Multiband thermal radiometry and related laboratory studies, indicating
possible origin and evolution of Ryugu
M. Hamm1,2, M. Grott2, H. Senshu3, J. Knollenberg2, J. de Wiljes1, V. E. Hamilton4, F. Scholten2, K. D. Matz2, H. Bates5,6, A.
Maturilli2, Y. Shimaki7, N. Sakatani8, W. Neumann2,9, T. Okada7, F. Preusker2, S. Elgner2, J. Helbert2, E. Kührt10,11, T.-M. Ho12,S. Tanaka7, R. Jaumann13,2, S. Sugita14
1

Institute of Mathematics, University of Potsdam, Potsdam, Germany, 2Institute of Planetary Research, German Aerospace
Center (DLR), Berlin, Germany, 3Planetary Research and Exploration Center, Chiba Institute of Technology, Narashino,Japan, 4Southwest Research Institute, Boulder, CO USA, 5Department of Earth Sciences, Natural History Museum, Cromwell
Road, London, SW7 5BD UK, 6Atmospheric, Oceanic and Planetary Physics, Oxford University, Parks Road, Oxford, OX1
3PU, UK, 7Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, Sagamihara, Japan,
8Department of Physics, Rikkyo University, Toshima, Japan, 9Universität Heidelberg, Heidelberg, Germany, 10Institute ofOptical Sensor Systems, German Aerospace Center, Berlin, Germany, 11Qian Xuesen Laboratory of Space Technology, China
Academy of Space Technology, Beijing, China, 12German Aerospace Center (DLR), Institute of Space Systems, Bremen,Germany, 13Freie Universität Berlin, Berlin, Germany, 14University of Tokyo, Bunkyo, JapanThe MASCOT lander’s radiometer [1] observed the diurnal variation of the surface temperature of single boulder on thesurface of (162173) revealing a low thermal inertia and high porosity [2]. While the instrument observes the surface throughsix filters, two broad band filter and four bands pass transmitting the region between 6 and 16 µm, only the data of the 8 – 12µm broadband filter was used in the previous studies [2, 3]. Coupling a thermophysical model to a combined DEM of boulderand landing site [4], and applying a data assimilation scheme for efficient parameter estimation [2], the analysis of the fullradiometer data becomes feasible. The thermal inertia and emissivity of the surface within the filter’s spectral ranges could beretrieved. The thermal inertia is estimated to be 255 – 265 J m-2 K-1 s-1/2, corresponding to a high porosity of 46 ± 1 %. Theemissivity in the broad filters is estimated to be 0.98 ± 0.1.
The emissivity estimates in the narrowband filters shows significant differences from band to band. In the 5.5 – 7 µm band theemissivity drops to band to 0.85 (-0.02, +0.01), reaches its maximum in the 8 – 9.5 µm band with 0.98 ± 0.01, drops in theadjacent 9.5 – 11.5 µm band to 0.94 (-0.02, +0.01), and rises again to 0.97 ± 0.01 in the 13.5 – 15.5 µm region. We form theratios of the emissivity within the 9.5-11.5 µm band and the 13.5 – 15.5 µm band with respect to the emissivity within the 8 9.5 µm band and compare them to equivalent emissivity ratios of mid-infrared spectra of powdered and thin section samples ofvarious carbonaceous chondrites. We find that respective ratios of aqueously altered CM and CI chondrites form a common
trend and our results for Ryugu lies within this cluster of CM and CI chondrites. The CI chondrites appear to be the best
spectral match in the mid-infrared. Our study indicates that despite the partially dehydrated appearance of the surface in thevisible to near-infrared wavelength range [5,6,7], the mid-infrared shows strong signs of aqueous alteration, and the Ryugu
materials might be less dehydrated than previously thought.

References[1] Grott, M., et al., 2017, Space Science Reviews, 208, 413–431. [2] Grott, M., et al., 2019, Nature Astronomy, 3, 971 – 976.
[3] Hamm, M., et al., 2020, Monthly Notices of the Royal Astronomical Society, 496, 3, 2776-2785. [4] Scholten, F., et al.,2019, Astronomy & Astrophysics, 632, L5. [5] Sugita, S., et al., 2019, Science, 364. [6] Kitazato, et al., 2019, Scienceeaav7432. [7] Kitazato, et al., 2021, Nature Astronomy, 5, 246–250.