Mars is one of the smaller planets in the solar system, but its geology is on a grand scale. With a surface sculpted by almost every major geological process known, it has vast chasms, broad lava plains, ancient impact basins, and the largest volcano in the entire solar system - Olympus Mons. But all this has evolved over distinct epochs, with different geological processes dominating at different times. To date these geological periods precisely will probably require a series of crewed trips to the Red Planet.

About 4.5 billion years ago, at the same time as the Earth was taking shape, another rocky planet began to form slightly farther out from the Sun. It steadily coalesced from the primordial solar nebula - a place brimming with small bodies hurtling around and often crashing together. Mars eventually emerged from this molten turmoil of objects after about 10 million years.

The new planet gradually cooled from its traumatic, fiery birth, separating into a core, a molten mantle and a solid crust. Over the next billion years, new material was added regularly: Asteroids were still numerous in the solar system and continued to strike the young planet. During this period, the massive impact craters scarred Mars for life, covering the planet with deep pits and ejecta - rocky debris created by the explosive impacts. These craters are best preserved in the ancient southern highlands of Mars.

By the time the solar system had settled down, impacts by asteroids had become rarer - and the impact rate has continued to decline. But then its cooling crust was leading Mars to an entirely new geological era. Volcanoes burst through fractures in the surface, and molten lava poured over the planet. The northern hemisphere, where the crust was thinner, took the brunt of the upheaval. The mountainous Tharsis ridge - the site of Mars' largest volcanoes - was formed.

At this volcanic time, Mars began to form a denser atmosphere than it has now. This was because volcanoes eject more than just Lava - they belch forth gases and water vapor into the air. The thicker atmosphere allowed liquid water to remain on Mars' surface in the form of rivers and lakes. But, the planet grew colder. When temperatures fell below the freezing point of water, ground ice and possibly glaciers began to form, cutting swaths through the terrain. Landslides tumbled down mountainsides Eventually the cold may have caused the water to freeze to the soil. Some of the carbon dioxide may have dissolved into the water, or became trapped in the surface rocks as carbon compounds. Much of Mars' atmosphere was also lost, blown away by massive impacts and stripped by the solar wind.

Next came another era of volcanism, covering most of the northern hemisphere with vast lava plains. The Tharsis ridge rose even higher. The Marineris canyon yawned apart, dwarfing Earth's Grand Canyon. New Faults released torrents of water carving channels and other features. Finally, the Tharsis region gave birth to the enormous volcanoes of Ascraeus Mons, Arisia Mons, Pavonis Mons, and Olympus Mons.

Mars in no longer volcanically active. Geologists believe that the last volcano became extinct a few hundred million years ago. Instead, wind is the main force that sculpts Mars' features. Giant sand storms regularly scour the entire planet. Mars may not have tectonic plates and abundant life that constantly reshapes the the Earth's surface, but it has enjoyed no shortage of geological activity - and the planet is still changing.