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White Mars: The story of the Red Planet Without Water
Melbourne - Oct. 19, 2000 A few paradoxes remain about this new Mars. Chief amongst these is the Carbonate Paradox. Basic chemistry requires that if Mars had liquid water at surface for any reasonable period of time, then rapid chemical reactions between the atmospheric CO2 dissolved in the water, and the rocks of Mars, would produce copious quantities of carbonate rock. The process should be so efficient that almost no CO2 should remain, and thick deposits of carbonates should fill all the lake and ocean beds of Mars. Despite decades of orbital and Earth-based imaging and spectroscopy, and 3 Landers, no trace of surface carbonates can be found. Plenty of young impact craters have drilled into and exposed the lake beds, yet no carbonates are seen. The only carbonates that we know of on Mars come from ALH84001, the infamous "Life on Mars" meteorite". This contains carbonate globules that defy analysis. Some investigators are adamant that they are high temperature hydrothermal carbonates formed as the original heat of the igneous rock dissipated, and water circulated through the grains. Others suggest a low-temperature and presumed near-surface origin. There is a Volumetric Paradox associated with the outburst floods. There is only a small amount of space available between the grains of a rock, even for a loose aggregate of grains. In order to flow catastrophically across the surface, many times more water than rock is required, otherwise a thick sluggish slurry develops, with very different flow duration and deposit morphology. In order to reproduce the observed features, about 10 times as much water is required, as the volume of the chaotic zone. It is hard to store and then release this amount of water - hence the complex recycling scenarios with many smaller floods, each tapping an enlarged aquifer. We also have to remember the Faint Young Sun Paradox. Our knowledge of stellar evolution and fusion processes suggests that the Sun has warmed over geologic time. When it was first born, it only produced 70% of its present output, increasing steadily with time. The young planets would have been colder than they now are and even more extreme greenhouse atmospheres are required to compensate for this. Perhaps we should take a note from history and see if we are not inadvertently starting down the same road of interpreting imperfect data about Mars to get the answer we would like, rather that which is most likely. Everything about Mars agrees that it is cold and dry, except for those outburst flood channels. Is there any other way to interpret them?
The role of CO2 on Mars Because of this, liquid CO2 cannot exist on Mars' surface despite the fact that the temperature range for Mars is perfect for liquid CO2 (except in polar regions where it is too cold). However, it is a well known fact that underground the pressure increases due to the weight of rocks overhead. One does not need to go down more than a few hundred metres before the pressure is sufficient for liquid CO2 to be stable. Of course, some sort of seal is required, otherwise the CO2 will escape up cracks and between the individual grains of Mars' regolith, but a thin layer of impervious rock, or more probably a clotting layer of water ice can easily provide such a seal.
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In all the excitement about water on Mars, we have forgotten about the other volatile on Mars. CO2 actively cycles between solid and vapor state on Mars, just as on Earth we have water ice (snow) at the poles and vapor in the atmosphere. Earth is dominated by its liquid water oceans, so why don't we see liquid CO2 on Mars? The answer is that CO2 is much more volatile than water. It liquefies at -56.6 oC, and requires relatively high pressure to force it into the liquid state. Over 5 atmospheres of pressure are required at this low temperature to stop the liquid CO2 exploding into gas. At zero Centigrade, liquid CO2 needs 35 atmospheres confining pressure while water needs less than 1% of one atmosphere. CO2 is over 5000 times more explosive than water. You can easily observe this for yourself by watching the discharge of a pressurised CO2 fire extinguisher. These actually contain liquid CO2 under hight pressure. When you fire one off, a jet of CO2 bursts out of the nozzle and sprays a cloud of CO2 at the flames.
