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.