[PPS07-P09] Temperature variations of boulders on Ryugu observed in Hayabusa2 decent operations
Keywords:Hayabusa2, Ryugu, C-type asteroid , The thermal infrared imager, Temperature variations, Boulder
The purpose of the Hayabusa2 mission is not only to obtain samples of the C-type asteroid 162173 Ryugu but also to acquire lots of information about Ryugu . After arrival at Ryugu, the Hayabusa2 has performed a variety of science observations using remote sensing instruments such as Optical Navigation Camera (ONC), Near-Infrared Spectrometer (NIRS3), and Light Detection and Ranging (LIDAR) to characterize Ryugu. The observations by the Thermal Infrared Imager (TIR) , a mid-infrared thermographic camera mounted on Hayabusa2, allow us to analyze the digital thermal images which indicate the thermal radiation from Ryugu in order to determine the physical state of the asteroid surface.
TIR has a field of view (FOV) of 16.7°×12.7° and the effective pixels of the detector of 328×248, resulting in the spatial resolution about 0.051° per pixel . A temperature range that TIR can cover is 150 to 460 K and the well-calibrated temperature range is 230 to 420 K.
Global thermal images of Ryugu indicate low thermal inertia, suggesting a highly porous physical state of the surface relative to typical carbonaceous chondrites, even for large boulders . Close-up thermal images taken during the descent operations show that most of the surface is also covered with highly porous boulders, but some boulders are obviously more consolidated like typical carbonaceous chondrites.
In this study, we investigated temperature variations of boulders and their physical state in the specific regions. We used the high-resolution thermal images taken below the altitude of 500 m during the descent operations for the release of MINERVA rovers on 21st September 2018, the release of MASCOT lander on 3rd October 2018, the touchdown rehearsals (TD1-R1A on 15th October 2018 and TD1-R3 on 25th October 2018), the first touchdown (TD1) on 21st February 2019, and the close-up observations around the second touchdown candidate region (DO-S01) on 8th March 2019. We measured the temperatures of individual boulders on these areas using the TIR images. We will report and discuss the details of the results of the temperature variations of surface boulders of these above regions on asteroid Ryugu.
Acknowledgments: The authors appreciate Dr. Koji Matsumoto and Kyoko Yamamoto at the National Astronomical Observatory of Japan for the use of the LIDAR corrected trajectory of the Hayabusa2 spacecraft. This study is partly supported by the JSPS Kakenhi No. JP17H06459 (Aqua Planetology) and the JSPS Core-to-Core Program “International Network of Planetary Sciences”.
References:  Watanabe S. et al., Science 464, 268-272 (2019),  Okada, T. et al., Space Sci. Rev., 208, 255-286 (2017),  Okada, T. et al., Nature (accepted).