Thermal Infrared Spectroscopy of Igneous Rocks at Simulated Mercury's Surface Environment

Abstract

Recent findings by the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft, corroborated by petrologic modeling, revealed the surface composition of Mercury as mainly constituted by low-Fe and Mgrich basalts [1, 2]. This interpretation dismisses the previously assumed widespread presence of more felsic materials - as on the Moon’s surface - leaving open the question of the crust petrogenesis of Mercury. Assessing the presence of a differentiated crust on the surface of Mercury is among the core objectives of the upcoming ESA/JAXA BepiColombo mission to Mercury [3]. The ESA Mercury Planetary Orbiter will carry the Mercury Radiometer and Thermal Imaging Spectrometer (MERTIS) covering the spectral range 7- 14 µm to map the surface mineralogy of the planet at 500 m spatial resolution. The interpretation of spectra collected along a wide range of daily surface temperatures is complicated by the shift of the location of the band minima depending on the temperature [4, 5]. In addition, the simultaneous presence of different minerals, each one with its own characteristic thermal expansion coefficient, results in a more difficult interpretation of the spectra. In this work we examine the thermal infrared (TIR) spectral variations of linear mixtures of plagioclase and pyroxenes that most likely could be present in the differentiated igneous crust of Mercury, in addition to the T-dependent spectral variations of the single constituents. Furthermore we show how two different thermal expansion coefficients combined in the same sample reveal in the resulting thermal infrared spectrum. To this aim we measure high-T emissivity - up to 450° C - of synthetic and natural compositions (e.g., anorthosite, gabbro, norite) and their components