Abstract
ABSTRACT Deriving thermophysical properties such as thermal inertia from thermal infrared observations provides useful insights into the structure of the surface material on planetary bodies. The estimation of these properties is usually done by fitting temperature variations calculated by thermophysical models to infrared observations. For multiple free model parameters, traditional methods such as least-squares fitting or Markov chain Monte Carlo methods become computationally too expensive. Consequently, the simultaneous estimation of several thermophysical parameters, together with their corresponding uncertainties and correlations, is often not computationally feasible and the analysis is usually reduced to fitting one or two parameters. Data assimilation (DA) methods have been shown to be robust while sufficiently accurate and computationally affordable even for a large number of parameters. This paper will introduce a standard sequential DA method, the ensemble square root filter, for thermophysical modelling of asteroid surfaces. This method is used to re-analyse infrared observations of the MARA instrument, which measured the diurnal temperature variation of a single boulder on the surface of near-Earth asteroid (162173) Ryugu. The thermal inertia is estimated to be 295 ± 18 $\mathrm{J\, m^{-2}\, K^{-1}\, s^{-1/2}}$, while all five free parameters of the initial analysis are varied and estimated simultaneously. Based on this thermal inertia estimate the thermal conductivity of the boulder is estimated to be between 0.07 and 0.12,$\mathrm{W\, m^{-1}\, K^{-1}}$ and the porosity to be between 0.30 and 0.52. For the first time in thermophysical parameter derivation, correlations and uncertainties of all free model parameters are incorporated in the estimation procedure that is more than 5000 times more efficient than a comparable parameter sweep.
Highlights
Thermal conditions on atmosphereless, small Solar system bodies are governed by the thermophysical properties of the surface material, e.g. thermal conductivity, heat capacity, and emissivity
The 20 ensemble square root filter (ESRF) simulations were run for 20 asteroid rotations starting with a randomly chosen thermal inertia each
The last thermal inertia estimates of all ensemble members over all 20 simulations were combined into the histogram, i.e. 1000 thermal inertia estimates make up the final result of = 299 ± 4 J m−2 K−1 s−1/2 with the uncertainty given by the 2σ bound, where σ is the standard deviation over the thermal inertia set
Summary
Thermal conditions on atmosphereless, small Solar system bodies are governed by the thermophysical properties of the surface material, e.g. thermal conductivity, heat capacity, and emissivity. Thermophysical and structural material properties can be derived from thermal infrared data. The thermophysical properties of numerous Solar system bodies have been investigated using telescopes (e.g. Masiero et al 2011; Muller, Hasegawa & Usui 2014; Harris & Drube 2016; Muller et al 2017), or satellite remote sensing data The mission included the MASCOT lander (Ho et al 2017) that, among other instruments, carried a thermal infrared radiometer (Grott et al 2017). Res., 112, E05007 Ogawa K., Hamm M., Grott M., Sakatani N., Knollenberg J., Biele J., 2019, Icarus, 333, 318 Okada T. et al, 2017, Space Sci. Rev., 208, 255 Okada T. et al, 2020, Nature, 579, 518–522 Paige D. Rev., 150, 125 Pelivan I., 2018, MNRAS, 478, 386 Pelivan I., Drube L., Kuhrt E., Helbert J., Biele J., Maibaum M., Cozzoni
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