. (2010) 41st LPSC, 1364. [2] Issacson, P.J. et al. (2011) JGR, 116. [3] Mustard, J.F. et al. (2011) JGR, 116. [4] Yamamoto, S. et al. (2010) Nat. Geosci., 3, 8, 533- 536. [5] Paige, D.A. et al. (2010) SSR, 150, 125-160. [6] Glotch, T.D. et al. (2010) Science, 329, 5998, 1510-1513. [7] Song, E. et al. (2012) 43rd LPSC, 2553. Greenhagen B.T. et al. (2010) Science, 329, 1507-1509.
Fig 3: SLE spectra of various silicates (from Glotch et al. 2010 [6]).
Fig 4: Estimating the CF position (from song et al. 2012 [7]).
Fig 4.
Fig 8: SLE emissivity spectra convolved to Diviner Ch 3,4&5 (left) compared with linear mixing (right).
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Fig 1a: Aristarchus crater Diviner CF map.
Fig 1b: M3 mafic index map . Red 1um band depth , green 2 um band depth, blue 1578 nm reflectance.
Fig 2a: Pr. Agarum Diviner CF map.
Fig 2b: M3 mafic index map . Red 1um band depth, green 2 um band depth, blue 1578 nm reflectance.
