The Mariner 9 Orbiter (launched in 1971) and the two Viking Orbiters (launched in 1975) took pictures of the surface of Mars, showing dry valleys and channels presumably eroded by liquid water. Geological studies of the valley networks suggest that running water gradually eroded and shaped them. Some valleys show a morphology suggesting the action of groundwater sapping, when a river is fed by a spring and the valley grows by headwords erosion. Others seem to have been produced by precipitation runoff.
Surface features on Mars that appear to have been formed by liquid water flow. (Courtesy: NASA)
The valley networks are nearly completely restricted to ancient upper highlands, dated 3.5 to 4.0 billionyears old (from the quantity of impact craters). From this it is inferred that environmental conditions on Mars must have been conducive to liquid water at this distant time. The surface temperature must have been above zero degrees Celsius to allow valleys to form, irrespective of whether this was by sapping or precipitation. In addition to the valley networks there is other evidence of high erosion rates on early Mars. Prominent in this is the degradation of ancient craters and similar surface features on the ancient terrain (about 3.8 billion years old or older). Terrain features formed later show a much lower rate of erosion.
The most likely mechanism for the formation of a warmer, wetter Mars in the past would be a large greenhouse effect. The greenhouse effect occurs when visible sunlight passes through the atmosphere and is absorbed directly on the planet's surface. The surface, in turn, radiates energy in the invisible, infrared; atmospheric greenhouse gases absorb some of this outward radiation, releasing heat and warming both the atmosphere and the surface. Today, Mars has only a very thin CO2 atmosphere (0.006 bar surface pressure,) and therefore but a modest greenhouse effect.
Computational climate calculations generate a discrepancy. According to computer simulations of how stars evolve over billions of years, the Sun in the time of Mars' infancy is estimated to have been 25-30% less luminous than today. However, from geological and biological evidence Mars and the Earth are thought to have been warmer than they currently are, raising a problem known as The Faint Young Sun Paradox. Ammonia (NH3), a strong greenhouse gas, was once thought to be the component of the early atmosphere of Mars and the Earth to account for this, but it is now realized that NH3 could not survive long enough to have an effect before its destruction by sunlight. So the theory was replaced by atmospheres of carbon dioxide (CO2) and water vapor (H2O). These gases are likely to have been vented from the hot convecting mantle of the planet and also contributed by periodic cometary or asteroid impacts.
North pole of Mars with elevations taken with the Mars Global Surveyor Altimeter. Blue is low, red is high. The massive depression at the center of the image may have been an ocean basin. (Courtesy: NASA)
Currently, climate simulations of early Mars have tried to explain the unequivocal evidence of running water 3.5-4.0 billion years ago with a thicker CO2 atmosphere. But we do not know whether the "warm, wet" period persisted for 0.5 billion years or was merely episodic for a much shorter period like millennia. Alternative possibilities are that the early Sun was brighter than current astrophysical theory suggests, or that valleys somehow formed in colder conditions. Resolving these issues will require further exploration of Mars.