On a planet that's colder than Antarctica and where water boils at ten degrees above freezing, how could liquid water ever exist? Scientists say a dash of salt might help.

by Dr Tony Phillips

When scientists revealed dramatic new pictures of flood-like gullies on Mars, the big surprise wasn't that the Red Planet might harbour water. Researchers have known for years that water exists there. There are trace quantities of water vapour in Mars' atmosphere and substantial amounts of water ice at the martian poles. There may even be enough frozen water beneath Mars' surface to fill a large ocean if melted. What was amazing is that water may be present as a liquid very near the planet's surface and occasionally on top of the surface when underground deposits burst forth for a brief flash flood.

"We have conditions on Mars that seem to forbid liquid water very close to the surface," said Michael Carr of the US Geological Survey (USGS) at a press conference in June 2000. "At high latitudes, where the gullies are located, the temperatures are 70 to 100 degrees centigrade below freezing. It's incredibly cold. We expect the ground to be frozen 3 to 6 km deep."

The low temperature of Mars conspires with the planet's thin atmosphere (it is 100 times thinner than Earth's) to make water possible in only two forms: solid ice and gaseous vapour. A cup of liquid water transported Star Trek-style to the surface of Mars would instantly freeze or boil (depending on the local combination of temperature and pressure). Researchers think that the water which carved the martian gullies probably boiled explosively soon after it erupted from underground.

more from GSFC Martian gullies in Newton Crater. Scientists hypothesize that liquid water burst out from underground, eroded the gullies, and pooled at the bottom of this crater as it froze and evaporated. If so, life-sustaining ice and water might exist even today below the Martian surface - water that could potentially support a human mission to Mars.

"The air pressure is so low on Mars that even in the most favourable spots, where the pressure is higher than average, liquid water is restricted to the range 0 to +10 °C," says Bob Haberle of the NASA/Ames Research Centre."Fresh water on Mars begins to boil at 10 °C. Here on Earth we can have water anywhere between 0 and 100 °C - that range is reduced by a factor of ten on Mars."

If the thought of boiling water at 10 degrees °C seems bizarre, simply consult a high-altitude cookbook for a reality check. On mountaintops where the air pressure is low, water boils at a lower temperature than it does at sea level. (At 9000 ft a 'three-minute' boiled egg takes about five minutes to fully cook!) Mars simply takes the principles of high-altitude cooking to an extreme.

Although any liquid water exposed to Mars' low-pressure atmosphere is likely to boil, vapour is not the most important repository of martian H₂O. If all the vapour in the present-day atmosphere rained down on one spot, it would barely fill a small pond. On the other hand, the martian poles contain lots of water in the form of a solid. The north polar cap, composed primarily of water ice, is 1200 km across and up to 3 km thick in some places. The water volume there is about 4% of the Earth's south polar ice sheet. Even more water ice is thought lie deep underground.

more from GSFC Water on Mars. A: A 3D view of the Martian north pole created from Mars Global Surveyor laser altimeter data. The cap is composed mainly of solid water ice. B: Wispy clouds of water ice hover over the Kasai Vallis region of Mars. C: Ground frost (or snow) consisting of water ice at the Viking 2 landing site on Utopia Planitia.

So, the big question is not whether water exists on Mars - it does - but rather is there liquid water despite the planet being so cold? The prospects for life on Mars, both human and martian, hinge on the answer.

"First of all, you have to remember that the average atmospheric pressure on Mars is very close to the triple point of water," explains Richard Hoover, an astrobiologist at the Marshall Space Flight Centre."You only have to increase the pressure a little bit to make liquid water possible."

The 'triple point' is the combination of pressure (6.1 millibars) and temperature (0.01 °C) at which water can exist simultaneously in all three states: a solid, a liquid and a gas. On Earth, our experience with the triple point is usually limited to ice skating. The temperature of ice on a skating rink is just a fraction of a degree from the triple point. A little bit of pressure on the solid ice can cause it to transform to a liquid. The weight of a skater applied to the ice along the blade of the skate therefore creates a thin layer of liquid water that lubricates the blade and makes gliding possible.

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."

The massive Hellas impact basin in the southern hemisphere of Mars is nearly 9 kilometres deep and 2,100 kilometres across. The air pressure at the bottom of the basin is about twice the global average. In this false-color image based on measurements from the Mars Global Surveyor laser altimeter, red and white colours denote high elevations and blue denotes low.

Conditions would be favourable for liquid water only during the martian day. The temperature falls precipitously at night, so any liquid would re-freeze. At the Viking lander sites, for example, instruments registered temperatures as high as -17 C in the air and +27 °C in the soil on sunlit summer days. After sunset, thermometer readings plunged back to -60 °C or below. [click for more information about martian temperatures]

"One thing we have to be careful of is our everyday experience that water always freezes at zero degrees," noted Hoover. "It doesn't. Water containing dissolved salts freezes at a significantly lower temperature. Don Juan Pond in Antarctica is a good example. It's a high salinity pond with liquid water at temperatures as low as -24 °C."

"Salts have the potential to significantly lower the freezing point of water," agrees Steve Clifford of the Lunar and Planetary Institute. "Indeed, there are some combinations of salts that can lower the freezing point by as much as 60 °C. However, thermodynamic and chemical stability arguments (arising from work by Benton Clark) suggest that, on Mars, the most potent freezing point-depressing brines are likely to be based on NaCl (common table salt)."

An analysis of a Martian meteorite by Arizona State University scientists suggests that ancient martian oceans - if they existed - contained a mix of salts similar to those in Earth's oceans today. That wasn't the first clue that Mars was salty, though. In 1976 the two Viking landers analyzed martian soil and found that it probably contained 10 to 20 percent salts. Martian rocks, like those on Earth, react to form salt and clay minerals when exposed to water. On our planet this process gives rise to a variety of brines in the western salt lakes of North America. The detailed chemistry of the brines depends on the composition of local rocks.

Another way to help keep water liquid - on Mars or Earth - is to keep it moving.

"If you know a hard freeze is coming where you live, what's the first thing you do?" asks Hoover. "You turn your faucets on a little to let water trickle out. This way your pipes won't freeze."

The same principle applies on Mars where salty water could be moving through subterranean aquifers. "Ice is a crystal," explains Hoover, "and it's harder to form crystals when the water is flowing."

Photos Courtesy Richard B. Hoover Sampling ice from a moulin in the tongue of Alaska's Matanuska glacier. Orange moss can be seen growing on broken rock debris on ice ledge

Hoover visited the Matanuska Glacier in Alaska to search for cold-loving microorganisms living in and around the ice.

"I chose the Matanuska Glacier to visit because it's accessible and has dark rock in contact with ice," says Hoover. "The sun shining on the rock causes the ice to melt. There are pools of liquid water where microorganisms grow in abundance. There is something very interesting and exciting about this picture of me taking samples from the edge of a moulin (a water-carved crevasse). Most of what we see is ice and the air temperature is below freezing, yet there is liquid water pouring out of the glacier. How is that possible? The water had broken free further back up the glacier where sunlit rocks melted the ice. Then it flowed beneath the ice until it broke through a hole in the wall of the ice. Everything the liquid water came in contact with was freezing, yet the moving water did not freeze.

"I have also seen liquid water running from snow melting on dark rocks heated by sunlight in Antarctica, even though the air temperature was below -20 °C."

There are many places on Earth where liquid water and ice co-exist in sub-zero conditions, says Hoover. The most famous example is Lake Vostok, an expanse of water roughly the size of lake Ontario lying 4 km beneath the Antarctic ice sheet. The ice sheet acts as a blanket, shielding the lake from Mars-like temperatures at the surface.

Will explorers one day discover oases like Lake Vostok beneath icy terrain on Mars? No one knows. But instead of "Follow the Water," the mantra of future colonists on the red planet might well be "Follow the Salt."

FirstScience Editorial on 'Missions to Mars'