5:15 PM - 7:15 PM
[PPS06-P12] LIDAR albedo of the asteroid Ryugu revisited
Keywords:albedo, asteroid, Ryugu, LIDAR
Normal albedos of the asteroid Ryugu have been calculated from ONC data by Yokota et al. [2021] and LIDAR data by Yamada et al. [2022, 2024]. Particularly, LIDAR albedo map will be publicized soon via ISAS/DART archive system [cf., Matsumoto et al., under re-view]. In this LIDAR albedo calculation, Yamada et al. [2022] discover unexpected varia-tions in the primary results of albedos, which are well correlated with laser-diode (LD) temperature. The period of this variation is approximately 400 seconds but is changeable. Yamada et al. [2022] remove this variation by applying a bandpass filter whose frequency range is between 0.002 ~ 0.0032 Hz [Yamada et al., 2024]. This elimination procedure is ra-tional. However, it possibly removes genuine albedo variation on the surface of Ryugu.
Instead, we use LD temperature variation as a filter and apply it to the unwanted albedo variation. We derive the spacecraft altitude condition as the first step of this data pro-cessing study. Ratios of received pulse intensity to transmitted pulse intensity are figured for every 1-km altitude bin, and their standard deviations are calculated (Fig. 1). On the other hand, Yamada et al. [2022] reveal that 1-sigma error of received pulse intensity is 9.7 %. Figure 1 shows that the calculated deviations compare to this 1-sigma error for the altitude be-low 9 km. Thus, we only use the LIDAR data acquired at an altitude lower than 9 km.
The correlation between the LD temperature and the primary albedo value is evident, but their causality is unclear. Therefore, we evaluate the correlation by moving the LD tem-perature variation from -150 to +150 seconds, and we adopt the highest correlation (or the lowest if the correlation is negative) and its time shift of LD temperature. Two coefficients, a multiple constant c1 and a constant addition c2, are determined to minimize the residu-als from albedo values in the manner of the least-square method. The resulting correla-tion is often as high as 0.5. However, the correlations are occasionally lower than 0.2, especially for the orbit arcs that are longer than a few hours, probably because the time shift is not stable at this duration.
In order to determine the reasonable duration of LD temperature filtering, we divide the long arcs into blocks whose duration is from 2000 to 5000 seconds. Then, the correlation and time shift for each block are calculated for the best fitting c1 and c2. For the short blocks, the correlation becomes high while the correlation varies widely. For the long blocks, the correlation varies less, but the correlation becomes constant losing the change of albedo values at the Ryugu surface. Then, we choose the 3000 seconds for the best du-ration to evaluate the best fitting c1 and c2 for every block (Fig. 2).
Matsumoto, K., Noda H., Senshu, H., et al., Software Interface Specification for the Haya-busa2 LIDAR Data Products, Version 2.0, under review. [Data set] Level 4 Albedo map (in preparation) https://data.darts.isas.jaxa.jp/pub/hayabusa2/lidar_bundle/
Yamada, R., Yamamoto, K., Oshigami, S. et al. Derivation of 1.064 μm normal albedos on the C-type asteroid Ryugu from laser pulse intensity measurement of the Hayabusa2 LI-DAR. Earth Planets Space 74, 166 (2022). https://doi.org/10.1186/s40623-022-01717-z
Yamada, R., Yamamoto, K., Oshigami, S. et al. Correction: Derivation of 1.064 μm normal albedos on the C-type asteroid Ryugu from laser pulse intensity measurement of the Hayabusa2 LIDAR. Earth Planets Space 76, 17 (2024). https://doi.org/10.1186/s40623-023-01949-7
Yokota Y, Honda R, Tatsumi E et al (2021) Opposition observation of 162173 Ryugu: nor-mal albedo map highlights variations in regolith characteristics. Planet Sci J 2(177):1–32
Instead, we use LD temperature variation as a filter and apply it to the unwanted albedo variation. We derive the spacecraft altitude condition as the first step of this data pro-cessing study. Ratios of received pulse intensity to transmitted pulse intensity are figured for every 1-km altitude bin, and their standard deviations are calculated (Fig. 1). On the other hand, Yamada et al. [2022] reveal that 1-sigma error of received pulse intensity is 9.7 %. Figure 1 shows that the calculated deviations compare to this 1-sigma error for the altitude be-low 9 km. Thus, we only use the LIDAR data acquired at an altitude lower than 9 km.
The correlation between the LD temperature and the primary albedo value is evident, but their causality is unclear. Therefore, we evaluate the correlation by moving the LD tem-perature variation from -150 to +150 seconds, and we adopt the highest correlation (or the lowest if the correlation is negative) and its time shift of LD temperature. Two coefficients, a multiple constant c1 and a constant addition c2, are determined to minimize the residu-als from albedo values in the manner of the least-square method. The resulting correla-tion is often as high as 0.5. However, the correlations are occasionally lower than 0.2, especially for the orbit arcs that are longer than a few hours, probably because the time shift is not stable at this duration.
In order to determine the reasonable duration of LD temperature filtering, we divide the long arcs into blocks whose duration is from 2000 to 5000 seconds. Then, the correlation and time shift for each block are calculated for the best fitting c1 and c2. For the short blocks, the correlation becomes high while the correlation varies widely. For the long blocks, the correlation varies less, but the correlation becomes constant losing the change of albedo values at the Ryugu surface. Then, we choose the 3000 seconds for the best du-ration to evaluate the best fitting c1 and c2 for every block (Fig. 2).
Matsumoto, K., Noda H., Senshu, H., et al., Software Interface Specification for the Haya-busa2 LIDAR Data Products, Version 2.0, under review. [Data set] Level 4 Albedo map (in preparation) https://data.darts.isas.jaxa.jp/pub/hayabusa2/lidar_bundle/
Yamada, R., Yamamoto, K., Oshigami, S. et al. Derivation of 1.064 μm normal albedos on the C-type asteroid Ryugu from laser pulse intensity measurement of the Hayabusa2 LI-DAR. Earth Planets Space 74, 166 (2022). https://doi.org/10.1186/s40623-022-01717-z
Yamada, R., Yamamoto, K., Oshigami, S. et al. Correction: Derivation of 1.064 μm normal albedos on the C-type asteroid Ryugu from laser pulse intensity measurement of the Hayabusa2 LIDAR. Earth Planets Space 76, 17 (2024). https://doi.org/10.1186/s40623-023-01949-7
Yokota Y, Honda R, Tatsumi E et al (2021) Opposition observation of 162173 Ryugu: nor-mal albedo map highlights variations in regolith characteristics. Planet Sci J 2(177):1–32