Modeling and Interpretation of In-Situ Radiometric Flux Measurements on the Surface of Asteroid (162173) Ryugu

Abstract

In the presented work, a thermophysical model was developed to calculate the temperature evolution of the surface of asteroid (162173) Ryugu. This asteroid is the target of the Hayabusa2 sample return mission, which carried a lander called Mobile Asteroid surface SCOut (MASCOT). The MASCOT RAdiometer (MARA) measured infrared fluxes emitted by the surface and these measurements were modeled and interpreted using the temperatures calculated with the thermophysical model. The thermophysical properties of Ryugu's surface material were retrieved, and the distribution of thermal forcing acting on the surface of Ryugu, which could cause rock breakdown through thermal fatigue, was studied. This thesis is a cumulative work comprising three studies that were published in peer-reviewed journals.

In the first study, the thermophysical model is presented along with a radiative heat transfer model that relates the simulated temperatures to the MARA measurements and which can account for an arbitrarily complex asteroid surface. The study also presents a method to derive the thermophysical properties of the surface by comparing the modeled fluxes to the observed ones. The presented method allows for the retrieval of thermal inertia, emissivity and rock abundance of the surface, and the study shows that the spectral slope between 6 and 15 µm should be derivable from MARA observations.

In a second study, the thermophysical model was used to study the thermal environment on a spherical atmosphereless body. The diurnal temperature range, which is the main driver of thermal fatigue, was investigated as a function of thermal inertia and rotation axis orientation. It was found that the latitude of the maximum diurnal temperature range is determined by the balance between length of night and insolation power and not limited to the equator. The results imply that most of Ryugu's surface is exposed to similar amounts of thermal forcing.

The third study presents the analysis of the data collected by MARA while observing the temperature evolution of a single boulder on the surface of Ryugu. Here, the model developed in the first study was used to calculate the temperature evolution of the surface within the field of view of MARA, taking the unknown surface orientation and complex illumination condition into consideration. By using the radiative heat transfer model and parameter estimation method presented in the first study, the thermal inertia of the boulder was constrained to a surprisingly low 247-375 J m-2 K-1 s-1/2. This result indicates a high porosity of the boulder material of 28 to 55 % which is consistent with CI or CM chondrites and which is also consistent with observations of other Hayabusa2 and MASCOT instruments. The high porosity implies that Ryugu formed from an aqueously altered and highly porous parent body.