Making a Splash 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.