Spectrophotometric mapping of Coprates quadrangle, Mars.

Abstract

This volume describes the application of quantitative image analysis as a means of investigating the geologic history of the Valles Marineris region of Mars. Three approaches are employed for extracting geological information from Viking Orbiter images. First, multispectral composites are studied for color variations among geologic units and to map their spatial distribution in Coprates Quadrangle. Multitemporal images are next examined for variable features of the martian surface and atmosphere. Finally, multiple phase-angle images are analyzed for variations in the photometric phase function of the surface. The results suggest that volcanism, tectonism, magmatic intrusion and eolian redistribution have each played a part in shaping the martian canyons. Several distinct spectrophotometric units are recognized in Coprates Quadrangle on the basis of the imaging data. The dark red Lunae plains are spectrally and photometrically related to the brighter dust-mantled plains, canyon wall rock and interior layered sediments, indicating a physical and not compositional difference between these materials. Spectrally exceptional sedimentary deposits are evidence for compositional diversity among the bright materials and suggest multiple origins for the layered sediments. Exposed competent bedrock surfaces include lowlying Hesperian volcanic plains and a series of cliffs interpreted as a possible sill. Dark gray, friable and incompetent materials include an in-situ Noachian wall rock layer and locally generated dune-forming sands. These materials are found to be markedly forward-scattering, casting doubt on notions that these dune deposits are composed of lithic fragments or dust grain aggregates, two of the proposed sources for the puzzling martian sands. Interpretation of this unit as a voluminous regional malic tuff deposit may have important implications for the style of past volcanism on Mars. Bright gray materials include young, possibly recent volcanics with a derived distribution consistent with the interpretation of extrusion along faults near the margins of the canyon floors. This result supports the hypothesis that the Valles originated through tectonic extension, which along with volcanism, generation of dark sands, and eolian deposition and erosion of bright dust from global storms, are processes which may continue in the Valles Marineris today.