5:15 PM - 6:45 PM
[PPS03-P06] CHRISTIANSEN FEATURE AND RESTSTRAHLEN FEATURE FOR FUTURE TIR EXPLORATION.
Keywords:thermal infrared exploration, Hera, meteorite, thermal alteration
Introduction: The asteroids that retain information about the early Solar System are diverse and are classified by spectral properties. Asteroids are altered by thermal and aqueous alteration. It is necessary to understand their effects on spectral properties.
The Christiansen feature (CF) and Resthalen feature (RF) may be changed by thermal and aqueous alteration. CF is a reflectance minimum around 7-9 µm, which depends mainly on the SiO2 abundance. RF is a reflectance maximum around 9-12 µm, depending mainly on crystallinity [1][2].
The ESA asteroid mission: Hera targeting the S-type asteroid 65803 Didymos and its satellite Dimorphos will perform multi-band thermal infrared spectroscopic observations using a thermal infrared imager: TIRI [4]. It is expected that CF and RF estimations by TIRI will provide information about the composition and degree of alteration of S-type asteroids.
Objectives of this study are the following points.
1. Investigate changes in the CF and RF due to composition and thermal and aqueous alteration.
2. Verify CF and RF estimation from TIRI observations simulatively.
Methods:
1. Investigate the CF and RF of rocky samples.
Agilent Handy FTIR 4300 was used to acquire spectral data. The FTIR can acquire spectral data of 650 - 4000 cm-1 with a resolution of 8 cm-1. Samples measured are minerals (olivine, pyroxene, quartz, labradorite, obsidian), rocks (anorthosite, hypersthene-augite andesite, olivine basalt, and peridotite), and meteorite samples. The meteorite samples are petrologic types 3-7 ordinary chondrites (H, L, LL), primitive achondrites (winonaites, acapulcoites, lodranites, ureilites), differentiated achondrites (diogenites, eucrites) and carbonaceous chondrites (CM2, CO3, CR2).
In the analysis of spectral data, for the CF analysis, fitting with a polynomial of the 10th order was performed. For the RF analysis, multiple fitting with Gaussian functions was performed.
2. Verify CF and RF estimation by TIRI.
The TIRI PFM was used to acquire spectral data. TIRI can acquire 8 - 14 µm spectral data (6 bands: 7.8 µm, 8.6 µm, 9.6 µm, 10.6 µm, 11.6 µm, 13.0 µm) with a resolution of 0.013 deg/pix over a 13.3 x 10.0 deg FOV [4]. Samples measured were minerals (olivine, pyroxene, quartz, labradorite, obsidian) and rocks (anorthosites, hypersthene-augite andesites, olivine basalts and peridotites), and meteorite samples (NWA 7676 (LL3.5), NWA 7187 (L3.6), Chelyabinsk (LL5), NWA 5490 (L3.7)).
In the analysis of spectral data, CF and RF were estimated by fitting analysis with quadratic functions.
Results: In ordinary chondrites, positions of CF and RF are shifted to longer wavelengths in the order H < L < LL in Figure 1.
RF depth tends to increase in the order of carbonaceous chondrites < ordinary chondrites < primitive achondrites. RF depth tends to increase as the degree of thermal alteration increases.
In the comparison of spectral data from TIRI and the FTIR, the RF positions were almost consistent for most of the samples.
Discussion: The positions of CF and RF shift to shorter wavelength with increasing SiO2 content [1]. The correlation between CF and RF positions found in this study can be considered to correspond to the SiO2 abundance. For ordinary chondrites, the SiO2 abundance is estimated to be on the order of LL < L < H.
RF is associated with Si-O bonds [1]. The correlation between the degree of thermal alteration and RF depth found in this study is possibly due to the absorbance of the Si-O bonds caused by the different degrees of crystallinity.
The almost consistent RF positions estimated from spectral data by TIRI and the FTIR suggest that TIRI's observations are effective for estimating the composition and alteration.
References: [1] Zeng, X. et al. (2019) J. Geophys. Res. Planets 124, 3267–3282. [2] Greenhagen, B. T. et al. (2010) Science 329, 1507–1509. [3] Michel, P. et al. (2022) Planet. Sci. J. 3, 160. [4] Okada, T et al. (2023) LPSC, Abstract #1604.
The Christiansen feature (CF) and Resthalen feature (RF) may be changed by thermal and aqueous alteration. CF is a reflectance minimum around 7-9 µm, which depends mainly on the SiO2 abundance. RF is a reflectance maximum around 9-12 µm, depending mainly on crystallinity [1][2].
The ESA asteroid mission: Hera targeting the S-type asteroid 65803 Didymos and its satellite Dimorphos will perform multi-band thermal infrared spectroscopic observations using a thermal infrared imager: TIRI [4]. It is expected that CF and RF estimations by TIRI will provide information about the composition and degree of alteration of S-type asteroids.
Objectives of this study are the following points.
1. Investigate changes in the CF and RF due to composition and thermal and aqueous alteration.
2. Verify CF and RF estimation from TIRI observations simulatively.
Methods:
1. Investigate the CF and RF of rocky samples.
Agilent Handy FTIR 4300 was used to acquire spectral data. The FTIR can acquire spectral data of 650 - 4000 cm-1 with a resolution of 8 cm-1. Samples measured are minerals (olivine, pyroxene, quartz, labradorite, obsidian), rocks (anorthosite, hypersthene-augite andesite, olivine basalt, and peridotite), and meteorite samples. The meteorite samples are petrologic types 3-7 ordinary chondrites (H, L, LL), primitive achondrites (winonaites, acapulcoites, lodranites, ureilites), differentiated achondrites (diogenites, eucrites) and carbonaceous chondrites (CM2, CO3, CR2).
In the analysis of spectral data, for the CF analysis, fitting with a polynomial of the 10th order was performed. For the RF analysis, multiple fitting with Gaussian functions was performed.
2. Verify CF and RF estimation by TIRI.
The TIRI PFM was used to acquire spectral data. TIRI can acquire 8 - 14 µm spectral data (6 bands: 7.8 µm, 8.6 µm, 9.6 µm, 10.6 µm, 11.6 µm, 13.0 µm) with a resolution of 0.013 deg/pix over a 13.3 x 10.0 deg FOV [4]. Samples measured were minerals (olivine, pyroxene, quartz, labradorite, obsidian) and rocks (anorthosites, hypersthene-augite andesites, olivine basalts and peridotites), and meteorite samples (NWA 7676 (LL3.5), NWA 7187 (L3.6), Chelyabinsk (LL5), NWA 5490 (L3.7)).
In the analysis of spectral data, CF and RF were estimated by fitting analysis with quadratic functions.
Results: In ordinary chondrites, positions of CF and RF are shifted to longer wavelengths in the order H < L < LL in Figure 1.
RF depth tends to increase in the order of carbonaceous chondrites < ordinary chondrites < primitive achondrites. RF depth tends to increase as the degree of thermal alteration increases.
In the comparison of spectral data from TIRI and the FTIR, the RF positions were almost consistent for most of the samples.
Discussion: The positions of CF and RF shift to shorter wavelength with increasing SiO2 content [1]. The correlation between CF and RF positions found in this study can be considered to correspond to the SiO2 abundance. For ordinary chondrites, the SiO2 abundance is estimated to be on the order of LL < L < H.
RF is associated with Si-O bonds [1]. The correlation between the degree of thermal alteration and RF depth found in this study is possibly due to the absorbance of the Si-O bonds caused by the different degrees of crystallinity.
The almost consistent RF positions estimated from spectral data by TIRI and the FTIR suggest that TIRI's observations are effective for estimating the composition and alteration.
References: [1] Zeng, X. et al. (2019) J. Geophys. Res. Planets 124, 3267–3282. [2] Greenhagen, B. T. et al. (2010) Science 329, 1507–1509. [3] Michel, P. et al. (2022) Planet. Sci. J. 3, 160. [4] Okada, T et al. (2023) LPSC, Abstract #1604.