Remote Boulder counting and thermal IR temperature curves constrain strength, microporosity, thermal conductivity and grain density of rubble pile Ryugu's rock

Abstract

In this paper, we summarize the findings and deductions for small asteroid Ryugu from Hayabusa2 remote sensing as well as from MASCOT radiometer (MARA) data. Observations cover the VIS (broadband) and MIR (broadband) wavelength ranges. For a typical rock on Ryugu's surface, we find a thermal inertia of 295±18 Jm-2K-1s-1/2 (Hamm+ 2020), a microporosity of 50 ± 3% (Grott+ 2020, Hamm+, 2020), and assuming a CM composition and thus an inferred specific heat capacity of cp = 890 Jkg-1K-1 (± 10%, at an average temperature of 277 K), we estimate a thermal conductivity of 0.069 ± 0.012 Jm-1K-1 at ~277 K. These estimates are based on MARA surface brightness temperature measurements of an arguably (Biele+ 2019) dust-free boulder at MASCOT's landing site obtained over a full diurnal cycle. Those values are consistent with the TIR instrument's global findings (Okada+ 2020). The main source of uncertainty in the thermal inertia estimate is due to the uncertain surface orientation of the boulder top that determines the insolation power. Including a Digital Terrain Model (DTM) of the observed boulder, embedded in the MASCOT landing site (Scholten+, 2019), into the thermal model could reduce this uncertainty significantly. The very high deduced microporosity lets us reasonably estimate the tensile strength of those abundant "cauliflower rocks" (Jaumann+, 2019), ~200-280 kPa (Grott+, 2019).