Online Astronomy eText: The Planets
The History of Mars Link for sharing this page on Facebook
      Mars' varied topography suggests that most features on the heavily cratered highlands date back to a period of bombardment in the earliest stages of Solar System history, with very little weathering and erosion save by winds during the 4 billion years since then, while volcanic features near the Equator, the debris-covered lowlands on the northern side of the planet, and numerous flow features -- admittedly mostly explainable in terms of flash flooding and the like -- suggests that very different conditions existed at other times and places. Developing a consistent theory of Martian geological history that fits these varied terrains has been difficult, and in the absence of surface and subsurface studies of the structure and mineralogy of Mars, very uncertain. Still, enough has been learned so far to suggest three major periods of Martian history.
      The first period, beginning with the formation of the planet 4.5 billion years ago, and continuing for perhaps seven to nine hundred million years, involved considerable amounts of water erosion, weathering and mineral alteration. Particularly in the lower areas to the north of the Martian Equator, water altered surface chemistry and geology so substantially that little evidence of the period of bombardment in the first few hundred million years of Martian history remains; but in the higher areas to the south of the Martian Equator, water was apparently less abundant and even where present, did little to alter the impacts created during the last stages of that bombardment.
      The second period, beginning about 4 billion years ago, involved considerable volcanic activity, primarily near the Martian Equator, as evidenced by the Tharsis Ridge structures and stress fractures surrounding these areas, particularly those responsible for the formation of Valles Marineris. The main episode of volcanic activity appears to have ended within a few hundred million years, but volcanism continues, albeit at a very reduced rate, right up to the present time.
      The third period began about 3.5 billion years ago and continues to the present, with primarily wind erosion, deposition, and re-erosion of the surface as its major surface-altering process. Water, originally a major component of Martian surface processes, has had little or no role in the development of the Martian surface during this time, save for flash flooding produced by localized vulcanism melting subsurface ices.