The Standard Model of Mars' atmosphere and surface evolution.

There is a great deal of information out there on the web which covers the standard model for water on Mars and the various small variations on it that are being discussed. I recommend you follow up some of the links, like Bill Arnett's Nine Planets tour (a real tour de force), the standard NASA websites and the wealth of new images and information about Mars.

Mars Exploration Program
Mars PathFinder
Mars Global Surveyor
Viking Archives
Interactive Mars Atlas
The Red Planet - NASA slideshow
Other NASA slideshows

In a nutshell, the standard model goes like this (links from beyond here are jpeg images illustrating the features described):-
 

Origin

Mars was created 4.55 billion years ago, as a terrestrial planet in the inner Solar System. It is smaller than Venus and Earth, therefore has less gravity and perhaps less volatiles like water and carbon dioxide, but still had an appreciable inventory. it is furthest away from the Sun, and therefore colder than Venus (which became a fiery dry hell as the greenhouse effect boiled its seas) and Earth, which is a mild temperate planet with extensive seas. Both Earth and Venus had a lot of water and Carbon Dioxide to start with. Both have changed a bit, but that's another story. Mars is thought to have started the same way, with plenty of volatiles.
 

Water on Early Mars

The earliest terrains on Mars are of Noachian age, and show signs of erosion including small valley networks and degraded crater rims. Small deltas are visible at the mouths of channels entering some old craters and banks of sediment are found round their edges. It is therefore inferred that on Early Mars water flowed at least occasionally and that lakes and rivers were present, if ephemeral. There is a small problem that it doesn't seem possible to get early Mars warm enough to acheive this because the early Sun was cooler than it is now by about 30%, but by adding every last possible greenhouse gas to the atmosphere it can just about be done. Unfortunalely, if Mars was this warm, then the early Earth would have experienced the same sterilising greenhouse that Venus now "enjoys", making life on Earth a real puzzle.

After about 500 million years to a billion years had passed, Mars became drier in the Hesperian epoch. The fluvial systems dried up and stopped, and Mars became dominated by impact cratering and surface vulcanism. It is generally assumed that Mars lost most of its early atmosphere either into ice at the poles or underground, or it was lost to space by a combination of solar wind erosion and major impacts blasting the atmosphere away. The dry cold Mars persisted for a further billion or two years until in the late Hesperian things began to change.
 

Amazonian Outburst Floods

Mars begins to show signs of a resumption of fluid flow on its surface during the late Hesperian and especially in the early Amazonian when a series of catastrophic floods appear to have burst out of the ground at canyon walls and heads, displacing huge blocks of rock up to 10km on a side, and rushing downslope in a torrent of water, mud, boulders and sand. The floods carved distinctive broad-based channels with pointed islands around hard or high ground, like impact craters. These outburst floods have been likened to catastrophic floods on Earth when ice-dammed lakes broke through and burst across the surface creating very similar topography.

Unfortunately, there is a storage problem with this water. It appears to emerge from underground (indeed, the surface was too dry and cold for liquid water), yet the volumes involved and the flow rates are so large that it is hard to sustain the flow from even the most porous aquifer. Also, there doesn't seem to be enough water on Mars to have supplied all the flows without a mechanism for returning the water back to the underground aquifers and recharging them. And the water seems to have disappeared from where it went to - largely the northern lowland plains. Some fairly complex models have been developed to explain the occurence of these floods, and balance the books with Mars' water but they get fairly creaky and ad-hoc. That doesn't stop the standard model of Mars being generally accepted, and work continuing on sorting out some of the problems by adding additional detail.
 

Mars Today - Red Mars

After the early Amazonian, the floods seem to have stopped happening. Perhaps the water dried up and was locked into icecaps or subsurface ice in polar regions, so it stopped being recycled, and maybe some was lost to space. Either way, Mars stopped working as a fluvial system and the surface reverted to a dry dusty crater-strewn plain. The detailed images of the Mars Global Surveyor mission show many examples of sand dunes on a variety of scales, crossing a cratered landscape. Mars looks like it has dried up and died over the last billion years of its evolution. Some lava flows are very young, so vulcanism may recur on Mars, but the floods seem to have gone away. This is the challenge facing the Mars exploration program. How can we explore and perhaps colonise this dry, dusty planet? The standard model of Mars leads us to a difficult position, but not an impossible one. Plenty of scientists and thinkers are working on this problem and there are genuine plans in some circles for a sustained exploration program leading to manned missions.
 

The New Mars

Instead of tinkering with the details of the standard model for volatiles on Mars, there is another way to address all those anoying little (and not so little) problems above. I argue that it is time to stop adjusting the standard model and instead move to a different paradigm where the active volatile is not water, but carbon dioxide. By doing this, at a stroke we resolve all the problems of the standard model, and also explain a few other features of Mars such as the widespread and thick layering of Hesperian terrain, and why the Pathfinder Rover (Sojourner) had to drive round so many boulders.

Photos on this page courtesy of NASA. Please follow links in introduction for full NASA websites and image archives.