Melbourne - Oct. 19, 2000
Water and Life
On Earth, life requires water, therefore our search for life beyond the Earth is a search for water. For this reason we target Europa, a moon of Jupiter that may have a liquid ocean beneath a thick icy crust, and also Mars where huge erosional channels suggest the flow of fluids across the surface in the geological past. Many scientists believe that rivers and lakes existed in the past on Mars, and perhaps even oceans. However, this search on Mars may be ill-founded. Despite intense research, the evidence for water on Mars is scarce. Now a new theory suggests that the single strongest line of evidence for water on Mars - the "outburst flood channels" may have been formed not by liquid water but by cold dry eruptions of gas, dust and rock, fuelled by exploding liquid CO2.
Modern Dry Mars
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.
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
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!
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