Voluminous volcanism on early Mars revealed in Valles Marineris

The relative rates and importance of impact cratering, volcanism, erosion, and the deposition of sediments to the early geological history of Mars are poorly known. That history is recorded in the upper crust of the pillet, w hich is best exposed along the 4,000-km-long canyon system called Valles Ms trincris. Previous studies of the stratfgraphy of this region have assumed that it consists of megabreccia and fractured bedrock resulting from impact s, overlain by or interbedded with relatively thin layers oflava, and tvith the layering restricted to the uppenaost level of the crust(1-6). IIere we report new high-resolution images that reveal ubiquitous horizontal layeri ng to depths of at least 8 km in the canyons. Megabreccia should be only co arsely layered and fractured bedrock should be unlayered, so these observat ions indicate that volcanic or sedimentary processes were much more importa nt in early martian history than previously believed. Morphological and com positional data suggest that the layers were formed mainly by volcanic floo d lavas, Mars was therefore probably very volcanically active during at lea st the first billion years and after the period when the heaviest impact bo mbardment had ended.