Surface physical condition of asteroid Ryugu using close-up optical and thermal images

In 2018, the Hayabusa2 spacecraft [1] successfully conducted some descend operations toward Ryugu’s surface. They included MINERVA rover release in September, MASCOT lander release and two touchdown rehearsals in October. During these operations, we acquired high-resolved optical and thermal images from altitudes below 100 m, us- ing Optical Navigation Camera (ONC-T) and Ther- mal Infrared Imager (TIR), respectively. Close-up optical images by ONC-T show detailed physical conditions of the surface materials, such as particle size distribution of pebbles, surface morphol- ogy of small boulders and craters. Moreover, close-up thermal images by TIR indicate thermophysical prop- erties of the surface materials and its regional differ- ence, which cannot be resolved by higher altitude observations (e.g., home-position observations from 20 km altitude). Combination between optical and thermal observations is of great importance to under- stand the nature of the asteroid surface materials. In this study, we investigate the surface particle size from close-up ONC images. Thermophysical property of the surface component materials inferred from TIR images is also discussed, especially for the range observed for the thermal inertia of boulders

Fine-grained region with low thermal inertia in craters on Ryugu

Introduction: The Hayabusa2 spacecraft is surveying asteroid Ryugu from June 2018. Global observation by thermal infrared imager TIR revealed low thermal inertia around 200-300J m-2 K-1 s-0.5 [1]. The expected grain size from this thermal inertia was several centimeters [2]. However, the surface of Ryugu is globally filled with larger boulders [3]. These observational results indicate highly porous nature of the surface boulders with low thermal inertia, unexpectedly different from the carbonaceous meteorites we have. Local observations by MARA radiometer onboard MASCOT lander supported it [4, 5]. In addition to the global observation, close-up imaging by TIR during descending operations, including MINERVA and MASCOT releases, touch-down operations and their rehearsals, were conducted. The high resolution TIR images revealed local temperature anomaly related to the boulders [6] and craters, which showed local variation in thermophysical properties.In this study, we show survey of thermally-unusual craters on Ryugu using these close-up images. The unique feature were found as hot spot around the center of some small craters