Evidence for water action on the surface of Mars
During the last forty years, over a dozen robotic spacecraft traveled to Mars. They studied it remotely from orbit and with direct measurements on the surface. Investigations of data returned by these vehicles produce evidence indicating an active hydrological surface history. There are signs of processes long dormant and glimpses of recent activity. Ancient landforms across the now dry Martian globe bear striking resemblance to erosional and depositional features here on Earth. However, multiple explanations exist for the features discovered so far.
On a global scale, topographical measurements suggest a large northern polar ocean (Figure 1). Surrounding and feeding this large basin are chasms torn by massive liquid flows. These chasm morphologies resemble the scablands of western North America, a product of surges in glacier melting on Earth 11,000 years ago. However, the edges of the candidate ocean basin lack characteristics that define familiar coasts . Both altimeter measurements and orbital photography have yet to record wave-cut cliffs, ancient beaches or barrier islands. Interestingly, "topography is smoother below this possible ancient shoreline than above it, consistent with smoothing by sedimentation" - this sedimentation occurs over long periods of time beneath an ocean.
Stretching across equatorial Mars, spanning a length greater than the continental United States, is the Valles Marineris. This great system of rift valleys resembles the Grand Canyon but is the result of tectonic rather than erosional forces. Finer examination the walls of these rift valleys show fluid erosional features (Figure 2). Notice the gullies running through the talus slope at the base of this cliff. This manner of fluvial erosion is characteristic of terrains like that in Afghanistan.
Compare with Figure 3 for recent fluid erosional activity. Here "the channels and aprons have very crisp, sharp relief and there are no small meteor impact craters on them, suggesting that these features are extremely young relative to the 4.5 billion year history of Mars. It is possible that these landforms are still being created by water seeping from the layered rock in the crater wall today." 
Similar cliff erosion affects craters left in the highlands of Mars. Just below the rims of many craters, seepage of subsurface fluid alters the exposed walls and drips down along the cliff to the bottom . On the crater floor the fluid ponds and leaves dark, easily recognizable features (Figure 4).
Sustained water flow has carved a number of river channel networks across the surface of Mars. Dry riverbeds like Nanedi Valles wind sinuously through the flat highlands exposing layered rock on the valley walls . Another extinct river, Nirgal Vallis, sports gully erosion and aprons along its banks, and eras of channeling on it bed (Figure 5). Evidently, this river had a steadily decreasing amount of fluid discharge over its lifetime. It is also likely that there are episodic returns of water flow to the river bottom producing the channel within a channel.
The single most convincing landform class that speaks of an active water cycle in Mars' past is sedimentary rock . Orbital photography of this landform shows incredible terrains of layered sediment (Figure 6). The sedimentation process on Earth is well understood. Oceans and large, slow rivers drop suspended and precipitate load. As sediments accumulate they compress, harden and over millions of years form rock. Not only do these Martian landform have the distinct stratification of sedimentary rock; repeat fluvial erosion has also denuded the bed. These are all components of an extended period of hydrology.
Thermal emission sensing of the Martian surface maps the signature of minerals formed through interaction with water . Crystalline hematite (a-Fe2O3) results from "low-temperature precipitation and hydrothermal mineralization" with water. Abundant quantities of this substance cover the Sinus Meridiani region spanning thousands of kilometers. This is coincident with exposed beds of sedimentary rock, also precipitated from a large body of water.
The Mars Pathfinder conducted in-situ exploration of a small acre of a very flat Martian plain . The lander and its rover studied boulders and smaller rocks evidently rounded by water action. Placed in a larger context, these rocks lie in a catastrophic flood plain carved by a brief but massive deluge of fluid. There are no indications of sustained water flow in the lander's region.
The geologic legacy from Mars' wet past ranges from microscopic crystals to massive hemispheric flood plains. Sedimentary landforms are the bedrock of this lengthy epoch of active hydrology. Today, scant remnants of this water trickle down the walls of craters and briefly drain in small streams dwarfed in scale by massive channel walls carved millions of years ago.
John Kavanagh is a Geography student at the University at Buffalo.