10:45 AM - 11:00 AM
[PPS07-13] Spectral and mineralogical properties of naturally-heated carbonaceous chondrites; implication to heating processes on the surface of hydrous asteroids
Keywords:carbonaceous chondrite, C-type asteroid, reflectance spectra, TEM analysis, Hayabusa2
Spectral and mineralogical analyses were performed using nine naturally-heated and dehydrated carbonaceous chondrite samples (seven CM, one CI, and one CV chondrites), which classified into heating stages (HS) from I to IV based on X-ray diffraction results . In-situ heating of samples at 120–400 °C was performed during spectral measurements and successfully removed absorption water and part of rehydrated water from chondrite samples. Lewis Cliff (LEW) 87022 (unheated CM), Jbilet Winselwan, and Dhofar 735 (heated at 600–900 °C ) were used for transmission electron microscope (TEM) analysis.
Reflectance spectra of HS-I samples show the positive slope in Vis- IR range and the significant 0.7- and 3-μm absorption bands. The 0.7-μm band appears in only HS-I sample spectra. With increasing heating degree, (1) Vis-IR slope decreases, (2) the 0.7- and 3-μm bands decrease, and (3) Mid-IR spectral feature (i.e. Christiansen feature (CF) and Reststrahlen bands (RB)) shifts toward longer wavelength. TEM/EDX analyses showed that the matrix of severely-heated chondrites consists of tiny olivine, low-Ca pyroxene, and fine Fe- and Ni-rich metal grains (<1 μm). It is indicated that in proportion to the heating degree, amorphization and dehydration of serpentine and tochilinite from HS-I to HS-II might cause the 0.7- and 3-μm band weakening, and spectral slope and albedo decreasing of spectra. Then, formation of secondary olivine and pyroxene, and FeNi-rich metal grains at HS-IV would be responsible for the 3-μm band depth decreasing, spectral slope and albedo increasing, CF peak shift, and RB changes of chondrite spectra.
We plan to compare other IR reflectance spectra of asteroids, such as those acquired by AKARI , to better interpret remotely sensed data of Bennu (NASA’s OSIRIS-REx mission target) and Ryugu. Future NIRS3 observations before/after the Small Carry-on Impactor (SCI) crater experiment and lab analyses of returned samples will reveal additional compositional properties of Ryugu.
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Acknowledgements: We appreciate support from the National Institute of Polar Research for providing carbonaceous chondrites, the JSPS Japanese-German Graduate Externship for Research on Deep Earth Volatile Cycle, and the JSPS Core-to-Core Program "International Network of Planetary Sciences”.