Calibration of the Hayabusa2 near-infrared spectrometer using laboratory spectra of Ryugu samples

On December 6, 2020, JAXA's Hayabusa2 spacecraft successfully returned to Earth a re-entry capsule containing samples from the carbonaceous asteroid (162173) Ryugu. The capsule, recovered in South Australia, was transported to JAXA's curation facility in Sagamihara, Japan, where the sample container was opened and initial characterization was conducted under contamination-controlled conditions. During this initial characterization, the near-infrared reflectance spectra of the Ryugu samples were obtained using a Fourier Transform Infrared (FTIR) spectrometer. These spectra exhibited similar spectral features and a higher signal-to-noise ratio compared to those obtained from Ryugu's surface by the spacecraft's onboard near-infrared spectrometer, NIRS3 [1,2]. This provides a valuable opportunity to refine the calibration of Hayabusa2's remote sensing instruments. NIRS3 is a point spectrometer that continuously acquires spectral data over the 1.8-3.2 µm wavelength range with a field of view of 0.1 degrees [3]. During Hayabusa2's proximity operations, the entire surface of Ryugu was mapped, and a narrow absorption feature at 2.72 µm, attributed to hydrated minerals, was detected [4]. However, due to uncertainties in the radiometric calibration of NIRS3, the presence of regional variations in the intensity of the 2.72 µm absorption feature remains unclear. Here, we recalibrated the NIRS3 data using laboratory spectra of the returned Ryugu samples and re-evaluated the spectral characteristics across Ryugu's surface.

 

FTIR spectra obtained from three sub-bulk samples stored in chamber A of the sample container were used. These spectra were resampled to match the wavelength resolution of the NIRS3 spectra and compared with the average NIRS3 spectra acquired near the equatorial region of Ryugu. Photometric corrections were applied to the NIRS3 spectra to match the geometric conditions with those of the FTIR. As a result, the reflectance values from NIRS3 were approximately 10% lower than those from FTIR. However, this discrepancy may be due to contamination from the sapphire sample dish and is within the uncertainty range of the NIRS3 radiometric calibration. Good agreement was observed in both the intensity and shape of the 2.72 µm absorption feature, as well as in the overall spectral slope. These results indicate no significant differences between the NIRS3 and FTIR spectra, supporting the validity of using the Ryugu sample spectra as a reference for recalibrating the NIRS3 data of the asteroid's surface.

 

We derived new radiometric calibration coefficients for NIRS3 using the spectra of the Ryugu samples and incorporated them into the data processing pipeline to produce an updated global spectral map of Ryugu. The recalibrated NIRS3 spectra show that both the band area of the 2.72 µm absorption feature and the spectral slope increase with increasing latitude, consistent with findings from OSIRIS-REx observations of asteroid Bennu [5]. These latitudinal trends may reflect variations in particle size, porosity, or surface roughness. In this presentation, we present the spectral characteristics of Ryugu's surface based on the recalibrated NIRS3 spectra, and discuss the results of applying this new calibration to previously acquired NIRS3 spectra of the Moon.

 

References:

[1] Yada et al. (2021) Nature Astronomy 6, 214-220. [2] Hatakeda et al. (2023) Earth, Planets and Space 75:46. [3] Iwata et al. (2017) Space Science Review 208, 317-337. [4] Kitazato et al. (2019) Science 364, 272-275. [5] Simon et al. (2020) Science 370, eabc3522.