Unmixinig of Laboratory IR Spectral Reflectance Measurements of Low-Mg Northern Volcanic Plains Analogs

Abstract

The MErcury Radiometer and Thermal Infrared Spectrometer (MERTIS) is part of the payload of ESA/JAXA’s BepiColombo mission, launched in Octo- ber 2018. MERTIS will observe the surface of Mercury in the wavelength range of 7 μm to 14 μm (IR spectrometer) and 7 μm to 40 μm (radiometer) . This range covers several spectral features, such as the Christiansen feature (CF), Reststrahlen bands (RBs), and Transparency feature (TF), which allow the deter- mination of Mercury’s surface mineralogy. MERTIS’ scientific objectives are to a) study Mercury’s surface composition, b) identify rock-forming minerals, c) globally map the surface mineralogy, and d) study surface temperature and thermal inertia. During its cruise to Mercury, the spacecraft has al- ready completed five swing-by maneuvers at Earth/Moon (2020), Venus (2020&2021), and Mercury (2021&2022). In the following years it will perform fur- ther swing-by maneuvers at Mercury . MERTIS already operated during the first swing-by maneuvers and acquired thermal infrared spectra of the lunar surface and Venus. Thus, these flybys offered the first opportunity to test the instrument under ‘real’ plan- etary observation conditions and allowed a first in-flight calibration . Starting orbital measurements in 2026, MERTIS will be the first instrument to observe the surface of Mercury in the IR wavelength range . The current knowledge of IR spectroscopy of Mercury is based on terrestrial telescopic observations . Based on obser- vations from the MESSENGER spacecraft several chemically distinct regions have been identified: Low- Mg Northern Volcanic Plains (Low-Mg NVP), High- Mg NVP, Smooth Plains (SP), Inter Crater Plains and Heavily Cratered Terrains (IcP-HCT), and High-Mg Province . The surface rocks are composed of a variety of dif- ferent minerals. The ~500 m/pixel ground resolution of MERTIS will therefore depict a mixture of these miner- als. In order to quantify the mineral abundances, we use a non-linear unmixing model, based on the Hapke re- flectance model . The “single-scattering albedo” describes the intrinsic reflectivity of an average single surface . Multiple scattering within a surface is gov- erned by the incidence and emission angle, which also depends non-linearly on the single-scattering albedo. The scattering behavior of an individual particle is de- scribed by a “single-particle scattering function”. The full set of equations is provided by . The unmixing model we use in this study has previ- ously been used for spectral unmixing of NASA RELAB data and lunar analog materials . In the framework of MERTIS it is applied to laboratory min- eral mixtures, including glasses and varying grain sizes