Abstract

Ares Vallis is one of the greatest outflow channels of Mars, which are large complexes of fluid-eroded troughs. Several models to explain the origin and the evolution of this valley were proposed in the past; nevertheless, high-resolution images and elevation data acquired by newest Martian missions allow studying Martian features more in detail than in earlier periods. Aims of this work are to realize a geomorphological map of Ares Vallis and its valley arms using data acquired by recent Martian missions, and to attempt to draft its geological evolution. Investigations outline that Ares Vallis is characterized by features shaped by several catastrophic floods emanated by chaotic terrains. These features are overlaid by glacial and periglacial morphologies. Catastrophic flood features consist of erosional terraces, grooved terrains, streamlined uplands, giant bars, and an impressive cataract-like feature. Ice-related morphologies consist of ice-contact features, thermokarstic depressions, and patterned grounds. Geomorphological evidence suggests that catastrophic floods was, at place, more than 500 meters deep; furthermore, they was ice covered, confirming that the climatic conditions of the planet were similar to those of present day. The amount of time intervening among different floods varies from hundreds to thousand of years. At the end of each catastrophic flood, ice masses some hundreds of meters thick grounded on the valley floor, forming a stagnant dead-ice body. Catastrophic floods events were followed by relatively brief periods of warmer-wetter climatic conditions, possibly triggered by greenhouse effect of water vapor and carbon dioxide released in the atmosphere during the catastrophic flood processes. During these periods, water in equilibrium with the atmosphere etched thermokarstic depressions and channels on areas previously sculpted by catastrophic floods. Water flowing on ice-walled streams emplaced ice-contact deposits. Finally, ice wasted by sublimation, indicating that the Martian atmosphere became again to dry-cold climatic conditions, similar to those of present day.

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