Water is probably the most peculiar substance in the Universe. It is immensely stable as a liquid, compared to its chemical analogues. It has powerful solvent properties, and it has a very unusual behaviour with pressure. We all know that ice floats and we forget that this is very unusual behaviour. Most materials are denser in their solid form and their "ice" would sink. Because of this, water melts under high pressure - indeed it is this phenomena that allows one to skate on ice and make snowballs (See "why you can't have a snowball fight on Mars" and "why you can't skate in Antartica"   for some interesting ramifications of this. For Mars, we can take this further...

The phase diagrams and material properties of Water and CO2 are important physical constraints on the behaviour of candidate fluids for the outburst "floods". A phase diagram shows what is the stable phase (gas, liquid, or solid) at any given combination of pressure and temperature (providing that enough time has passed for everything to equilibrate). For each volatile, at very low pressures they are found exclusively as a gas (vapour) whatever the temperature. For moderate pressures and low temperatures the volatiles exist as solids. Solid water is known as ice, and solid CO2 as "dry ice". At moderate pressures and higher temperatures, each volatile exists as a liquid. Liquid water is ubiquitous on Earth. Liquid CO2 is unusual in our experience, although cylinders of pressurised CO2 for industrial use and fire extinguishers actually contain liquid CO2.

The three lines on each phase diagram show the conditions where one phase changes to the other as pressure and temperature change. This change of phase may take some time to achieve, since energy must be either added to or lost from the system before the new form stabilises. Water in particular takes a lot of energy to change phase. There is a point where the three lines meet. This unique point (the "Triple Point") is one where all three phases can coexist stably. Another special point is marked with a bold dot. This is the "Critical Point" - Above the "Critical Temperature", neither volatile can be liquefied, whatever pressure is imposed. Instead, an indeterminate "supercritical" gas-like phase exists. Steam engines and turbines use supercritical steam as their working fluid and some dry cleaning processes use supercritical CO2 as a solvent.

CO2 is more volatile than water. By this we mean that it more readily forms a liquid and vapour than does water. It takes less energy per unit mass to do so, and the transition occurs at lower temperatures and higher confining pressures.

The arrow on each diagram shows what would happen if underground sources of either volatile were abruptly exposed to lower pressures near the surface, as would happen in an outburst "flood" or a collapse of a chaos zone. Water at depth will tend to freeze when it is decompressed, so any outburst of water will partially freeze as it emerges from the rock. This freezing will act to plug up any opening and block off the flow. Proponents of water as the working fluid of outburst floods have to perform careful balancing of the volumes of fluid so that enough remains to be able to transport any debris at all, rather than just its own load of "pressure ice".

It will also not escape the astute reader that the melting point of water is rather warm by Mars' standards. Much of Mars' surface has a mean annual temperature around 200 K (at the present epoch). Typical geothermal gradients on Mars are expected to be a mere 5 to 10 K / km (Earth is typically 30 - 35 K / km). Therefore, on Mars, liquid water is not expected above depths of 5-10 km unless volcanic activity is warming the ground. On the other hand, liquid CO2 would have been ubiquitously stable in equatorial regions of Mars about 2 billion yers ago, at burial depths of 50m+.

CO2 can also exist as a stable solid at depth, due to its pressure solidification behaviour, especially in the past when surface temperatures were lower. When a cliff collapse occurs at a chaos zone, this solid CO2 depressurises and (partially) melts. Thus liquids can be formed spontaneously from solid material, merely because it collapses. The liquid CO2 then floods out through the dusty regolith and literally explodes into Mars' thin atmosphere with an internally-generated pressure of at least 5 bars. The effect is like letting off a fire extinguisher into a bag of flour. The fine dust and sand of Mars' regolith is blasted into a huge dust cloud which roars down the valley like a fluffy avalanche. However, this "fluff" is powerful stuff. Like volcanic ash clouds on Earth, it can carry so much load in suspension that it flows like a liquid downhill and erodes and destroys anything in its way, carving the outburst "flood" channels. Even solid rocky layers are blasted into fragments by the explosion and carried downstream as boulders. In this way the impressive boulders at the Mars Pathfinder landing site were transported and deposited by clouds of gas and dust, not by water!

The existence of life on Earth is entirely due to the chemistry of water. Similarly, the story of life on Mars is neccesarily linked with the story of water on Mars. If Water has been abundant and frequent on the surface of Mars, then so should life be. If instead, water has been absent on the surface for the last 3.5 billion years, then so too must life be restricted to subterranean refuges where limited amounts of liquid water percolate through the rock near igneous intrusions. Mars has no planetary network of  active  plate boundaries with associated volcanoes. Instead, volcanism is limited to a few key zones where prolonged foci of volcanism have continued throughout Mars history. Life on Mars, if it is found anywhere, will be found in the volcanic provinces of Tharsis and Elysium. On Mars, Mons Olympus may be not the abode of the Greek Gods, but of Martian microbes.

      Created: June 2002
      Last modified: June 2002
      Authorised by:  Head, Earth Sciences

      Maintained by: Nick Hoffman
      Email: nhoffman@unimelb.edu.au