11:00 〜 13:00
[PPS03-P10] The 1.064μm albedo features on the C-type asteroid Ryugu from observation by Hayabusa2 LIDAR
キーワード：C型小惑星、はやぶさ2、レーザ高度計、アルベド
In June 2018, the Japanese asteroid explorer Hayabusa2 has arrived at C-type asteroid 162173 Ryugu after cruising of about three and half years. The first global albedo analysis of Ryugu has been performed using the optical navigation camera (ONC) images in visible range (0.4-0.9 μm) and it reported that the asteroid has relatively uniform surface reflectance and that geometric albedo is 0.045±0.002 at 0.55 μm [1]. The global observation of Ryugu by near infrared spectrometer 3 (NIRS3) has also revelated its low reflectance in near infrared range (1.8-3.2 μm) and the reflectance at 2.0 μm is 0.017±0.002 [2]. The 1.064 μm laser wavelength of the Haysabusa2 LIDAR is in the observation gap between ONC and NIRS3 ranges. Hayabusa2 LIDAR can record transmitted intensity for each laser shot and corresponding returned pulse intensity simultaneously. We have calculated the albedo on the surface of Ryugu at 1.064 μm using the recorded transmitted and returned laser energies.
In this study, we propose a method to calculate the surface albedo using small data sets of the LIDAR. A key of accurate derivation of the surface albedo is precise energy estimation of the returned pulse which are disturbed by rough asteroid surface. Although the received energy is represented by integration of the returned pulse over time and space, the Hayabusa2 LIDAR does not have function to monitor time profile of the returned pulse. Therefore, we have constructed a method to calculate the returned pulse considering properties of the transmitted laser pulse, altitude of the spacecraft, law of reflectance, and surface topography. Then, we have also investigated response of the LIDAR receiver to the disturbed returned pulse using the LIDAR-EM. We will describe the calculation method of albedo based on results of returned pulse calculation and the LIDAR-EM experiment.
The 1.064 μm albedo have been derived using 15-day observation data taken at altitude lower than 9 km before conjunction phase of Hayabusa2. The albedo map can cover areas between 40°S and 20°N, and the distribution is evaluated as average within a square grid of 2°×2° due to overlapping of neighboring laser footprints. The average and standard deviation of albedo in all evaluated 3946 grids is 0.0406±0.031 at 1.064 μm, and about 98 (%) of the averaged grids have values between 0.03 and 0.05. The average of 1.064 μm albedo overlaps the global average of the normal albedo obtained by the ONC-T at 0.945 μm (0.0399±0.0005) [3] within in 1σ, and the low and uniform albedo feature at 1.064 μm is consistent with observation of ONC and NIRS3.
On the other hand, there are 33 exceptions at grids which have albedo over 3σ of the global average in our albedo map. In this presentation, we indicate distribution and albedo values of the exceptions on the map and discuss the causes of the higher and lower values. Then, we will also investigate topographic signatures using the ONC images around the exceptions on the 1.064 μm map.
[1] S. Sugita, R. Honda, T. Morota et al., The geomorphology, color, and thermal properties of Ryugu: implications for parent-body processes. Since 364, eaaw0422, pp. 1-11 (2019)
[2] K. Kitazato, R. E. Milliken, T. Iwata et al., the surface composition of asteroid 162173 Ryugu from Hayabusa2 near-infrared spectroscopy. Since 364, pp. 272-275 (2019)
[3] Y. Yokota, R. Honda, E. Tatsumi et al., Opposition observation of 162173 Ryugu: normal albedo map highlights variations in regolith characteristics. The Planetary Science Journal, 2:177, pp. 1-32 (2021)
In this study, we propose a method to calculate the surface albedo using small data sets of the LIDAR. A key of accurate derivation of the surface albedo is precise energy estimation of the returned pulse which are disturbed by rough asteroid surface. Although the received energy is represented by integration of the returned pulse over time and space, the Hayabusa2 LIDAR does not have function to monitor time profile of the returned pulse. Therefore, we have constructed a method to calculate the returned pulse considering properties of the transmitted laser pulse, altitude of the spacecraft, law of reflectance, and surface topography. Then, we have also investigated response of the LIDAR receiver to the disturbed returned pulse using the LIDAR-EM. We will describe the calculation method of albedo based on results of returned pulse calculation and the LIDAR-EM experiment.
The 1.064 μm albedo have been derived using 15-day observation data taken at altitude lower than 9 km before conjunction phase of Hayabusa2. The albedo map can cover areas between 40°S and 20°N, and the distribution is evaluated as average within a square grid of 2°×2° due to overlapping of neighboring laser footprints. The average and standard deviation of albedo in all evaluated 3946 grids is 0.0406±0.031 at 1.064 μm, and about 98 (%) of the averaged grids have values between 0.03 and 0.05. The average of 1.064 μm albedo overlaps the global average of the normal albedo obtained by the ONC-T at 0.945 μm (0.0399±0.0005) [3] within in 1σ, and the low and uniform albedo feature at 1.064 μm is consistent with observation of ONC and NIRS3.
On the other hand, there are 33 exceptions at grids which have albedo over 3σ of the global average in our albedo map. In this presentation, we indicate distribution and albedo values of the exceptions on the map and discuss the causes of the higher and lower values. Then, we will also investigate topographic signatures using the ONC images around the exceptions on the 1.064 μm map.
[1] S. Sugita, R. Honda, T. Morota et al., The geomorphology, color, and thermal properties of Ryugu: implications for parent-body processes. Since 364, eaaw0422, pp. 1-11 (2019)
[2] K. Kitazato, R. E. Milliken, T. Iwata et al., the surface composition of asteroid 162173 Ryugu from Hayabusa2 near-infrared spectroscopy. Since 364, pp. 272-275 (2019)
[3] Y. Yokota, R. Honda, E. Tatsumi et al., Opposition observation of 162173 Ryugu: normal albedo map highlights variations in regolith characteristics. The Planetary Science Journal, 2:177, pp. 1-32 (2021)