The Mariner 9 orbiter (launched in 1971) and the two Viking orbiters (launched in 1975) took pictures of the surface of Mars which revealed unequivocal evidence of dried-up valleys and channels thought to have been eroded by liquid water. Outflow channels strongly resemble terrestrial flood channels, such as those in eastern Washington state of the US, and could form under present climatic conditions by catastrophic release of groundwater. Such floods were perhaps triggered by large impacts or "marsquakes". On the other hand, geological studies of the valley networks suggest that these must have been gradually eroded by running water: some show morphology suggesting formation by groundwater sapping (i.e. when a river is fed by a spring and the valley grows by headward erosion); others seem to have been produced by precipitation runoff. The valley networks are nearly all restricted to ancient upper highlands, dated as 3.5 to 4.0 billion years old from the quantity of impact craters, so it is postulated that environmental conditions on Mars must have been conducive to liquid water at this time. This means that the surface temperature must have been close to 0 deg C or greater (compared to about -60 deg C today) to allow valleys to form, irrespective of whether this was by sapping or precipitation.

Distributary Fan. Evidence for water? (MOC2-543a

In addition to valley networks, high erosion rates on early Mars are considered to be strong evidence of a warmer, wetter climate. High erosion rates are manifest on the terrain greater than 3.8 billion years old as degradation of craters and other surface features. Terrain formed later shows a much lower rate of erosion.

The most likely mechanism for a warmer, wetter Mars in the past is if Mars had a larger greenhouse effect. The greenhouse effect arises as follows: visible sunlight passes through the atmosphere and is absorbed directly on the planet's surface which then radiates the energy in the (invisible) infrared; atmospheric greenhouse gases absorb some of this upwelling radiation which warms the atmosphere; the atmosphere radiates because of its finite temperature and some of this radiation is absorbed on the surface. The overall effect is to raise the surface temperature above that which would result in the absence of an atmosphere because the surface receives radiation not just from the Sun but from the atmosphere also.

For the inner planets of the solar system, Mercury, Venus, Earth and Mars, the greenhouse effect increases their average surface temperatures by 0, 500, 35 and 7 deg C respectively. Mercury has no appreciable atmosphere, Venus has an exceedingly dense atmosphere (90 bar surface pressure) of carbon dioxide (CO2), and Earth has a moderately dense atmosphere (1 bar surface pressure) which includes greenhouse gases such as water vapor (H2O) and carbon dioxide (CO2). Today, Mars has only a very thin CO2 atmosphere (0.006 bar surface pressure) and so has a modest greenhouse effect.

Detailed view of distributary fan (MOC2-543c)

Climate calculations were done in the 1970s to see how Mars' atmosphere could have supported a greenhouse effect in the past sufficient for liquid water. As today, such calculations face a difficulty: according to computer simulations of how stars evolve over billions of years, the Sun is estimated to have been 25-30% less luminous than today, and yet Mars and the Earth are thought to have been warmer (from geological and biological evidence): a problem known as The Faint Young Sun Paradox. Ammonia (NH3), a strong greenhouse gas, was initially suggested as a possible component of the early atmosphere of Mars and the Earth to account for this problem. However, it is now realized that NH3 could not survive long enough due to its destruction by sunlight. Subsequently, atmospheres of carbon dioxide (CO2) and water vapor (H2O) were considered --- such gases are likely to have been vented from the mantle of the planet and are produced by periodic cometary or asteroid impacts.

By the end of the 1980s, further calculations indicated that a 5 bar CO2 atmosphere could sustain warm and wet conditions long enough to form valley networks. However, in the 1990s, it was realized that if the atmospheric pressure were to be raised so high, CO2 would condense in the atmosphere.

Indeed, the early Mars atmosphere was calculated to hold no more than 1.5 bars of CO2 with an average surface temperature close to its present day value, about -60 deg C. Furthermore, where would such an early thick atmosphere have gone? The presumed mechanism was that CO2 would be gradually dissolved in rivers and lakes on the surface to eventually form carbonate rock. As Martian volcanos became less active and eventually extinct, this locked-up CO2 would fail to be recycled to the atmosphere (like it is on today's Earth), the atmospheric pressure would fall, the greenhouse effect would diminish, and the planet would evolve to its cold, dry condition today. However, if a thick CO2 atmosphere was converted into carbonate rock in order to evolve to the present-day atmosphere, a global layer, 80 m thick, of calcite, would result. Currently, there is no unambiguous detection of large carbonate beds on Mars but carbonates could be buried and unseen.

Recently, climate simulations have suggested that a thick atmosphere of CO2 could be supported in two possible ways. Firstly, CO2 clouds behave differently from water clouds because they are highly reflective to infrared radiation. Hence, if CO2 did condense in a thicker early atmosphere it could actually reflect infrared rays emanating from the surface back down to the surface and sustain a warmed planet. Secondly, small amounts of sulfur dioxide (SO2) high up in the atmosphere (where CO2 might condense) would absorb ultraviolet light from the sun and warm the atmosphere of early Mars (rather like the terrestrial ozone layer warms the stratosphere). Consequently, a thick CO2 atmosphere becomes viable. The SO2 would originate from volcanoes, which we know were active on early Mars. Currently, then, 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 which could be sustained by at least a couple of mechanisms. But no one theory can yet be singled out and we do not know whether the "warm, wet" period persisted for 0.5 billion years or was merely episodic for a much shorter periods like millenia. 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.

Detailed view of distributary fan (MOC2-543d)