examining spectral variations in localized lunar dark mantle deposits
46th Lunar and Planetary Science Conference (2015)
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EXAMINING SPECTRAL VARIATIONS IN LOCALIZED LUNAR DARK MANTLE DEPOSITS. E. R. Jawin1,2, S. Besse2, L. R. Gaddis3, J. M. Sunshine4, J. W. Head1, and S. Mazrouei5, 1Dept. Earth, Environ., Planetary Sci., Brown Univ., Providence, RI 02912 USA, 2European Space Res. & Tech. Ctr, Noordwijk, Netherlands, 3 USGS Astrogeo. Sci. Ctr, Flagstaff, AZ, USA, 4Dept. Astronomy, Univ. of Maryland, College Park, MD, USA, 5 Dept. Earth Sci., Univ. of Toronto, ON, Canada ([email protected]). analyses from the DMDs in Alphonsus, J. Herschel, Introduction: Lunar deposits of pyroclastic materiand Oppenheimer craters are compared to spectra of al, referred to as dark mantle deposits (DMDs), are the DMDs in Lavoisier and Walther A craters [8,14]. fine-grained, low-albedo volcanic materials [1]. Previous studies identified over 100 DMDs across the lunar Results: Alphonsus DMD. Spectra were extracted surface, ranging from ~10-50,000 km2 [2,3]. The largfrom the three largest sub-deposits in the Alphonsus est of these are known as “regional” DMDs, while the DMD (Fig. 1). There are variations in albedo within smaller are “localized” DMDs [4]. DMDs are believed each sub-deposit (inset, Fig. 1A). Spectra of the subto represent the eruption of gas-rich magmas which deposits were taken at various distances from the vent originated from mafic parent melts sourced from primto examine this variation. Unit names (Light, Intermeitive, unfractionated magmas [5]. Regional DMDs diate, etc.) indicate changes in albedo across the DMD. were found to contain iron- and titanium-rich glass, Spectra taken close to the vent show 1 μm bands and mafic devitrified beads [4,6,7]. Iron-rich volcanic shifted to longer wavelengths and deeper absorption glass has also been identified in localized DMDs [8,9]. depths (Fig. 1). This suggests that inside the vent and Volcanic glasses are important in characterizing the in proximity (Dark unit), spectra are dominated by lunar interior and understanding the origin and evolumafic pyroclastic materials. As distance from the vent tion of basaltic magmatism on the Moon. Understandincreases, spectra become contaminated by noritic ing the mineralogical diversity and variations in cryscrater floor material. This variation is interpreted to tallinity in localized DMDs can aid in identifying the represent variable thickness of the deposit. DMD thinsources and eruption mechanics of these deposits. ning also agrees with the albedo variation. Three localized DMDs were analyzed using data Spectra were also taken in Mare Nubium (Fig. 1D). from the Moon Mineralogy Mapper (M3) [10] to (a) The maria show a slightly shorter 1 μm band position, document DMD mineralogy, (b) characterize the mineralogical variation within and between deposits, and (c) compare the mineralogy of DMDs to nearby mare basalts, in an attempt to constrain mineralogical diversity in localized DMDs. Data and Targets: Spectral analyses of the localized DMDs are based on observations from M3, a visible and near-infrared spectrometer covering the spectral range 0.4-3 μm at a spatial resolution of ~140280 m/pixel [10–12]. A continuumremoval was applied to the spectra, approximated by a straight line fit between 0.73-1.62 μm and 1.62-2.58 μm for the 1 and 2 μm bands, respectively [8]. Mosaics of context images from the Kaguya (SELENE) Terrain Camera (TC) support the spectroscopic products analyzed here [13]. Figure 1. Alphonsus DMD. (A) Absolute reflectance of DMDs; inset: outlines of each spectral unit in The localized DMDs occur in the W sub-deposit. White dashed lines outline the volcanic vents, while areas outlined in red are insolatfloor-fractured craters, including ed portions used in the analysis. (B) Continuum-removed spectra from A. (C) RGB color composite map Alphonsus, J. Herschel, and Oppenof the crater: IBD1000 nm; G: BD1900 nm; B: R1580 nm. (D) Continuum-removed spectra of the heimer crater. Each localized DMD DMD, crater floor, and Mare Nubium basalt. is located near a mare unit. Spectral
46th Lunar and Planetary Science Conference (2015)
a slightly longer 2 μm band position, and a narrower, symmetric band shape relative to the DMDs. This distinction in spectra signifies a variation in mineralogy or internal structure (crystallinity) between the two units. Previous analyses [15] interpreted the wide, asymmetric 1 μm absorption in DMDs such as Alphonsus to indicate olivine. However, olivine does not have a 2 μm absorption feature, and the inclusion of olivine or other mafic minerals would not explain the band center shift to shorter wavelengths at 2 μm. We conclude this spectral signature is indicative of amorphous materials in the form of iron-enriched volcanic glass (e.g., [8]). J. Herschel and Oppenheimer DMD. The J. Herschel DMD spectra show similar features to Alphonsus: the shape and location of the 1 and 2 μm bands are distinct from the crater floor and nearby maria, and are consistent with mafic materials containing glass (Fig. 2). Two of the smallest DMD sub-deposits in Oppenheimer crater (N, SSE) show a similar variation in albedo with distance as in the Alphonsus DMD, which indicates spectral variation and thinning of the deposit. The DMD spectra appear distinct from the surrounding mare deposit, with a similar shift in the 1 and 2 μm bands to longer and shorter wavelengths, respectively, as in Alphonsus and J. Herschel DMDs. This indicates volcanic glass in the Oppenheimer DMDs, although the relative strength of the 1 and 2 μm bands is different than that in the Alphonsus and J. Herschel glassy spectra. In addition, two new DMDs were identified to the northeast of Oppenheimer, in Dryden S and T craters. These DMDs are spectrally similar to the Oppenheimer DMDs. Discussion: Assuming the spectral signatures in the DMDs represent the presence of volcanic glass, it is possible to distinguish between glass compositions using a combination of the 1 and 2 μm band positions and 2 μm to 1 μm band depth ratios (not shown here). Based on these two metrics, the spectral signatures of the Alphonsus and J. Herschel DMDs are similar to the green glass returned from the Apollo 15 landing site [16], while the Oppenheimer spectra resemble orange glass returned from the Apollo 17 landing site [7]. These variations may indicate the presence of olivine (in the case of Alphonsus and J. Herschel DMDs) and/or ilmenite (Oppenheimer DMD). In addition to a thinning of the DMD, the variation in spectral signatures across the isolated sub-deposits may indicate a variable amount of volcanic glass in the DMDs, with a higher concentration of glass closer to the central vent. This may be evidence of a sporadic, potentially repetitive explosive emplacement style typical of vulcanian eruptions on Earth. In larger, more continuous Hawaiian eruptions, temperatures and opti-
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cal densities increase towards the center of the eruptive plume, leading to more crystalline material closer to the eruptive vent [16]. However, vulcanian eruptions are smaller and sporadic, with eruptive optical densities and temperatures low enough to rapidly quench erupting magma, creating glass [17]. Assuming this formation mechanism is representative of most localized DMDs, volcanic glass may be detectable in other similar deposits on the Moon. References: 1. Wilson and Head, JGR 86, 2971–3001 (1981). 2. Gaddis et al., Icarus. 161, 262–80 (2003). 3. Gustafson et al., JGR 117 (2012). 4. Gaddis et al., Icarus. 61, 461–89 (1985). 5. Delano and Livi, LPSXII (1981), 226–8. 6. Pieters et al., JGR 78, 5867–75 (1973). 7. Adams et al., Proc. Lun. Conf. 5th (1974), 171–86. 8. Besse et al., JGR 119, (2014). 9. Horgan et al., Icarus. 234, 132–54 (2014). 10. Pieters et al., Curr. Sci. 96, 500–5 (2009). 11. Boardman et al., JGR. 116 (2011). 12. Besse et al., Icarus. 222, 229– 242 (2013). 13. Haruyama et al., EPSL. 60, 243 (2008). 14. Souchon et al., Icarus. 225, 1–14 (2013). 15. Hawke et al., LPSC, 19th, 255–68 (1989). 16. Delano, LPSC, 10th, 275–300 (1979). 17. Weitz et al., Meteorit. Planet. Sci. 34, 527–40 (1999). 18. Head and Wilson, LPSC 10th, 2861–97 (1979).
Figure 2. (A) Continuum-removed spectra of the DMDs, including Walther A and Lavoisier DMDs [11,16], and Mare Frigoris (near J. Herschel). (B) Continuum-removed spectra of the glassy DMDs, relative to lunar pyroclastic orange glass and green glass. Spectra are offset for clarity.
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EXAMINING SPECTRAL VARIATIONS IN LOCALIZED LUNAR DARK MANTLE DEPOSITS. E. R. Jawin1,2, S. Besse2, L. R. Gaddis3, J. M. Sunshine4, J. W. Head1, and S. Mazrouei5, 1Dept. Earth, Environ., Planetary Sci., Brown Univ., Providence, RI 02912 USA, 2European Space Res. & Tech. Ctr, Noordwijk, Netherlands, 3 USGS Astrogeo. Sci. Ctr, Flagstaff, AZ, USA, 4Dept. Astronomy, Univ. of Maryland, College Park, MD, USA, 5 Dept. Earth Sci., Univ. of Toronto, ON, Canada ([email protected]). analyses from the DMDs in Alphonsus, J. Herschel, Introduction: Lunar deposits of pyroclastic materiand Oppenheimer craters are compared to spectra of al, referred to as dark mantle deposits (DMDs), are the DMDs in Lavoisier and Walther A craters [8,14]. fine-grained, low-albedo volcanic materials [1]. Previous studies identified over 100 DMDs across the lunar Results: Alphonsus DMD. Spectra were extracted surface, ranging from ~10-50,000 km2 [2,3]. The largfrom the three largest sub-deposits in the Alphonsus est of these are known as “regional” DMDs, while the DMD (Fig. 1). There are variations in albedo within smaller are “localized” DMDs [4]. DMDs are believed each sub-deposit (inset, Fig. 1A). Spectra of the subto represent the eruption of gas-rich magmas which deposits were taken at various distances from the vent originated from mafic parent melts sourced from primto examine this variation. Unit names (Light, Intermeitive, unfractionated magmas [5]. Regional DMDs diate, etc.) indicate changes in albedo across the DMD. were found to contain iron- and titanium-rich glass, Spectra taken close to the vent show 1 μm bands and mafic devitrified beads [4,6,7]. Iron-rich volcanic shifted to longer wavelengths and deeper absorption glass has also been identified in localized DMDs [8,9]. depths (Fig. 1). This suggests that inside the vent and Volcanic glasses are important in characterizing the in proximity (Dark unit), spectra are dominated by lunar interior and understanding the origin and evolumafic pyroclastic materials. As distance from the vent tion of basaltic magmatism on the Moon. Understandincreases, spectra become contaminated by noritic ing the mineralogical diversity and variations in cryscrater floor material. This variation is interpreted to tallinity in localized DMDs can aid in identifying the represent variable thickness of the deposit. DMD thinsources and eruption mechanics of these deposits. ning also agrees with the albedo variation. Three localized DMDs were analyzed using data Spectra were also taken in Mare Nubium (Fig. 1D). from the Moon Mineralogy Mapper (M3) [10] to (a) The maria show a slightly shorter 1 μm band position, document DMD mineralogy, (b) characterize the mineralogical variation within and between deposits, and (c) compare the mineralogy of DMDs to nearby mare basalts, in an attempt to constrain mineralogical diversity in localized DMDs. Data and Targets: Spectral analyses of the localized DMDs are based on observations from M3, a visible and near-infrared spectrometer covering the spectral range 0.4-3 μm at a spatial resolution of ~140280 m/pixel [10–12]. A continuumremoval was applied to the spectra, approximated by a straight line fit between 0.73-1.62 μm and 1.62-2.58 μm for the 1 and 2 μm bands, respectively [8]. Mosaics of context images from the Kaguya (SELENE) Terrain Camera (TC) support the spectroscopic products analyzed here [13]. Figure 1. Alphonsus DMD. (A) Absolute reflectance of DMDs; inset: outlines of each spectral unit in The localized DMDs occur in the W sub-deposit. White dashed lines outline the volcanic vents, while areas outlined in red are insolatfloor-fractured craters, including ed portions used in the analysis. (B) Continuum-removed spectra from A. (C) RGB color composite map Alphonsus, J. Herschel, and Oppenof the crater: IBD1000 nm; G: BD1900 nm; B: R1580 nm. (D) Continuum-removed spectra of the heimer crater. Each localized DMD DMD, crater floor, and Mare Nubium basalt. is located near a mare unit. Spectral
46th Lunar and Planetary Science Conference (2015)
a slightly longer 2 μm band position, and a narrower, symmetric band shape relative to the DMDs. This distinction in spectra signifies a variation in mineralogy or internal structure (crystallinity) between the two units. Previous analyses [15] interpreted the wide, asymmetric 1 μm absorption in DMDs such as Alphonsus to indicate olivine. However, olivine does not have a 2 μm absorption feature, and the inclusion of olivine or other mafic minerals would not explain the band center shift to shorter wavelengths at 2 μm. We conclude this spectral signature is indicative of amorphous materials in the form of iron-enriched volcanic glass (e.g., [8]). J. Herschel and Oppenheimer DMD. The J. Herschel DMD spectra show similar features to Alphonsus: the shape and location of the 1 and 2 μm bands are distinct from the crater floor and nearby maria, and are consistent with mafic materials containing glass (Fig. 2). Two of the smallest DMD sub-deposits in Oppenheimer crater (N, SSE) show a similar variation in albedo with distance as in the Alphonsus DMD, which indicates spectral variation and thinning of the deposit. The DMD spectra appear distinct from the surrounding mare deposit, with a similar shift in the 1 and 2 μm bands to longer and shorter wavelengths, respectively, as in Alphonsus and J. Herschel DMDs. This indicates volcanic glass in the Oppenheimer DMDs, although the relative strength of the 1 and 2 μm bands is different than that in the Alphonsus and J. Herschel glassy spectra. In addition, two new DMDs were identified to the northeast of Oppenheimer, in Dryden S and T craters. These DMDs are spectrally similar to the Oppenheimer DMDs. Discussion: Assuming the spectral signatures in the DMDs represent the presence of volcanic glass, it is possible to distinguish between glass compositions using a combination of the 1 and 2 μm band positions and 2 μm to 1 μm band depth ratios (not shown here). Based on these two metrics, the spectral signatures of the Alphonsus and J. Herschel DMDs are similar to the green glass returned from the Apollo 15 landing site [16], while the Oppenheimer spectra resemble orange glass returned from the Apollo 17 landing site [7]. These variations may indicate the presence of olivine (in the case of Alphonsus and J. Herschel DMDs) and/or ilmenite (Oppenheimer DMD). In addition to a thinning of the DMD, the variation in spectral signatures across the isolated sub-deposits may indicate a variable amount of volcanic glass in the DMDs, with a higher concentration of glass closer to the central vent. This may be evidence of a sporadic, potentially repetitive explosive emplacement style typical of vulcanian eruptions on Earth. In larger, more continuous Hawaiian eruptions, temperatures and opti-
1398.pdf
cal densities increase towards the center of the eruptive plume, leading to more crystalline material closer to the eruptive vent [16]. However, vulcanian eruptions are smaller and sporadic, with eruptive optical densities and temperatures low enough to rapidly quench erupting magma, creating glass [17]. Assuming this formation mechanism is representative of most localized DMDs, volcanic glass may be detectable in other similar deposits on the Moon. References: 1. Wilson and Head, JGR 86, 2971–3001 (1981). 2. Gaddis et al., Icarus. 161, 262–80 (2003). 3. Gustafson et al., JGR 117 (2012). 4. Gaddis et al., Icarus. 61, 461–89 (1985). 5. Delano and Livi, LPSXII (1981), 226–8. 6. Pieters et al., JGR 78, 5867–75 (1973). 7. Adams et al., Proc. Lun. Conf. 5th (1974), 171–86. 8. Besse et al., JGR 119, (2014). 9. Horgan et al., Icarus. 234, 132–54 (2014). 10. Pieters et al., Curr. Sci. 96, 500–5 (2009). 11. Boardman et al., JGR. 116 (2011). 12. Besse et al., Icarus. 222, 229– 242 (2013). 13. Haruyama et al., EPSL. 60, 243 (2008). 14. Souchon et al., Icarus. 225, 1–14 (2013). 15. Hawke et al., LPSC, 19th, 255–68 (1989). 16. Delano, LPSC, 10th, 275–300 (1979). 17. Weitz et al., Meteorit. Planet. Sci. 34, 527–40 (1999). 18. Head and Wilson, LPSC 10th, 2861–97 (1979).
Figure 2. (A) Continuum-removed spectra of the DMDs, including Walther A and Lavoisier DMDs [11,16], and Mare Frigoris (near J. Herschel). (B) Continuum-removed spectra of the glassy DMDs, relative to lunar pyroclastic orange glass and green glass. Spectra are offset for clarity.
