The surface of Mars can be divided into two major regions: (a) the densely cratered, more ancient highlands in the southern hemisphere, and (b) the younger, lower plains in the north. Mars probably possesses an internally differentiated structure with a metallic, core, a thick mantle composed of iron-rich silicate minerals and a thin crust. Cratering has been a major geologic process on Mars, and a record of an early period of intense meteoritic bombardment has been preserved. Volatiles outgassed from the interior formed an atmosphere and a simple hydrologic system. Later, as temperatures lowered, liquid water became locked up in the polar caps and in the pore spaces of rocks and soil, as groundwater or ice, and was only occasionally released in large floods. Eolian processes have been observed in action on Mars, and many surface features have been modified by wind erosion or deposition. Evidence for internal activity are:

Mars has several huge shield volcanoes. Olympus Mons is the largest mountain in the Solar System, rising 24 km (78,000 ft.) above the surrounding plain. Its base is more than 500 km in diameter.

A huge bulge on the Martian surface (Tharsis) is about 4000 km across and 10 km high.

A system of canyons (Valles Marineris) 4000 km long and from 2 to 7 km deep

Water is the chief agent of weathering and erosion on Earth. Mars is a much drier, colder planet on which liquid water cannot exist very long at the surface because it will immediately begin to boil, evaporate, and freeze--all at the same time. However, new pictures from the Mars Orbiter Camera (MOC) onboard the Mars Global Surveyor (MGS) have provided an astonishing observation which suggests that liquid water may have played a role in shaping some recent gully-like features found on the slopes of various craters and troughs.

The landforms both on Earth and Mars are divided into three parts: the alcove, the channel, and the apron. Water seeps from between layers of rock on the wall of a cliff, crater, or other type of depression. The alcove forms above the site of seepage as water comes out of the ground and undermines the material from which it is seeping. The erosion of material at the site of seepage causes rock and debris on the slope above this area to collapse and slide downhill, creating the alcove.

The channel forms from water and debris running down the slope from the seepage area. The point where the top of the channel meets the bottom of the alcove is, in many cases, the site where seepage is occurring. Channels are probably flushed-clean of debris from time to time by large flash floods of water released from behind an ice barrier that might form at the site of seepage during more quiescent times.

The aprons are the down-slope deposits of ice and debris that were moved down the slope and through the channel. Whether any water--likely in the form of ice--persists in these deposits is unknown. The fact that the aprons do not go very far out onto the floors of craters and troughs indicates that there is a limit as to how much water actually makes it to the bottom of the slope in liquid form. Most of the water by the time it reaches the bottom of the slope has probably either evaporated or frozen.

Martian sedimentary rocks are just now beginning to reveal clues about the planet's complex early history. "Early Mars" refers to a time thought to have been more than 3.5 billion years ago, a period when the planet was young and impact craters---created by meteors, asteroids, and comets hitting the surface---were forming more frequently than they do today.

The history suggested by the martian sedimentary rocks may have included warm, wet climates with thousands of crater lakes (i.e., with liquid water) that persisted for millions of years. Alternatively, the rocks might be recording climate changes and thick deposits of airborne dust formed on a much colder, drier world than many have suspected.