The Martian surface is varied and contains a significant north-south asymmetry. The southern hemisphere is marked with an abundance of craters and probably dates back into the planet's history to about four billion years ago. Craters larger than 20 kilometers in diameter exist in high density here, similar to the density of such craters in the lunar highlands (Cattermole 53). Contrastingly, the northern hemisphere of Mars is covered by plains with much smaller crater densities. These plains have been observed to possess numerous characteristics, such as volcanism, wind action, ground-ice presence, as well as fluvial erosion.

The atmosphere of Mars is quite similar to that of Earth in that both atmospheres have regional and seasonal weather patterns, clouds, global wind directions, and water vapor-saturated atmospheres. The atmospheres of Mars and Earth do differ, however. They have different compositions since Mars's surface atmosphere is more than 95% carbon dioxide, and the Martian atmosphere is also much thinner than that of our planet (Cattermole 38).

Factors like atmosphere are vital to understanding the possibility of water near the Martian surface, a topic which will be discussed later. A thinner atmosphere also means that more meteroids can invade Mars since there is not as much of an atmospheric shielding effect as there is on Earth.

Impact craters on Mars are important to our understanding of the evolution of the planet, in that craters can give a good indication of the nature and configuration of the surface content as well as contain a record of erosion and deposition events. Most prominent, craters are a tool in determining the relative ages of surfaces. Older surfaces are usually more cratered, for instance, and this principle is the foundation for much of what is known about the history of planets other than ours.

Some older surfaces, however, are not abundant in craters because of resurfacing caused by infilling or erosion. The residence time of craters on Mars is generally longer than that of craters on Earth, because craters on our planet are rapidly destroyed by resurfacing. Earth's surface is comparatively young with the oceanic crust, for example, being only around 250 million years old because it has been completely resurfaced. Terrestrial impact craters are relatively rare because Earth's atmosphere acts as an efficient shield for small bodies. Our atmosphere decelerates these small meteoroids, breaks them up, and ablates the body before it ever reaches our surface.

Even those impacts that are quickly "erased" can still be detected and recorded because an impact scar of deformed or recrystallized rock will remain. Although the majority of crater studies have been on lunar craters since they are among the best preserved meteorite impacts scientists have been able to observe, there is still an adequate amount of information on Martian impacts to generate data. Craters on the red planet have the same size groupings as craters found elsewhere. Small craters are bowl-shaped and simple, larger ones are complex with central peaks and terraces, and the largest craters are multiringed basins (Mutch 116).

A basic difference between Martian impacts and Earth impacts is that the larger of the former craters are generally deeper than those on Earth. This is due mostly to the discrepancy in gravity between the two planets, but this enlargement process might also be controlled by a volatile substance, such as ground ice, present in the regolith. Craters on Mars have a fluid ejecta pattern that is nothing like the patterns on our planet and suggests that there is ground ice present under the red planet's surface. The ejecta patterns that surround craters on Mars are unanimously the most distinguishing characteristic of these impacts. With craters that are less than around five kilometers in diameter, the ejecta blanket is much like that found around Earth craters, so the ejecta appears to have been deposited by ballistic trajectories. Any impact structures larger than five kilometers, however, differ greatly from Earth impact ejecta. It is at this scale that Martian ejecta patterns take on a characteristic fluid appearance, and the larger they are, the more complex the ejecta patterns get.

Fluidized ejecta is created when water or partially melted ice mixes with debris upon impact. It is believed that a layer of ice and water lies beneath the surface of Mars, and if meteorites hit hard enough, they can break through to this layer, where the debris mixes with the fluid. This ejecta continues to flow outward from the impact on it's own after the splash down (Carr 52), creating, in some cases, a flower-like pattern around the crater. This "flower" pattern has several thin ejecta layers. Some are so thin, in fact, that faint remnants of the underlying surface can be seen.

The other type of fluidized ejecta crater has a much thicker ejecta layer, so thick that it obscures the terrain beneath. This layer also has a very textured surface.

There are various other features like channels, intercrater plains, and volcanoes that mark the Martian surface along with craters. Over large portions of the red planet, the surface is cut by branching valley networks which vary in distinctness. These branching valleys are much more common in areas of lower crater density located between large craters. These valleys are surely of fluvial origin and their formation occurred when Mars's atmosphere was denser and richer in volatiles than present-day Mars. Degradation of the valleys is varied and this indicates that these channels underwent early meteorite attacks during the heavy bombardment stage just as craters did (Cattermole 64). Martian terrain is also marked by intercrater plains, which occur in areas with less crater cover and between large craters and on their floors. These plains can be quite diverse, ranging between eroded layered deposits, curvilinear ridges, and hundreds of small pits.

Whereas craters are most abundant in the southern hemisphere, volcanism is more prevalent in the northern hemisphere. Although there is no volcanic activity on the red planet today and no signs of fresh lava flows, studies and observations prove there was definitely an abundance of volcanic activity in the planet's past. (Cattermole 65) Geologists have also determined that Mars appears to have no tectonic plates and therefore no tectonic plate activity influencing volcanoes. Erupting volcanoes in the northern half of Mars resurfaced the hemisphere in the past with their lava flows, thus destroying evidence of most of the meteorite impacts that once existed there. This leaves us with a Mars with only around half of its craters left for us to now observe.

The craters that remain on the surface of Mars as well as the other geologic structures, when put together, present us with a big picture of the red planet terrain and history. The larger northern polar cap consists of both water and carbon dioxide ice. The smaller polar cap in the south consists of mostly carbon dioxide and changes dramatically in size with the seasons. The seasons of Mars, which include a long and frigid northern winter and an exceptionally warm and long southern summer, probably are a result from the red planet's tilt and highly elliptical orbit (Raeburn 110-111).

The most prominent structures on the red planet are two giant impact basins, the Hellas (picture of Hellas region) and Argyre, which are located in the heavily cratered southern hemisphere of the planet and are estimated to have formed four billion years ago. Hellas is, at around 1600 kilometers in diameter, the largest basin structure on Mars. It consists of many isolated mountain peaks found in a circular design. Valleys and concentric parallel ridges are also observed within this structure, and several faint rings have been identified, suggesting this impact structure has highly degraded. (Mutch 120) Argyre, although almost half the diameter of the Hellas basin at only 900 kilometers, has features that are much more defined. The circular diameter of this multi-ringed basin "is surrounded by a rugged mountainous Region", which is quite rare in Earth craters. (Mutch 119) These Hellas and Argyre basins are just two of the numerous fascinating structures found on the red planet that can teach us many things about Mars. For the past several decades and continuing today, many people are especially intrigued by the possibility of life on Mars, which the planet's geology may be able to help us answer someday.