Presentation Information
[PPS03-P01]Phase Angle Dependence of Ryugu and Bennu Sample Reflectance Spectra: Implications for MMX/OROCHI Observations
*Suzuha Hirota1, Roger Stabbins2, Koki Yumoto3,4, Ryota Fukai4, Shingo Kameda1,4 (1.Rikkyo University, 2.Natural History Museum, 3.LIRA-Observatoire de Paris, CNRS, Université PSL, Sorbonne Université, Université Paris Cité, 5 Place Jules Janssen, 92195, 4.Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency)
The reflectance spectrum of a small body's surface reflects basic surface information, such as the composition of rocks and minerals, and plays an important role in exploration.
The OROCHI satellite camera, installed on the Martian Moons Exploration (MMX) mission to observe surface topography and material information, uses seven cameras to enable simultaneous multi-wavelength observations. However, phase angle differences due to camera position become significant during close-range observations. The reflectance spectra of asteroids and moons are known to vary as a function of the phase angle, which is the angle between the Sun, the surface, and the camera. In fact, a decrease in reflectance and phase reddening have been observed with increasing phase angle on the asteroids Ryugu and Bennu. However, the physical origin of this phase angle dependence remains unclear due to complex interactions involving particle size, composition, space weathering, and other factors. While a phase reddening trend has been reported on Phobos, the target of MMX observations, the degree of phase reddening varies by more than twofold depending on the region. Furthermore, observations are not global. Therefore, corrections are necessary to obtain accurate spectra from OROCHI's close-range observations.
In this study, we clarify the phase angle dependence of reflectance spectra for asteroid samples and investigate the necessity and feasibility of phase angle correction in OROCHI observations. To this end, we measured the reflectance spectra of Ryugu and Bennu samples using a device simulating OROCHI. To evaluate changes in reflectance and spectral slope due to differences in camera position, we did not use a bandpass filter and instead used a monochromator light source to illuminate the samples in the 450-750 nm wavelength range. Measurements were performed using Ryugu sample(C9003) and Bennu sample(ORX-19000). Reflectance was calculated by taking a ratio to a 99% standard reflector.
As a result, dark, flat reflectance spectra were obtained for both samples, and it was confirmed that reflectance decreases with increasing phase angle. Furthermore, it was found that the degree of this decrease differs depending on the wavelength. The change in spectral slope with increasing phase angle was evaluated in the 450-550 nm and 550-750 nm ranges using values normalized to the reflectance at 550 nm. The Ryugu sample showed a dominant decrease in the spectral slope at longer wavelengths, suggesting a trend toward phase bluing. On the other hand, the Bennu sample showed a dominant increase in the spectral slope at shorter wavelengths, suggesting a trend toward phase reddening. Thus, within the phase angle range of approximately 22-40° assumed for close-range observations by OROCHI, the relationship between phase angle and color tends to differ even for similar materials such as the Ryugu and Bennu samples. This result emphasizes the need for appropriate phase angle correction to obtain accurate reflectance spectra with OROCHI during close-range observations.
The OROCHI satellite camera, installed on the Martian Moons Exploration (MMX) mission to observe surface topography and material information, uses seven cameras to enable simultaneous multi-wavelength observations. However, phase angle differences due to camera position become significant during close-range observations. The reflectance spectra of asteroids and moons are known to vary as a function of the phase angle, which is the angle between the Sun, the surface, and the camera. In fact, a decrease in reflectance and phase reddening have been observed with increasing phase angle on the asteroids Ryugu and Bennu. However, the physical origin of this phase angle dependence remains unclear due to complex interactions involving particle size, composition, space weathering, and other factors. While a phase reddening trend has been reported on Phobos, the target of MMX observations, the degree of phase reddening varies by more than twofold depending on the region. Furthermore, observations are not global. Therefore, corrections are necessary to obtain accurate spectra from OROCHI's close-range observations.
In this study, we clarify the phase angle dependence of reflectance spectra for asteroid samples and investigate the necessity and feasibility of phase angle correction in OROCHI observations. To this end, we measured the reflectance spectra of Ryugu and Bennu samples using a device simulating OROCHI. To evaluate changes in reflectance and spectral slope due to differences in camera position, we did not use a bandpass filter and instead used a monochromator light source to illuminate the samples in the 450-750 nm wavelength range. Measurements were performed using Ryugu sample(C9003) and Bennu sample(ORX-19000). Reflectance was calculated by taking a ratio to a 99% standard reflector.
As a result, dark, flat reflectance spectra were obtained for both samples, and it was confirmed that reflectance decreases with increasing phase angle. Furthermore, it was found that the degree of this decrease differs depending on the wavelength. The change in spectral slope with increasing phase angle was evaluated in the 450-550 nm and 550-750 nm ranges using values normalized to the reflectance at 550 nm. The Ryugu sample showed a dominant decrease in the spectral slope at longer wavelengths, suggesting a trend toward phase bluing. On the other hand, the Bennu sample showed a dominant increase in the spectral slope at shorter wavelengths, suggesting a trend toward phase reddening. Thus, within the phase angle range of approximately 22-40° assumed for close-range observations by OROCHI, the relationship between phase angle and color tends to differ even for similar materials such as the Ryugu and Bennu samples. This result emphasizes the need for appropriate phase angle correction to obtain accurate reflectance spectra with OROCHI during close-range observations.