Mars Astrobiology: Recent Status and Progress - Planetary Exploration and Science: Recent Results and Advances - page 158

Fig. 8.5 Delta in Eberswalde Crater - Mars Global Surveyor ( http://www.msss.com/
mars_images/moc/2003/11/13/ - NASA/JPL-Caltech/MSSS, November 13th, 2003)

8.3.3.3 Groundwater

By 1979, it was thought that outflow channels formed in single, catastrophic
ruptures of subsurface water reservoirs, possibly sealed by ice, discharging colossal
quantities of water across an otherwise arid Mars surface. In addition, evidence
in favor of heavy or even catastrophic flooding is found in the giant ripples in
the Athabasca Vallis. Many outflow channels begin at chaos or chasma features,
providing evidence for the rupture that could have breached a subsurface ice seal.
The branching valley networks of Mars are not consistent with their formation by
sudden catastrophic release of groundwater, both in terms of their dendritic shapes
which do not come from a single outflow point and in terms of the discharges
which apparently flowed along them. Instead, some authors have argued that they
were formed by slow seepage of groundwater from the subsurface essentially as
springs. In support of this interpretation, the upstream ends of many valleys in
such networks begin with box canyon or “amphitheater” heads, which on Earth
are typically associated with groundwater seepage. There is also little evidence
of finer-scale channels or valleys at the tips of the channels, which some authors
have interpreted as showing the flow appearing suddenly from the subsurface with
appreciable discharge rather than accumulating gradually across the surface.