A Dry Rusty Dead World?
Over a century ago astronomers peered at the blurred image of Mars, and thought they saw faint lines marked across its surface. Initially dubbed "canali" or channels, Percival Lowell called them canals and wrote evocatively of a doomed civilisation on an arid Earth-like planet, bringing water from the poles to survive. Although we now know that the canals were artefacts of their telescopes, Lowell's book, "Mars as the Abode of Life" began (or encouraged) a train of thought that brought us H.G. Wells "War of the Worlds", Edgar Rice Burrough's "Barsoom" and countless 1950's B-Movie and pulp fiction space invaders.
Modern Dry Mars
With improving telescopes and spectroscopy, we came to learn in the 20th Century that Mars is in reality a cold dry and dusty planet. Colder than Antarctica and drier than the Sahara, it has less than 1% Earth's atmospheric pressure, and 97% of that is suffocating CO2. Mars is so cold that every winter, over 25% of the atmosphere itself condenses in a flurry of snow and ice at the pole caps to form a layer of dry ice - solid CO2. There is also water on Mars, or at least H2O - but this is almost exclusively in the solid form in the permanent Northern polecap, and perhaps also in polar permafrost. The mean surface temperature on Mars is around -50 oC! Much of the surface of Mars is revealed by orbiter and flyby images to be heavily cratered, rather like the Moon or Mercury. Very little erosion has occurred in these areas since Mars first emerged from the turmoil of accretion over 4 billion years ago.
In the 1970's, Viking Lander mass spectrometer measurements showed that intense depletion of Hydrogen has occurred over geologic time from Mars' thin atmosphere. Although the extent of the depletion is debatable, it is clear that Mars has lost 70-90% of its available water inventory. CO2 has been less depleted over time due to its higher molecular weight and other gases such as Nitrogen and the noble gases are also variously depleted according to their individual molecular masses. One of the chief barriers to life on Mars might not be water, or photosynthesis, but available Nitrogen!
The SNC meteorites are strikingly deficient in H2O compared to Earth analogs, and two meteorites have even been found to contain inclusions of liquid CO2, not liquid water! The picture that emerges from this compendium of evidence is of a characteristically cold, dry, dusty planet, dominated by the behaviour of CO2, and yet there is one piece of evidence that does not fit this story of a dry Mars.
At the same time that Viking Landers were apparently putting the last nail in the coffin of Earth-like Mars, new images from the orbiters completely changed our understanding of the planet. In a number of places across Mars, huge erosional channels were detected, emerging from chaotic collapse zones, and gouging hundreds to thousands of km across the surface of the planet before debouching in the northern lowlands. Indeed, the entire northern plains are much younger and less cratered than the southern highlands, suggesting that extensive resurfacing by sedimentation took place somewhere around 1 to 2.5 billion years ago.
Earth analogs for these channels were soon found in the channelled scablands of Washington State, USA where researchers mapped very similar features in the erosional scars from giant glacial lakes when ice dams burst, releasing floods across the countryside. For Mars, it seemed clear that water was trapped underground in a pressurised aquifer and then burst out, breaking up the ground into the jumble of km-scale blocks of the chaos zones, then rushing away downhill into the northern plains, eroding the "Outburst Flood" channels as they went.
Although some problems persisted with the outburst flood model, such as how to recharge the aquifer on a planet without rain, it was adopted as the standard model for Mars. The planet was still seen as cold and dry at the surface, but liquid water ran in veins within its crust and had once emerged at the surface to form rivers, lakes, and perhaps even a short-lived northern ocean. Although Lowell's "canals" had been discredited, another kind of channel had been proven on Mars and new stirrings of the imagination began.
Warm Wet Mars
With water at surface, even if only for part of the geologic history of the planet, new speculations became possible. Of course, the story of Water on Mars is also the story of Life on Mars. On Earth, life originated deep within hot rocks, fuelled by chemical solutions from volcanoes and geothermal springs, before it entered the oceans, developed photosynthesis and finally emerged on land. A similar process is expected for Mars and surface water almost inevitably means surface life. During the 80's and 90's scientists vied with each other to make new discoveries about water, and life, on Mars.
In 1996, President Clinton announced that NASA scientists had found bacteria-like fossils in one of the Mars meteorites - ALH84001. This statement released a storm of emotional and intellectual inquiry into life on Mars, and elsewhere in the universe. The Mars meteorite "fossils" have since been re-evaluated and considerable doubt has been cast on them. However, in the process we have learned a great deal about unusual organisms on Earth that live in caves, or underground - often in scalding hot environments - and metabolise methane or hydrogen, or strange chemical species that would poison "normal" organisms. Strangely, we have found that these "extremophile" organisms are actually more primitive than "normal" organisms, which is why we now believe life originated in these circumstances.
For Mars, the flood of scientific papers into how Mars could have been warmer in the past (through exotic greenhouse atmospheres) and how water could recycle to supply the outburst floods has led to a progressive movement of the consensus scientific position on Mars. The question of late has become not "Was Mars Warm and Wet?" but "How Warm and How Wet?" Indeed, Michael Malin and Ken Edgett recently stunned the planetary science community with news that young channel features had been found on Mars, that indicated flow of fluids in geologically recent times - something almost unimaginable for a cold dry Mars!
White Mars Part Two