![Map of the (top) phase ratio I(5◦ )/I(15◦ ) at 2.0 µm and (bottom) phase reddening (with spectral slope computed between 2.1 µm and 2.5 µm). The maps were computed using a 10◦ × 10◦ grid at the Ryugu surface. The particularly high phase ratio and low phase reddening values for some grid elements located at latitude above 10◦N are due to a narrower phase angle range (see Appendix F, Fig. 20). All grid elements above this latitude were discarded for the computation of the average east and west properties.-- [astro-ph.EP]](https://i0.wp.com/astrobiology.com/wp-content/uploads/2026/09/Updated.png?fit=1200%2C677&ssl=1)
[astro-ph.EP] The Near InfraRed Spectrometer (NIRS3) onboard Hayabusa2 observed asteroid (162173) Ryugu, searching especially for signatures of water-bearing minerals.
However, during the mission, absolute reflectance measured by the Telescopic Optical Navigation Camera (ONC-T) and NIRS3 showed a systematic offset. Additionally, touchdown operations have modified the instrumental response, which has made post-second-touchdown data less reliable.
Correction of these issues is crucial to interpret the subtle spectroscopic differences across the surface, better understand the origin and evolution of carbonaceous asteroids, and support the Hayabusa2♯ mission’s future observations of the asteroids (98943) Torifune and 1998 KY26.
We used Ryugu observations at different mission phases for relative calibrations and lunar observations for the absolute calibration. Moon spectra acquired during the first Earth swing-by were compared to simulated ones from photometric models based on lunar orbiter and ground-based data. We then applied the updated calibration to the full Hayabusa2 proximity phase dataset, enabling disk-resolved global and regional photometric analyses.
We showed that the data obtained following the first and second touchdowns required downscaled corrections of 5.4±1.8% and 10.3±0.4% at 2 μm, respectively, while the absolute calibration revealed that NIRS3 reflectance was underestimated by about 8% compared with the original calibration.
Our new photometric parameters are consistent with previous studies and indicate a weak east-west dichotomy in phase ratio and phase reddening, pointing to higher surface roughness and higher abundance of fine-grained regolith in Ryugu eastern hemisphere.
Antonin Wargnier, Eri Tatsumi, Koki Yumoto, Mayumi Ichikawa, Shin-ya Murakami, Yuichiro Nagai, Kazuhiro Honda, Yasuhiro Yokota, Toru Kouyama, Takahiro Iwata, Moe Matsuoka, Satoshi Tanaka, Kohei Kitazato
Comments: 23 pages, 25 figures, accepted for publication in PSJ
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Instrumentation and Methods for Astrophysics (astro-ph.IM)
Cite as: arXiv:2609.04973 [astro-ph.EP] (or arXiv:2609.04973v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2609.04973
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Related DOI:
https://doi.org/10.3847/PSJ/ae9fe7
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Submission history
From: Antonin Wargnier
[v1] Fri, 4 Sep 2026 10:23:34 UTC (6,269 KB)
https://arxiv.org/abs/2609.04973
Astrobiology,
