1. Scarps, Ridges, Troughs, and Other Lineaments on Mercury. 2. Geologic Significance of Photometric Variations on Mercury

Volcanic and tectonic implications of the surface morphology of Mercury are addressed in two separate sections. In Part 1, mercurian scarps, ridges, troughs, and other lineaments are described and classified as planimetrically linear, arcuate, lobate, or irregular. A global pattern of lineaments is interpreted to reflect modification of linear crustal joints formed in response to stresses induced by tidal spindown. Large arcuate scarps on Mercury most likely record a period of compressional tectonism near the end of heavy bombardment. Shrinkage owing to planetary cooling is the mechanism preferred for their production. Two planimetrically lobate escarpments probably formed by uplift along intersecting elements of the global mercurian lineament pattern. One may subsequently have been modified by extrusive igneous activity along its trace. Most irregular scarps inside craters are interpreted to be tectonic features formed in response to local stresses, perhaps induced by subsurface magma movements. Large linear ridges on Mercury may record a period of volcanism responsible, at least in part, for intercrater plains formation. Linear ridge production is speculatively attributed to accumulation of extruded material along linear vents, and to differential erosion around relatively resistant dikes intruded into near-surface materials. Linear, open-ended troughs are well-developed in a distinct terrain unit on Mercury characterized by intense modification of pre-existing landforms. Regional trends defined by these troughs are consistent with those of the global mercurian lineament pattern. Combined with their regional setting, this suggests that the troughs formed by differential erosion along linear crustal fractures. A few are radial from nearby large craters, and may be highly modified chains of secondary impact craters. Scarps, ridges, and troughs in and around Caloris Basin define trends radial from the basin center and concentric with its rim. A radial system of linear ridges outside Caloris probably reflects the combined effects of ejecta deposition and erosion during the basin-forming event. Planimetrically irregular ridges developed in smooth plains inside Caloris may owe their origin to regional subsidence, perhaps in response to magma withdrawal from below to form smooth plains outside the basin rim. Gravitational readjustment owing to loading by plains material may be responsible for scarp and ridge formation outside Caloris. Finally, isostatic readjustment to basin excavation may have caused regional uplift inside the basin to form a system of planimetrically irregular troughs. In Part 2, measurements of local normal albedo are combined with computer-generated photometric maps of Mercury to provide constraints on the nature of mercurian surface materials and processes. If the mercurian surface obeys the average lunar photometric function, its normal albedo at 554 nm is .16±.03. This is roughly 40% higher than the corresponding lunar value, but the difference may be largely attributable to differences in the photometric function s of the two bodies, and to unmodelled effects such as multiple scattering. The existence of relatively bright smooth plains confined to crater floors is most easily reconciled with a volcanic origin for some mercurian smooth plains. Lack of photometric contrast across most large escarpments on Mercury is consistent with the tectonic origin for these features inferred from morphologic studies. Local photometric and transectional relationships in two instances suggest mantling of preexisting topography by younger, perhaps volcanic, material. Brightness of several extremely localized patches in large craters is attributed to enhanced backscatter owing to multiple reflections relative to surrounding plains and craters. These patches are generally "bluer" than typical mercurian plains, and some are surrounded by material which is "redder" than typical plains. Chemical alteration of crustal rocks, perhaps related to fumarolic activity along impact-induced fractures, is the preferred explanation for these uniquely mercurian features.