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White Mars: The Paradoxes

Do "outflow" channels always point to water, or can could another agent be responsible for carving deep scars in the surface of Mars
by Dr Nick Hoffman
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
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.

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.

click for full size chart
In temperature and pressure terms, the subsurface of Mars is perfect for liquid CO2. Remember those fluid samples from Mars meteorites that turned out to be liquid CO2? Imagine a planet where the rocks were streaming wet with, not water, but liquid CO2. We use the term "Liquifer" for this, in analogy with a water-bearing Aquifer on Earth. When we explore beneath the surface of Mars, temperatures gradually rise as we go deeper, due to internal heat flow. Liquid CO2 forms at -56.6 oC, water does not melt until zero Centigrade, so the first-formed liquid in the subsurface of Mars will be liquid CO2. Although saline water melts more easily than fresh water, this is only worth 10 or 15 degrees at best and there is a further complication. Under pressure, mixtures of CO2 and H2O form a new type of joint ice - a Clathrate, or Gas Hydrate. This form of ice takes 8 CO2 molecules for every 44 of H2O and forms a very stable ice, that won't melt until +10 oC, even in saline conditions. Therefore small amounts of CO2 can lock up large amounts of H2O. Excess CO2 will form extensive liquifers. If water is not going to be available in the subsurface, then how can we generate the outburst floods? The obvious idea is to see what happens when a pocket of liquid CO2 breaks through to the surface.

CO2 Outbursts

click for full size chart
Just like the fire extinguisher on Earth, the CO2 will explosively vaporise and blast outwards, like a small volcano. The rocks around the CO2 "spring" will be blasted apart and thrown into the air forming a cloud of debris. This debris cloud will be much denser than the thin Martian atmosphere and will tend to slump and flow downhill. As the outburst proceeds, the ground will be undermined and huge blocks will collapse - forming the chaos zone and releasing even more liquid CO2. Over a very short period of time, large volumes of fluidised debris can be produced. The outburst will continue until the CO2 pocket is exhausted, or the escape route becomes choked by the load of rubble.

Earth Analogs

click for full size chart
We see processes a little like this around volcanoes on Earth. Clouds of Ash are thrown out by an eruption and unless they are very hot and buoyant, they collapse downhill. Huge fluidised clouds of Ash, dust, gas, and larger rocks tumble and pour downhill as a soft but deadly surge. These "Pyroclastic Flows" can travel at near supersonic speed. They form channels, both because they flow down into valleys where they are focussed and because they have immense erosive and destructive power to scout and cut into the ground with their debris load of large boulders. They can demolish buildings, snap mature trees like matchsticks, and burn and destroy everything in their path. In 1902 on the Caribbean island of Martinque, over 29,000 people were killed by a single such flow, leaving only 2 survivors from the entire town of St. Pierre. The flank collapse of Mount St. Helens in 1980 triggered a powerful pyroclastic flow that killed many local residents and sightseers.

These flows are part of a suite of "Density Flows", so named because they consist of a dense fluidised cloud that flows downhill as if it were a fluid. They have interesting properties since they are compressible, travel at very high speeds, and can also move up and over large obstacles. On Earth, giant flows of this type have crossed ranges of hills nearly 1 km high and reformed on the far side to continue their destructive advance. Other examples are Snow Avalanches, Submarine Turbidites, and Long Run-Out Rock Avalanches. These flows are capable of travelling long distances if they are either initially very large, or if they contain a source of volatiles to sustain their fluidity. Boulders of metre scale or more are easily carried for many kilometres on a cushion of hot air!

White Mars Part Three

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