[PPS03-P06] Thermal Infrared Imaging of C-type Asteroid 162173 Ryugu with Porous Surfaces
Keywords:Hayabsua2, Thermal Infrared Imager, Thermal Inertia
TIR is the same design as the LIR on Akatsuki Venus climate orbiter , with 8 to 12 μm wavelength range, 328 x 248 effective pixels, and the FOV and IFOV of 16.7° x 12.7° and 0.051° per pixel, respectively. The first image of Ryugu detected by TIR was a point source on 6 June 2018, as was previously predicted . Disc-resolved thermal images of Ryugu was taken from the HP on 30 June. The surface temperature was measured at 300 to 370K at about 1AU from the Sun, and gradually decreased with the solar distance. Comparison of the results with modeling  shows that the surface of Ryugu is not like fine regolith nor base rocks. Most of large boulders on Ryugu show temperatures almost the same as the surrounding surfaces. Diurnal temperature profiles of Ryugu are rather flatter than expected for a smooth terrain. Apparent thermal inertia is ~300 tiu or larger, but probably lower than the value when the surface roughness is considered. All these facts are consistent with a very rough and porous surface with low porosity (~35 ± 10 %)  and the surrounding surface is dominated by porous rocks larger than several centimeters. Macro porosity should be 30 to 50 % to account for the bulk density of Ryugu ~1200 kg m-3.
During the descent operations for the release of landers and the touchdowns for sample collection, TIR has taken close-up thermal images of the local sites, continually from the altitude of 500 m until the start of final descent at 10 or 20 m altitude. Close-up thermal images show the surface physical state and a variety of boulders. Most of the boulders in close-up images have temperatures similar to the surroundings but with more variety. A small number of boulders are remarkably colder than the surroundings, indicating dense rocks with lower porosity.
These facts indicate that Ryugu is probably a rubble pile of loosely-bound porous rocks, which might originate from the outer layer of parent body where compaction did not occur effectively, while the dense boulders might originate the compacted center region of parent body or from the different parent body. A different composition (e.g., organic rich materials) cannot be ruled out so far to account for such a low thermal inertia (and low density) of Ryugu.
Acknowledgments: The authors appreciate all the members of the Hayabusa2 Project and supporting staff for their technical assistance and scientific discussions. This research is partly supported by the JSPS KAKENHI No. J26287108, No. J17H06459 (Aqua Planetology), and the Core-to-Core program “International Network of Planetary Sciences”. MDB acknowledges support from CNES.
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