Mercury is hot: A fractal thermal roughness Model for MERTIS spectral calibration

Kurzfassung

The BepiColombo mission is a joint project of the European Space Agency (ESA) and the Japan Aerospace Exploration Agency (JAXA). The Mercury Planetary Orbiter (MPO) carries the Mercury Radiometer and Thermal Infrared Spectrometer (MERTIS) that is planned to acquire infrared spectra of Mercury in the range of 7−14μm. The instrument is designed to characterize Mercury's surface composition, identify rock-forming minerals, map the surface mineralogy, and study the surface temperature variations and the thermal inertia. For mineralogical analysis, the emissivity has to be extracted from radiance measurements. In a laboratory setting, for a smooth surface or observations of planetary bodies under small emission and incidence angles, simple division of the measured spectral radiance by the Planck function for a single temperature value is sufficient. However, airless planetary bodies are rough and have comparatively low thermal inertias such that strong thermal gradients establish on scales of a few millimeters. Consequently, the thermal radiation detected by an infrared sensor significantly deviates from a simple black body spectrum, mainly when observed under oblique illumination geometry. The radiation due to thermal emission is no longer a simple Planck function but a non-linear superposition of many Planck functions with a unique mathematical structure. Consequently, surface roughness, self-heating, shadowing, and the spatial scales on which these effects occur must be considered. Various approaches to model surface roughness have been discussed before. We implemented a comprehensive thermal roughness model for airless bodies that enables accurate emissivity calibration and roughness analysis of the Moon and Mercury. Several methodic improvements such as fractal surfaces, fast self-heating computation, and angular dependent bolometic albedo make the model accurate and versatile. We present the most recent update of our model applied to two interesting datasets: Lunar infrared images taken by the Chinese weather satellite Gaofen 4 and MERTIS lunar measurements acquired during a gravity assist maneuver in April 2020. These datasets serve to test the model and prepare it for the prospective analysis of Mercury.