Spectroscopy of silica and the remote detection of astrobiologically rich environments

Details Spectroscopy of silica and the remote detection of astrobiologically rich environments Spectroscopy of silica and the remote detection of astrobiologically rich environments

Apr 2003

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003eaeja.....2057m&link_type=abstract

EGS - AGU - EUG Joint Assembly, Abstracts from the meeting held in Nice, France, 6 - 11 April 2003, abstract #2057

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Scientific paper

Fundamental to the exploration of Mars and other solid planets for astrobiology is the desire to locate sites where rocks have been aqueously altered or where aqueous sediments exist. One material that is common and abundant on Earth, and that is likely to occur in any environment where silicate rocks have interacted with water, is silica. Silica is an important material for astrobiology because of its long crustal residence time and relative abundance on Earth, and because most of the early terrestrial microfossils are entombed in it. Presumably, if opaline deposits or microcrystalline silica are discovered on Mars, they will be high priority sites for further investigation. Not only is silica important in preserving biological materials directly, it is also an important mineralogical indicator of geological environment. For example, remote detection of the high temperature polymorphs tridymite or cristobalite would be suggestive of volcanic or hydrothermal-fumarolic activity. Discovery of various silica polymorphs on Mars would be important for understanding the geologic context of deposits, as well as for identification of astrobiologically interesting materials. Given the availability of thermal infrared spectra of Mars from the NASA TES and THEMIS experiments, and the anticipated results of Mini-TES from the surface of Mars, an investigation of the thermal emission spectra of silica minerals and siliceous materials is timely. We present results of a thermal infrared spectral analysis of silica polymorphs and some siliceous rocks. The spectral effects of crystal structure, crystallinity, composition, and surface roughness are examined. All amorphous forms, including silica glass, hyalite (opal-AN), and opal (A), exhibit very similar emission spectra. Partially crystalline silica, such as opal-CT is distinguishable from amorphous forms by the depth and shape of the major reststrahlen feature. Spectra of cristobalite and tridymite bear unique Si-O vibrational absorption features from spectra of quartz due to differences in their crystal structures. Microcrystalline quartz has different spectral shape than coarsely crystalline quartz due to scattering from surface roughness along fractured and natural faces. In most cases, pure microcrystalline quartz (chert) can be distinguished from coarse quartz, but the presence of abundant optically thin, microcrystalline minerals, such as carbonates in the chert, may cause volume scattering to dominate the spectral shape. These results demonstrate that emission spectroscopy provides information about the structure and texture of silica, and suggest that hyperspectral thermal emission remote sensing may be used to characterize silica forms on planetary surfaces.

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