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Thermal Infrared emissivity measurements of Lunar simulants: Application to the MERTIS/BepiColombo data

Presentation #220.10 in the session Laboratory Investigations (Poster + Lightning Talk)

Published onOct 23, 2023
Thermal Infrared emissivity measurements of Lunar simulants: Application to the MERTIS/BepiColombo data

The Mercury Radiometer and Thermal Infrared Spectrometer (MERTIS) is part of the ESA BepiColombo Mercury Planetary Orbiter (MPO) payload. MERTIS consists of a push-broom IR-spectrometer (TIS) and a radiometer (TIR) which operate in the wavelength domains of 7-14 μm and 7-40 μm, respectively. During its long cruise to Mercury, BepiColombo have been flyby the Earth on April 10th, 2020. MERTIS had the opportunity to observed the Moon through its space baffle during this flyby. The instrument provided the first hyperspectral observations of the Moon in the thermal infrared wavelengths from space. Laboratory measurements on Lunar analogs are needed to interpret this dataset. Here, we present thermal infrared emissivity and hemispherical reflectance measurements of lunar sample and simulants under simulated Moon conditions over the range of wavelength of MERTIS. Samples consist of an Apollo 16 powder, several mixtures (e.g. DLR mixture), terrestrial analogs (Mojave and Eifelsand) and simulants from NASA, USGS and the Exolith lab. Measurements are performed in the Planetary Spectroscopy Laboratory (PSL) at the Deutsches Zentrum für Luft- und Raumfahrt (DLR). PSL offers the capability to measure emissivity of powder materials, in air or in vacuum, across a large range of temperatures (100-1000°C) and over an extended spectral domain (0.2-200 µm). Measurements are made by using a Bruker Vertex 80V spectrometer operating in the UV-VIS-NIR and in the Mid- to far-IR. The instrument is purged with dry air and equipped with a liquid-nitrogencooled HgCdTe (MCT) detector. The spectrometer is attached to a vacuum emissivity chamber for measurements from 50° to at least 600°C. Here, thermal infrared emissivity is measured on samples heated to 130°C inside steel cups under the vacuum chamber. Hemispherical measurements are made at ambient temperature on samples before and after heating at 130°C. Hemispherical reflectance can be converted to emissivity following the Kirchhoff’s law (emissivity = 1 – reflectance) whereas this law does not apply to bidirectional reflectance. Emissivity and hemispherical reflectance measurements are compared to the MERTIS hyperspectral data. Laboratory spectra allow a better understanding of the MERTIS data of the Moon but also prepare the observations of the Mercury surface, the main objective of MERTIS. The surface composition of the Moon and Mercury have been frequently compared in the literature. MERTIS spectral domain will be crucial to determine the composition of the Mercury’s surface and a good interpretation of its dataset is needed.

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