Meter-scale characteristics of Martian channels and valleys

Mars Global Surveyor images, with resolutions as high as 1.5 m pixel, enabl e characterization of martian channels and valleys at resolutions one to tw o orders of magnitude better than was previously possible. A major surprise is the near-absence of valleys a few hundred meters wide and narrower. The almost complete absence of fine-scale valleys could be due to lack of prec ipitation, destruction of small valleys by erosion, or dominance of infiltr ation over surface runoff. V-shaped valleys with a central channel, such as Nanedi Vallis, provide compelling evidence for sustained or episodic flow of water across the surface. Larger valleys appear to have formed not by he adward erosion as a consequence of groundwater sapping but by erosion from water sources upstream of the observed sections. The freshest appearing val leys have triangular cross sections, with talus from opposing malls meeting at the center of the valley. The relations suggest that the width of the v alleys is controlled by the depth of incision and the angle of repose of th e walls. The flat floors of less fresh-appearing valleys result primarily f rom later eolian fill. Several discontinuous valleys and lines of craters s uggest massive subsurface solution or erosion. The climatic implications of the new images will remain obscure until the cause for the scarcity of fin e-scale dissection is better understood. (C) 2000 Academic Press.