dc.description.abstract
Mercury is the closest planet to the Sun and its formation still remains unclear. It is an important boundary condition for formation models of our solar system and planetary system in general. Among the exoplanets many planets have been discovered in similar distances from their host star. Due to the closeness to the Sun Mercury experiences extreme temperature variations (-150degC – 450degC) during its day-night cycles. Spectroscopy is one of the most powerful techniques to study the surface mineralogy of any planetary body from its orbit. Various spectral ranges provide different insights to the surface we look at. In order to spectrally interpret the surface mineralogy, it is important to understand the spectral behavior of the planet’s analogues under its respective surface environment conditions in a controlled laboratory setup.
This thesis focuses primarily on understanding the composition of Mercury from orbital remote sensing observations, especially with the Mercury Radiometer and Thermal Imaging Spectrometer (MERTIS) instrument onboard ESA-JAXA BepiColombo mission to Mercury. MERTIS will be the first radiometer and the first thermal infrared (TIR; 7-14 μm) hyperspectral spectrometer to orbit Mercury. To this purpose a specialized spectral library was created of various Mercury analogues under their extreme environmental conditions as a function of temperature under vacuum, the stability and spectral signature of a range of sulfides was studied, the derivation of various spectral parameters to facilitate the mapping of surface composition was evaluated and with these the analysis of data in the visible and near-infrared by the NASA MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) was revisited. The specialized emissivity spectral library of 7 synthetic sulfides and 10 terrestrial silicates, as potential Mercury analogues, has been developed as a function of temperature ranging from 100degC to 500degC under vacuum. Put together, they will aid the mapping of the crustal and volcanic mineralogy of the Mercury surface. The new spectral parameter developed in the presented work on laboratory silicate mineralogy study shows that olivine, pyroxenes, and feldspar groups can be successfully separated by MERTIS.
MESSENGER mission carried two visible infrared spectrometers (0.45-1.45 μm), Mercury Atmospheric and Surface Composition Spectrometer (MASCS; hyperspectral-point spectrometer), and Mercury Dual Imaging System (MDIS; multispectral-imaging spectrometer), to map the surface mineralogy of Mercury. However, due to Fe-poor nature of the Mercury surface materials, the spectrometers revealed featureless red-sloped spectra across major surface units. This limited the direct mapping of the silicate mineralogy of the Mercury’s crustal and volcanic rocks, and therefore, remains only as estimated mineralogy using the data obtained from geochemistry suite. In the presented study, the global multivariate analysis of MASCS data was carried out to understand the spectral diversity of the planet and its relation to the known geochemical terrains mapped by geochemistry suite. The study for the first time spectrally distinguished between low-Mg and high-Mg northern volcanic plains.
One of the major discoveries of NASA MESSENGER mission is the discovery of widely spread hollows on Mercury surface, which are indicative of possible sublimation processes of the volatile-rich minerals. As sulfides are strongly proposed as the potential candidates of hollow materials, MgS, FeS, CaS, CrS, TiS, NaS, and MnS are studied for its emissivity spectra as a function of temperature in vacuum, further, the ultraviolet- to far-infrared reflectance spectroscopy of the fresh and thermally weathered sulfides under vacuum are also investigated in the presented work. This unique spectral library of sulfides will enable mapping of volatile-rich surface mineralogy of Mercury by spectrometers onboard MESSENGER and BepiColombo missions. Further, the extended study presented in this thesis investigates the emissivity spectral behavior of CaS for four simulated Mercury days. The results from this study prove CaS is the most stable sulfides that survives the extreme thermal environment of Mercury and is an important tracer for other sulfides those might be lost in the hollow-forming process dominated by sublimation. The emissivity spectra reported here are significant for the detection and mapping of CaS associated with hollows and pyroclastics using MERTIS datasets.
This thesis is a cumulative work comprising of six peer-reviewed manuscripts out of which three are published and three are in correspondence with their respective journals for publications.
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