The details of an outflow channel in Ares Valles show the site of catastrophic flooding on Mars, as seen by the Mars Reconnaissance Orbiter.

Sometime in the past, water released from the subsurface of so-called “chaotic terrain” to the south of this channel. Impact craters on the channel provide a way for scientists to estimate the age of the flooding event by roughly matching the number of craters to age. However, the channel bottom may also have been covered by younger material such as lava. Sand dunes have likewise formed since the flooding ceased.

No evidence has emerged of recent running water on the surface of Mars, although scientists believe liquid water may exist beneath the surface. The Phoenix Mars Lander is currently starting to dig near the North Pole for ice and soil samples, so that scientists can study that history of water on Mars.