Presentation Information
[PPS03-P10]Calibration update of the NIRS3 spectrometer onboard Hayabusa2 and global near-infrared photometric properties of asteroid (162173) Ryugu
*Antonin Wargnier1, Eri Tatsumi2, Koki Yumoto1,3, Mayumi Ichikawa1, Yuuichiroh Nagai1, Kazuhiro Honda1, Shin-ya Murakami1, Yasuhiro Yokota4, Toru Kouyama5, Takahiro Iwata1, Kohei Kitazato6 (1.Institute of Space and Astronautical Science, JAXA, Japan, 2.Instituto de Astrofísica de Canarias, Spain, 3.LIRA, Paris Observatory, France, 4.Institute of Science Tokyo, Japan, 5.AIST, Japan, 6.University of Aizu, Japan)
Keywords:
Surface,Ryugu,Photometry,Spectroscopy
The carbonaceous asteroid Ryugu, visited by the JAXA Hayabusa2 mission, was extensively observed during the proximity phase with the NIRS3 near-infrared spectrometer (1800 – 3200nm) [1,2]. The radiometric calibration of NIRS3 was initially obtained from ground-based measurements of a commercial blackbody. After the two touchdown (TD) operations of the Hayabusa2 spacecraft, the instrumental response of the NIRS3 instrument was altered likely due to contamination by dust particles, and the radiometric calibration was modified accordingly, thanks to the measurements from the RAD internal lamp. However, thorough analysis of all the Ryugu data obtained during the proximity phase showed systematic residual variations between data obtained during mission phases, indicating the need for further refinement of the radiometric calibration. The calibration process was first dedicated to the correction of the data obtained between TD1 and TD2, and after TD2, in order to match the data before TD1. For this, we derived a wavelength-dependent scale factor based on a least-squares fit of the two phase curves – between TD1 and TD2, and after TD2 – to the phase curve before TD1. The data obtained after TD1 were then found to be about 3% brighter than the data before TD1, and the data after TD2 about 11% brighter. After improving the relative calibration of the NIRS3 dataset at the different mission phases, the second step was to check and improve the absolute calibration of the instrument using the Moon observations acquired in December 2015 during the Hayabusa2 Earth swing-by. We used the method presented in [3]: (i) we used the photometrically-corrected radiance (i0=30°, e0=0°, α0=30°) lunar map derived by [4]; (ii) we applied the ROLO correction factor [3], which we updated to apply it to the SP data up to 2053 nm; (iii) from the NIRS3 footprint position on the Moon, we computed the average SP spectra and projected them to the NIRS3 observation geometries using the SP model [4]; (iv) we finally derived a scale factor to match the SP model and the NIRS3 data in the wavelength range 1820 – 2053 nm.These new radiometrically corrected data enabled us to derive updated global near-infrared photometric properties of Ryugu, using all data from the proximity phase acquired between June 2018 to November 2019. After removing observations acquired with extreme illumination/observation geometries (i,e>70°) and performing a 3D averaging binning (i,e, α) in a similar way as [5], we derived the global disk-resolved phase curves and fitted these phase curves with the Hapke IMSA model for all wavelength channels, hence obtaining the variations of the Hapke parameters with the wavelength. The obtained parameters at 1.8 mm with this full dataset are consistent with previous studies [6,7]. We chose to use the Hapke model because it provides the best fit compared to other tried models such as linear-magnitude-Lommel-Seeliger or exponential-McEwen. The photometric parameters are then used to produce photometrically corrected (i0=30°, e0=0°, α0=30°) data. These radiometric and photometrically corrected NIRS3 data will be released through NASA PDS and JAXA DARTS.
Acknowledgments: This work is supported by the JAXA Hayabusa2# International Visibility Enhancement Project.
References: [1] Kitazato et al. (2019), Science, 364, 6437 [2] Iwata et al. (2017), SSR, 208 [3] Kouyama et al. (2016), PSS, 124 [4] Yokota et al. (2011), Icarus, 215, 2 [5] Yokota et al. (2021) LPSC 52nd, Abstract #2105. [6] Domingue et al. (2021), PSJ, 2:178 [7] Pilorget et al. (2021), Icarus, 355
Acknowledgments: This work is supported by the JAXA Hayabusa2# International Visibility Enhancement Project.
References: [1] Kitazato et al. (2019), Science, 364, 6437 [2] Iwata et al. (2017), SSR, 208 [3] Kouyama et al. (2016), PSS, 124 [4] Yokota et al. (2011), Icarus, 215, 2 [5] Yokota et al. (2021) LPSC 52nd, Abstract #2105. [6] Domingue et al. (2021), PSJ, 2:178 [7] Pilorget et al. (2021), Icarus, 355