5:15 PM - 7:15 PM
[PPS03-P10] Mid-IR spectral variations depending on composition and metamorphism of ordinary chondrites for the Hera mission.
Keywords:infrared spectroscopy, meteorite, chondrite, Hera
Introduction:
The mid-infrared (MIR) region of the spectrum exhibits characteristic reflectance minima and maxima, which are influenced by mineral composition and physical conditions [1]. The Christiansen Feature (CF), appearing around 7–9 µm, corresponds to a reflectance minimum and is affected by the SiO2 content and crystal structure. In contrast, the Reststrahlen Feature (RF), observed near 10 µm, corresponds to a reflectance maximum and is influenced by particle size.
In remote sensing of the Moon and asteroids, studies using CF and RF have focused on the Moon and carbonaceous asteroids [2], [3]. Research on CF and RF in S-type asteroids is limited. Therefore, it is crucial to investigate CF and RF in ordinary chondrites, which are similar to S-type asteroids. Furthermore, few studies have compared different meteorites using the same methods. This study examines CF and RF in various meteorites, focusing on ordinary chondrites, to support future MIR spectral remote sensing, including observations by the Thermal InfraRed Imager (TIRI) onboard the ESA Hera mission [4].
Methods:
We measured 207 polished thick sections of meteorites embedded in epoxy and thin sections mounted on silica glass with epoxy. These included 175 ordinary chondrites (H: 78, L: 79, LL: 18), 21 primitive achondrites (pri-AC) (Acapulcoites: 4, Lodranites: 4, Ureilites: 10, Winonaites: 3), 3 differentiated achondrites (diff-AC) (Diogenites: 1, Eucrites: 2), and 8 carbonaceous chondrites (CC) (CM2: 5, CO3: 2, CR2: 1). Spectroscopic measurements were conducted using a Handy FTIR 4300 (Agilent) with a Coarse Silver Cap reference (spot size: Φ5 mm).
Results and Discussion:
The distribution trends of CF and RF positions vary among meteorite types (Figure 1a). CF positions are clustered around ~7.3, ~7.6, ~7.9, and ~8.4 µm. The CFs of minerals in the meteorites are ~7.9 µm (plagioclase), ~8.4 µm (pyroxene), and ~8.8 µm (olivine). The CFs of meteorites at ~7.9 µm and ~8.4 µm closely match those of plagioclase and pyroxene, respectively. This suggests that the CF of a meteorite is influenced by its dominant constituent minerals. The absorption bands contributing to the CF of minerals extend to shorter wavelengths, potentially overlapping and deepening due to the absorption bands of other minerals. This may explain why the CF of olivine, which occurs at longer wavelengths, is not observed in meteorites. The ~7.3 µm feature of meteorites may be attributed to silica glass under the thin sections of meteorites.
RF positions tend to shift toward longer wavelengths in the order of diff-AC < pri-AC < CC < H < L < LL (Figure 1a). We also found a correlation between RF positions and fayalite content. This suggests that RF in meteorites is influenced by olivine chemistry, as olivine’s RF, located at longer wavelengths, is less affected by Si-O stretching absorption bands of other minerals.
We found that RF reflectance increases in the following order: CC < OC (LL, L, H) < diff-AC < pri-AC. Additionally, RF reflectance increases with petrologic type from 3 to 4 (Figure 1b). These results suggest that larger crystal sizes are associated with higher RF reflectance.
References:
[1] Salisbury, J. W., Walter, L. S., Vergo, N. & D’Aria, D. M. Infrared (2.1–25 um) Spectra of Minerals (The Johns Hopkins University Press, 1991).
[2] B. T. Greenhagen et al., Science, vol. 329, no. 5998, pp. 1507–1509, Sep. 2010, doi: 10.1126/science.1192196.
[3] M. Hamm et al., Nat Commun, vol. 13, no. 1, p. 364, Jan. 2022, doi: 10.1038/s41467-022-28051-y.
[4] P. Michel et al., Planet. Sci. J., vol. 3, no. 7, p. 160, Jul. 2022, doi: 10.3847/PSJ/ac6f52.
The mid-infrared (MIR) region of the spectrum exhibits characteristic reflectance minima and maxima, which are influenced by mineral composition and physical conditions [1]. The Christiansen Feature (CF), appearing around 7–9 µm, corresponds to a reflectance minimum and is affected by the SiO2 content and crystal structure. In contrast, the Reststrahlen Feature (RF), observed near 10 µm, corresponds to a reflectance maximum and is influenced by particle size.
In remote sensing of the Moon and asteroids, studies using CF and RF have focused on the Moon and carbonaceous asteroids [2], [3]. Research on CF and RF in S-type asteroids is limited. Therefore, it is crucial to investigate CF and RF in ordinary chondrites, which are similar to S-type asteroids. Furthermore, few studies have compared different meteorites using the same methods. This study examines CF and RF in various meteorites, focusing on ordinary chondrites, to support future MIR spectral remote sensing, including observations by the Thermal InfraRed Imager (TIRI) onboard the ESA Hera mission [4].
Methods:
We measured 207 polished thick sections of meteorites embedded in epoxy and thin sections mounted on silica glass with epoxy. These included 175 ordinary chondrites (H: 78, L: 79, LL: 18), 21 primitive achondrites (pri-AC) (Acapulcoites: 4, Lodranites: 4, Ureilites: 10, Winonaites: 3), 3 differentiated achondrites (diff-AC) (Diogenites: 1, Eucrites: 2), and 8 carbonaceous chondrites (CC) (CM2: 5, CO3: 2, CR2: 1). Spectroscopic measurements were conducted using a Handy FTIR 4300 (Agilent) with a Coarse Silver Cap reference (spot size: Φ5 mm).
Results and Discussion:
The distribution trends of CF and RF positions vary among meteorite types (Figure 1a). CF positions are clustered around ~7.3, ~7.6, ~7.9, and ~8.4 µm. The CFs of minerals in the meteorites are ~7.9 µm (plagioclase), ~8.4 µm (pyroxene), and ~8.8 µm (olivine). The CFs of meteorites at ~7.9 µm and ~8.4 µm closely match those of plagioclase and pyroxene, respectively. This suggests that the CF of a meteorite is influenced by its dominant constituent minerals. The absorption bands contributing to the CF of minerals extend to shorter wavelengths, potentially overlapping and deepening due to the absorption bands of other minerals. This may explain why the CF of olivine, which occurs at longer wavelengths, is not observed in meteorites. The ~7.3 µm feature of meteorites may be attributed to silica glass under the thin sections of meteorites.
RF positions tend to shift toward longer wavelengths in the order of diff-AC < pri-AC < CC < H < L < LL (Figure 1a). We also found a correlation between RF positions and fayalite content. This suggests that RF in meteorites is influenced by olivine chemistry, as olivine’s RF, located at longer wavelengths, is less affected by Si-O stretching absorption bands of other minerals.
We found that RF reflectance increases in the following order: CC < OC (LL, L, H) < diff-AC < pri-AC. Additionally, RF reflectance increases with petrologic type from 3 to 4 (Figure 1b). These results suggest that larger crystal sizes are associated with higher RF reflectance.
References:
[1] Salisbury, J. W., Walter, L. S., Vergo, N. & D’Aria, D. M. Infrared (2.1–25 um) Spectra of Minerals (The Johns Hopkins University Press, 1991).
[2] B. T. Greenhagen et al., Science, vol. 329, no. 5998, pp. 1507–1509, Sep. 2010, doi: 10.1126/science.1192196.
[3] M. Hamm et al., Nat Commun, vol. 13, no. 1, p. 364, Jan. 2022, doi: 10.1038/s41467-022-28051-y.
[4] P. Michel et al., Planet. Sci. J., vol. 3, no. 7, p. 160, Jul. 2022, doi: 10.3847/PSJ/ac6f52.