Abstract
We conducted ground-based performance evaluation tests of the Near-Infrared Spectrometer (NIRS3) onboard Hayabusa2 spacecraft in November 2013 and from April to May 2014 and established a method for evaluating its measured reflectance spectra. Reflectance spectra of nine powdered carbonaceous chondrite samples were measured by both NIRS3 and a Fourier transform infrared (FT-IR) spectrometer. We have established two methods for correcting the NIRS3 data by comparing them with the corresponding FT-IR data because raw data obtained by NIRS3 underwent spectral distortion caused by systematic offsets in sensitivity of individual pixels. The corrected NIRS3 spectra of carbonaceous chondrite samples are comparable with their FT-IR spectra. The depth of each band component Dλ is defined for each wavelength λ (μm) to characterize the absorption bands in NIRS3 spectra. It is suggested that the relationship between the D2.72/D2.79 ratio and the D2.76/D2.90 ratio would be useful for estimating the degree of heating of the asteroid surface, if contributions of terrestrial adsorbed water on D2.79 and D2.90 are properly corrected. The degrees of heating and space weathering are also comprehensively evaluated by the relationship between D2.90 and the D2.76/D2.90 ratio. Reflectance spectra of asteroid Ryugu, the target asteroid of Hayabusa2, to be recorded by the NIRS3 instrument are expected to reveal the characteristics of the surface materials by using the evaluation technique proposed in this paper. Such information will be used for choosing the touchdown points for sampling and also for investigating the distribution of the materials similar to the returned samples on Ryugu.Graphical abstract.
Highlights
C-type asteroid 162173 Ryugu was selected as a target of Hayabusa2 mission of the Japan Aerospace Exploration Agency (JAXA) to study the origin and evolutional history of the solar system and the origin of organic matters available for life
Ground-based observations of asteroid Ryugu indicated that its surface materials would possibly consist of unheated CM chondrite (Vilas 2008) based on an observation of the 0.7μm band, or heated carbonaceous chondrite because its Vis–NIR spectrum corresponds with the spectrum of Murchison CM2 chondrite heated at 550–900 °C
It is known that the depth of the 3-μm band of hydrous carbonaceous chondrites drastically decreases by heating due to the dehydration and decomposition of phyllosilicates (Hiroi et al 1996), indicating that the 3-μm band depth can be applied as an index of the heating stage
Summary
C-type asteroid 162173 Ryugu was selected as a target of Hayabusa mission of the Japan Aerospace Exploration Agency (JAXA) to study the origin and evolutional history of the solar system and the origin of organic matters available for life. Ground-based observations of asteroid Ryugu indicated that its surface materials would possibly consist of unheated CM chondrite (Vilas 2008) based on an observation of the 0.7μm band, or heated carbonaceous chondrite because its Vis–NIR spectrum corresponds with the spectrum of Murchison CM2 chondrite heated at 550–900 °C. Their spectral slopes around 0.7 μm are very similar to each other (Sugita et al 2013; Perna et al 2017). Belgica (B-) 7904 CM2 and Y-86720 CM2 are in this class
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