The thermal infrared spectral characteristics of hematite and its origin in Aram Chaos, Mars.

摘要

The identification of gray crystalline hematite on Mars by the Mars Global Surveyor (MGS) Thermal Emission Spectrometer (TES) instrument has resulted in the publication of many theories regarding its formation, and the landing of the Mars Exploration Rover Opportunity at Meridians Planum, the site of the largest hematite deposit. This dissertation focuses on three distinct problems regarding the origin of martian crystalline hematite. First, I compared a large suite of thermal infrared spectra from a variety of synthetic and natural hematite samples to the average martian hematite spectrum. Analysis of these spectra indicates that volcanically-derived hematite is a poor spectral match to the martian hematite spectrum. Thermal infrared spectra of goethite-derived hematite samples provide good spectral matches to the martian hematite spectrum. The lack of a spectral match between volcanically-derived hematite spectra and the martian hematite spectrum, as well as the match between the martian hematite spectrum and a goethite-derived hematite spectrum is presented as evidence for an aqueous origin for the martian hematite.; Of particular interest when examining martian thermal infrared hematite spectra is the region near 390 cm-1. Some hematite spectra have a small absorption feature in this region, while others, including the martian hematite spectrum, lack the feature. I used dispersion theory and modified Fresnel equations to model the thermal infrared spectra of platy hematite crystals of variable thickness. The results of this study suggest that for very platy hematite crystals (90% or more of total emission parallel to the [001] axis) a 390 cm-1 feature does not appear even at the highest emission angles. As the hematite crystal increases in thickness and the total contribution of [001] axis emission decreases, a 390 cm-1 feature appears at increasingly shallow emission angles.; Finally, I examined data from the Thermal Emission Imaging System (THEMIS), as well as TES, over Aram Chaos, the site of the second-largest known hematite deposit. Superposition relationships between the various mapped units in Aram Chaos indicate that the layered units, one of which includes gray hematite, were deposited subsequent to the chaos-forming event. The presence of hematite, unit superposition relationships, as well as the tentative identification of sulfate minerals in the Aram Chaos layered units are presented as evidence that the layered units were deposited in a lacustrine environment.