Dr. Nick Hoffman is a Senior Research Fellow of the active structural geology group at La Trobe University in Melbourne. His current research interests include the 3D structural and thermal evolution of basins, mountain belts and individual structures such as hydrocarbon prospects and economic deposits around Australia.

If you have any questions for Nick, post them to the Astronomy News forum.
 
 

The role of Carbon Dioxide on Mars

It has been well known for over 50 years that the atmosphere of Mars is dominated by CO₂, and for over 30 years that the seasonal polecaps are dry ice - solid Carbon Dioxide. What has been little explored till now is the ways that liquid carbon dioxide may be responsible for much of the paradoxical surface features of Mars. A new chapter in the understanding of the Red Planet is beginning to open with researchers suddenly discovering that a whole field of study has been neglected. Here, I tell a little of my part in the story of CO₂ on Mars and how I came to be in the right place with the right experience to suggest how different Mars really is than the planet we keep trying to make it be.

The outburst flood channels of Mars are one of the most extreme paradoxes of the planet. According to the standard model of Mars' evolution, about 3 to 3.5 billion years ago huge floods burst out from underground storage, leaving huge chasms in the ground. The floods rampaged across the surface of Mars in a volume equivalent to all the rivers on Earth in flood simultaneously. Then the waters spread out, froze, and gradually sublimed away to the atmosphere. Over a period of a few hundred million years, several separate areas each experienced these "outburst floods", and carved a pattern of channels across Mars leading from near-equatorial regions to the northern plains where the waters might even have merged into a boreal ocean.

This is the single greatest line of evidence for a wet Mars, and numerous mission proposals exist to explore the rivers, lakes, and ocean beds in the search for life signs and water-bearing minerals. One of the two Viking Landers set down in part of this channel system and the Pathfinder mission landed in another channel nearby. The images from those missions showed that huge boulders up to 1 metre in size had been transported by the floods, emphasising their raging ferocity. On the strength of this, and other interpretations of water on Mars, NASA has adopted as its fundamental goal in the exploration of Mars to "Follow The Water". Numerous NASA websites such as http://mars.jpl.nasa.gov/index.html offer information in support of this "Blue Mars" model of a planet that was formerly wet and has a high chance of having fossil or extant life relatively near the surface.

However, there are a few technical difficulties with these Blue Mars models. One difficulty is that it is hard to store water underground. There is only limited space in the pores between rock grains. My experience in the petroleum industry, dealing with the fluids trapped in sedimentary rocks has made me quite familiar with what one can and cannot do with rocks and water. If you try to push too much water into the rocks, they collapse. If you try to suck the water out too fast, they collapse, If you try to transport too much sediment in a watery flow, everything turns to porridge-like mud and freezes up.

When you account for this behaviour, then unless the subsurface of Mars is riddled with caves like a Swiss cheese, then you simply cannot get enough water out of the collapse zones without them collapsing to much larger extents. Most scientists who postulate the escape of water from the subsurface of Mars get around this difficulty by requiring multiple flow episodes down each channel. Each time a small collapse occurs, and a small flow. Five, ten, or even a hundred such flows add up to the observed total, and each time the aquifer gets recharged. This model for Mars requires strong aquifer recharge mechanisms, yet there is no evidence for global precipitation (rain or snow), so strange subsurface mechanisms are often discussed. Mars is also extremely cold today, so Blue Mars supporters often discuss a "Warm and Wet" Mars.

There is also a big problem with Blue Mars models regarding carbonates. Mars should have had a large inventory of CO₂ which would dissolve in the water and react with the rocks and dust of the regolith to form carbonates. Up to 1 km of carbonate rock should be present in the northern plains if it ever held an ocean, yet to date we have failed to observe any carbonates at the surface of Mars.

At about this point, I began studying the puzzles of Mars in earnest. My background is in petroleum exploration where I studied deep water sedimentary processes and the surface features they produce. During the late 1980's and 1990's, the petroleum industry moved from shallow water exploration to deep and ultra-deep water (2000 metres or more) in the search for oil. In the process, they left behind the province of shelfal marine sands in relatively shallow water depths (less than 200m) and moved to turbidite sand reservoirs. Not only the reservoirs were turbidites, but also the well drilling locations were sited in the flow zones of turbidites so for safety reasons the industry began to study flow processes and seabed effects of these flows. The results were surprising. Large rocks and gravel were discovered hundreds of kilometres out to sea in the cores of turbidite channels. These dense materials were being efficiently transported by suspension in dense clouds of mud and water.

Personally, I was also discovering the internet as a source of technical information. I found many stunning datasets and images to use in my work on the ocean floors, and also started to teach myself Astronomy in my spare time. I was fascinated by the Pathfinder mission and the cute Sojourner rover but dissatisfied with the answers proposed for how the landscape around the rover was produced. Although I am familiar with flood deposits and agree that the landscape is compatible with such an origin, it seemed very difficult to get a flood on Mars.

At the same time as the Pathfinder landing, the Soufriere Hills volcano in the Carribbean island of Martinique was undergoing a series of spectacular eruptions. These were publicised on the internet with a series of equally striking graphic images and movies as giant clouds of hot gas and ash (pyroclastic flows) poured down the flanks of the volcano, overwhelming the town of Plymouth and transporting metre-sized boulders on a cushion of hot air. Another memorable pyroclastic event of recent years was the eruption of Mount St. Helens. Pyroclastic flows are amongst the most damaging and destructive of volcanic phenomena due to their large size, high speed (some may be supersonic) and unpredictability.

As a geologist, I was aware of the similarity in process between deep-sea turbidite flows and volcanic pyroclastic flows, despite their very different environments and active volatiles. Both are density flows where a mixture of coarse and fine material is fluidised by volatiles (water, volcanic gas, or hot air). The resulting mixture is denser than the surrounding medium (the ocean or the atmosphere) and therefore flows downhill as a bottom-hugging flow that can carve channels, transport debris, and deposit vast carpets of smooth sediment. The ocean floors of Earth are one of the smoothest terrains in the solar system - and this smoothness is largely produced by turbidite flows which fill in all the lows and smooth out any small bumps. Other examples of density flows include snow avalanches, atmospheric weather fronts, and dust storms (on both Earth and Mars).

In the proverbial flash of inspiration, I put these widely disparate pieces together. I visualised that instead of a flood of water on Mars, these giant channels could have been carved by a gas-supported flow with CO₂ as the active volatile. Suddenly, everything made sense about Mars. This way one could get giant flows despite the mean subsurface temperature being -20 to -40 degrees C. The boulders at the Pathfinder site could have been transported by a cushion of cold CO₂, and Mars need never have had liquid water at its surface.

I spent some time working on the details. Initially I believed that the subsurface of Mars contained dry ice permafrost. Unlike water ice, solid CO₂ is stabilised by pressure, and I believed that when a cliff face failed and unloaded the ice it would undergo decompression melting and source large volumes of CO₂ gas to start a flow. However, calculations showed that this effect was relatively inefficient as a source of CO₂ vapour and in any case, the subsurface of Mars was too warm for solid CO₂, especially in the equatorial areas where the collapses occurred and floods originated. Instead, the subsurface of Mars is at an ideal temperature and pressure for liquid CO₂ to be stable.

Instead of liquid water, what is stored underground on Mars is liquid CO₂ and when a collapse occurs, this boils almost instantly and explosively to CO₂ vapour, blasting the rock and regolith to dust, except for the most resistant fragments such as igneous rocks. The rest of the regolith is composed of dust and gravel, weakly cemented by water ice. On Mars, water is not a fluid, but behaves as a mineral in most situations. Grains of ice would be tumbled along in the cryogenic flows, and transported as passive solids just like quartz grains are transported as sand by rivers on Earth.

To some extent the past tense should have been used above. Most of the flow features of Mars are over 3 billion years old and younger features seem to be much smaller and less energetic. Perhaps Mars emerged from a planetary deep freeze some 3.5 billion years ago and deposits of subsurface permafrost gradually thawed to produce unstable liquid CO₂ in the regolith. When it collapsed, the CO₂ escaped to the atmosphere then was transported to the poles where it froze in place as layers of thick permafrost intercalated by dust - the extensive Polar layered Terrain. Perhaps very little subsurface liquid CO₂ remains in the equatorial regions of modern Mars.

Collectively, I describe my ideas as "White Mars" (http://irian.geology.latrobe.edu.au/~nhoffman/Mars/Enter.html), to emphasise the role of ices and cryogenic volatiles rather than "Blue" liquids. I see Mars as always having been cold and dry. Indeed, due to the way that stars warm up as they evolve, Mars is probably warmer and more pleasant now than it has ever been in the past. The lack of carbonates on Mars is due to a lack of surface water. Ever.

These ideas were not initially popular. I have been rejected by most major journals at one time or another, yet I have had sufficient support to eventually publish major articles in significant journals. Other people are beginning to take an interest in the role of CO₂ on Mars and a number of institutions are producing new work within the "White Mars" paradigm and to work with me on aspects of the model. It is quite possible that within the next decade a major paradigm shift will occur and water on Mars will be relegated to a minor role compared to carbon dioxide.

However, those rare and local occasions where liquid water occurs will always be of intense interest because of their implications for life and unusual chemistry. On Mars, if one drills deep enough, and avoids the dangers of a liquid CO₂ blowout, eventually one will get deep enough to find liquid water at depths of 5 to 10 km at low lattitudes. Here, a deep dark biosphere may exist and extant life may await discovery. The surface Mars is inhospitable, and probably always was, but the deep rocks are warm and inviting.

Follow The Water:-
http://mars.jpl.nasa.gov/index.html
More about White Mars:-
http://irian.geology.latrobe.edu.au/~nhoffman/Mars/Enter.html
Make your own density flow:-
http://www.beloit.edu/coterie/SEPM/public_html/Water_Works/density_currents.html

by Dr. Nick Hoffman
28 May 2001.

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