10 - Water

Summary

All indurated rocks and most earths are bound together by a force of cohesion which must be overcome before they can be divided and removed. The natural processes by which the division and removal are accomplished make up erosion. They are called disintegration and transportation. Transportation is chiefly performed by running water.

…A portion of the water of rains flows over the surface and is quickly gathered into streams. A second portion is absorbed by the earth or rock on which it falls, and after a slow underground circulation reissues in springs. Both transport the products of weathering, the latter carrying dissolved minerals and the former chiefly undissolved.

G.K.Gilbert, Geology of the Henry Mountains (1880)

The Earth’s surface is dominated by landforms that have been carved by running water. Fluvial landforms are usually apparent in even the driest deserts. Running water is such an effective agent of erosion because of its density: Almost 1000 times denser than air, it exerts greater shear stress, buoys the weight of entrained particles, and is driven more forcefully by gravity than an equivalent volume of air.

Where rainfall is possible, even small amounts of water trump any other agent of erosion. Although rain is not possible on Mars under current conditions, its landscape plainly bears the scars of rainfall in the distant past. Some things are different: Mars has seen enormous floods that are comparable to the largest floods known on Earth, and groundwater sapping plays (or played) a far larger role than it does on our planet. We still do not understand how Mars’ floods originated or how such large volumes of water came to be suddenly released. Nevertheless, once released, the water followed the same laws as water on Earth and produced landforms for which terrestrial analogs exist.