Map of ancient water and ice on Mars

Polygonal ground, a network of cracks, forms when seasonal freezing and thawing move the soil, or when surface material dries.

Rain has never been observed, but a spacecraft’s laser scanner detected snowfall in the polar region.

Mars is extremely dry today. Water occurs as ground ice, in the polar ice caps and in the atmosphere, in clouds and fog.

Streams eroded these valleys. Water flowed through them during the Hesperian period, millions of years apart; they were never all active at once. Valley networks gradually covered equatorial regions. The same valley may have carried water repeatedly in different periods. Valleys are absent from volcanic regions because later lava covered them; near the cold poles, they may never have formed. Deltas form where a river deposits sediment at the shore of a lake or sea. Martian deltas, like the valleys, are more than 3.5 billion years old. Glacier-like features formed in ice-rich debris or beneath ice. Like the valleys, they occur within particular latitude zones, indicating the role of local zonal climate. The climate of Mars changes when its spin axis tilts more than it does today. Traces of glaciers, including terminal moraines, also occur at the equator near the great volcanoes. As the Sun grows warmer over the coming hundreds of millions of years, the climate will change significantly and permanently, and today’s ice will sublime or melt.

Brian Hynek mapped the valleys at the University of Colorado, Boulder. Joseph Levy and colleagues mapped the icy features at the University of Texas, Austin, USA.

The two circular depressions shown in black in the south are enormous impact basins that may once have held lakes.