## 4 TIR Observations in Asteroid Proximity Phase
### Close-up Thermal Images of Ryugu  
Close-up thermal images of local sites of Ryugu have been taken by the TIR at the altitudes below 100 m during the release of landers and during sampling activities. Those close-up thermal images are the first data sets in planetary missions. Close-up thermal images show the surface physical state and a variety of boulders at centimeter scale. As was seen by optical navigation cameras and the surface landers, it was evident that the surface of Ryugu is widely covered with several centimeter- to meter-sized rocks and boulders, but not with fine regolith [20; 27; 28]. The surroundings in the global and local thermal images correspond to the rough surface terrains that are covered with boulders down to the tens-of-centimeter scale in the close-up thermal images (see Figure 5).  
These close-up thermal images have proven that the thermal inertia of large boulders and their surroundings are almost the same, since the surroundings consist of rocks larger than the typical thermal skin depth of centimeter scale. We also recognized several boulders as \"cold spots\", remarkably colder by more than 20 K compared to the surroundings [4; 26]. The \"cold\" boulders should be the dense rocks with lower porosity, and probably have typical thermal inertia of carbonaceous chondrite meteorites of 700 to 1000 tiu [14].  
As shown in Figure 5, close-up thermal images also show that surface boulders have fine structure in them, indicating rugged rocks with very low thermal inertia of \(<300\) tiu. Their porosity should be more than 30 % (probably \(-50\%\)) [4]. These low-density and high-porosity boulders indicate different origin from terrestrial rocks or even meteorites. Considering the planetary formation processes, the formation started with fluffy dust in the solar nebula, and bodies grew by coagulation and accumulation to form planetesimals. Continued accumulation resulted in larger bodies to become the parent body of Ryugu. It should be a porous body because it has never experienced thermal alteration nor igneous processes, except for a low-level consolidation. At the innermost layer of the body, materials should be more consolidated due to higher lithostatic stress and temperatures. After impact fragmentation, the fragments are accreted together to form the present-day asteroid Ryugu. That is why most of  
Figure 4: Thermal image of Ryugu taken by TIR on 1 August 2018 from the Mid-Altitude (5 km from Ryugu), with the spatial resolution of 4.5 m per pixel. The largest crater Uroshima is observed at the equator, and the largest boulder Othime is seen at the South Pole. Boulders larger than 20 m are all identified in this image set, showing almost the same temperature with the surroundings. The temperature scale is in Kelvin. (Image: hyb2_tir_20180801_152032_1Z.fit, ©)AXA).  
surface materials are highly porous, except for dense rocks from higher consolidated materials such as meteorites. They should be sediments of impact fragments slightly consolidated by the lithostatic stress in the deep inside of the parent body. It cannot be ruled out that the dense boulders might have originated from the impacted meteoroid and have survived during the impact fragmentation processes [4].