[PPS07-09] Highly Porous Asteroid 162173 Ryugu Derived from Lowest Temperatures in a Day
Keywords:Hayabsua2, Thermal Infrared Imager, Thermal Inertia, Lowest Temperature
Thermal Inertia Estimate by TIR: There are several ways to derive the thermal inertia of an asteroid: 1) peak temperature in a day, 2) delay of peak temperature from the local noon, 3) fitting the diurnal temperature curves (hopefully considering the surface roughness), 4) cooling rate after sunset, 5) warming rate after sunrise, 6) lowest temperatures just before sunrise. For the initial analysis, we have taken the methods 1) to 3) to derive thermal inertia, but not considering the surface roughness, which is enough to characterize the thermal inertia much higher than the case of sandy or granular regolith (<100 tiu), and much lower than the typical primitive meteorites (> 600 tiu) or even dense rocks (> 1000 tiu) . Afterwards, we considered the surface roughness at the same time to fit the diurnal temperature profiles much better to derive it thermal inertia of 225 ± 45 tiu, with the roughness of 0.41 ± 0.08 .
In this study, we used the method 6), when we observed the asteroid from a large phase angle of > 40° and at the solar distance of 0.98 to 0.99 au, during Box-B observations on 22 to 24 August 2019. The surface roughness is not considered but it is less effective in the nighttime. Temperatures before sunrise correspond to the apparent thermal inertia of 200 to 300 tiu, which is basically the same as the values derived from the daytime observations.
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. J17H06459 (Aqua Planetology), and the Core-to-Core program “International Network of Planetary Sciences”.
References:  Okada T, et al. (2017) SSR 208, 255-286,  Okada, T. et al. (2020) Nature in print,  Shimaki Y. et al. submitted to Icarus.