Groundwater formation of martian valleys

The martian surface shows large outflow channels, widely accepted as having been formed by gigantic floods that could have occurred under climatic con ditions like those seen today(1-5). Also present are branching valley netwo rks that commonly have tributaries(1-8). These valleys are much smaller tha n the outflow channels and their origins and ages have been controversial. For example, they might have formed through slow erosion by water running a cross the surface, either early or late in Mars' history(9-13), possibly pr otected from harsh conditions by ice cover(14-16). Alternatively, they migh t have formed through groundwater or ground-ice processes that undermine th e surface and cause collapse, again either early or late in Mars' history(3 ,4). Long-duration surface runoff would imply climatic conditions quite dif ferent from the present environment. Here we present high-resolution images of martian valleys that support the view that ground water played an impor tant role in their formation, although we are unable as yet to establish wh en this occurred.