Making a Splash on Mars

A phase diagram of water. The 'triple point' (labeled "C" in the diagram) is the temperature and pressure where all three types of water can exist at once. In the diagram, note that liquid water cannot exist below 6.1 millibars. This fact is significant because the atmospheric pressure at the martian surface hovers just below that value. Any water that might form on a warm afternoon from melting water would quickly disappear in the desiccated martian atmosphere.

On Mars the globally-averaged surface pressure of the planet's atmosphere is only slightly less than 6.1 millibars.

"That's the average," says Haberle, "so some places will have pressures that are higher than 6.1 millibars and others will be lower. If we look at sites on Mars where the pressure is a bit higher, that's where water can theoretically exist as a liquid."

The atmospheric surface pressure on Mars is remarkably close to the triple point pressure 6.1 millibars. Is that a coincidence? Some scientists think not. If the global pressure were higher and liquid water was widespread on Mars's surface, CO2 in the atmosphere would dissolve in water and react with silicate rocks, trapping atmospheric carbon dioxide in carbonate minerals. This process would thin out the atmosphere until the pressure dropped below the triple point. Thus, the martian atmosphere could be self-limiting in this respect. [more information]

Haberle has developed a sophisticated climate model for Mars based in part on Mars Global Surveyor topography data. A simple version of the model is the basis for daily martian weather forecasts at the Ames Mars Today web site.

"I used the model to look for regions that meet the minimum requirements for liquid water - above the triple point and below the boiling point," explained Haberle. "According to the model, the highest surface pressure, 12.4 millibars, occurs at the bottom of the Hellas Basin (a low-lying area created by an ancient asteroid strike). The problem is that the boiling temperature there is only +10 °C. It can't get very hot or the water will boil away."

Evaporation of water in contact with Mars' dry atmosphere is also a problem, says Haberle. "Liquid water can be stable against freezing and stable against boiling, but unstable with respect to evaporation. The situation is analogous to Earth's oceans. Liquid water on the surface does not freeze ... or boil, yet it can evaporate if the atmosphere is not saturated with water vapour.

"There are 5 five distinct regions where we might sometimes find surface water: in the Amazonis, Chryse and Elysium Planitia, in the Hellas Basin and the Argyre Basin. Together they comprise about 30% of the planet's surface. That's not to say that liquid water really does exist in those places, just that it could."