Geometric Correction for Thermal Imaging of Asteroid Ryugu Observed by TIR onboard Hayabusa2

The thermal infrared imager (TIR) onboard the Hayabusa2 spacecraft performed thermographic observations of the asteroid 162173 Ryugu (1999 JU3) from June 2018 to November 2019. In this study, we performed a geometric correction for TIR images by making a one-to-one correspondence between the observed areas and the surface coordinates derived from a shape model of Ryugu. The pointing direction, which is an alignment direction of TIR, was adjusted by rotating the TIR frame relative to the base of the Hayabusa2 frame using a least-squares fit. This geometric correction allows us to identify observed local areas within one pixel, which corresponds to 5 m error in a 5 km altitude observation. The corrected temperature images projected onto the shape model were constructed. Hot temperature regions were found at the base of Ejima Saxum and Otohime Saxum, for instance. A simulation result indicates that multiple radiations from the surrounding terrains generate hot regions. The estimated thermal inertia of the base of Ejima Saxum as characteristic shape area is approximately 300 Jm -2 s -0.5 K -1 within the error bars of the observed temperature profile. This estimation is succeeded by performing the geometric correction in case that the surface topographic features are greater than the spatial resolution of the pixel. However, thermal inertia estimations of smooth terrains, such as the floor of Urashima crater, were difficult probably because of surface roughness effects. Our results suggest the necessity to develop a hybrid thermophysical model that implements large- and small-scale surface roughness.