The terrain in the southern hemisphere of Mars is ancient and heavily cratered. The pattern of ejected material from many Martian craters shows that the ejecta were very fluid and may have contained a high proportion of water.
Widespread volcanic flows have eradicated the ancient, cratered terrain in the northern hemisphere of Mars.
Mars has many volcanos. The largest Martian volcanos are located in the Tharsis region, a continent-sized bulge. The great sizes of the Tharsis volcanos imply that the Tharsis region has remained over its magma source for billions of years. The heights of the Tharsis volcanos show that the magma that formed them rose from great depths, where pressure was large enough to lift the magma to the summits of the volcanos. This shows that the lithosphere of Mars is thicker than the lithosphere of the Earth.
We can estimate the ages of Martian volcanos and lava flows by measuring crater densities. Volcanic activity occurred all over Mars about 2.5 billion years ago but has gradually become restricted to a smaller and smaller region of the planet.
The many faults and chasms on Mars show that there has been considerable crustal motion. The most spectacular example of crustal motion is the Valles Marineris canyon system. Much of the crustal motion seems to have occurred as a consequence of the weight of the Tharsis bulge. Long ago, Mars experienced a period of plate tectonic activity.
Two kinds of channels on Mars appear to have been cut by running water. Runoff channels, resembling terrestrial river systems, were formed when underground water or rainfall was collected from a large region. Outflow channels were cut by great floods that took place when large pools of water suddenly were produced in a local region. Small outflow channels may have been formed very recently.
Both the north and south poles of Mars have permanent polar caps. The larger northern cap is made primarily of water while the southern cap also contains a large percentage of carbon dioxide (dry ice). A seasonal cap of dry ice forms about each permanent polar cap in winter.
The polar regions of Mars are covered with thick layers of sediment formed from dust blown poleward by strong winds. It takes tens of thousands of years to accumulate a single layer tens of meters thick. The layering probably is caused by the precession of Mars's polar axis.
Pictures taken by the Viking landers show rolling countryside littered with rocks ejected from nearby impact craters. The regions around the landers resemble rocky deserts on the Earth.
The Pathfinder lander and the Sojourner rover explored Ares Vallis for about 3 months in 1997. Pathfinder images showed abundant evidence of ancient floods while Sojourner measured the chemical composition of rocks and soil.
The atmosphere of Mars consists primarily of carbon dioxide. The atmosphere is too thin to insulate the surface, where daily temperature fluctuations are as much as 60 K. Atmospheric pressure is lowest in the summer and winter when carbon dioxide is trapped in one of the polar caps. Pressure is highest in the fall and spring when carbon dioxide is released into the atmosphere.
Frequent dust storms keep dust suspended in Mars's atmosphere at all times. Great dust storms sometimes spread a thick layer of dust over the entire planet. The seasonal changes in dark markings are caused by deposition and removal of dust by seasonal winds. Large dust particles are skipped along the surface by winds and are very effective at eroding the Martian surface.
Although Mars contains a smaller percentage of iron than the Earth does, it has a core made of iron or iron mixed with sulfur. The crust of Mars is rich in iron, giving Mars its reddish color.
After Mars formed, radioactive decays heated its interior until iron melted and accumulated in its core. Eventually the rocks in the mantle melted partially to produce magma, which flooded the surface and produced large volcanos. Mars expanded as it grew hotter, perhaps causing its crust to pull apart and produce Valles Marineris. Mars has cooled and become less active for the last several billion years.
Little water (in solid, liquid, or gaseous form) can be found on Mars today. However, surface features such as its channels show that Mars once had much more water than we have been able to measure. It is likely that most of Mars's water is trapped as permanently frozen, subsurface ice.
Early in its history, Mars may have had an atmosphere thick enough for a greenhouse effect to occur. The atmosphere may have been warm and thick enough for liquid water to exist. As on the Earth, carbon dioxide was incorporated into surface rocks. Unlike the Earth, however, the lack of plate tectonics prevented carbon dioxide from being recycled into the atmosphere. The atmosphere dwindled, water froze, and Mars eventually reached its present condition.
The Viking biology experiments showed that the Martian soil is chemically active, but probably does not harbor life.
The evolution of the terrestrial planets can be pieced together using information from each of them. Impacts and radioactive decays heated each planet until much of its iron sank to form a core, and light rocky material floated to the surface to become the crust. The extent to which each planet has cooled and become volcanically inactive depends on size. The smaller planets quickly became inactive while the larger planets remain tectonically and volcanically active.
