MARS has finally shaken off its dry, barren image. Vast reservoirs of water ice almost certainly lie just a few centimetres below its dusty red surface, according to data released by researchers working with NASA鈥檚 Mars Odyssey, the probe now orbiting the planet.
It鈥檚 hard not to find the new data ironic. NASA has effectively admitted that if only Viking 2 had been equipped with a small gardening trowel when it landed on Mars in 1976, it would almost certainly have hit the jackpot then. But this week researchers are rushing to draw up design proposals for landers that can dig and drill, vying to be the first to pierce the Martian 鈥渨ater table鈥�.
Speculation is rife about how the results will affect the prospects for a crewed mission. If astronauts can survive the 2-year bombardment with solar radiation on their journey to Mars, the planet may at least be able to offer them a good, long drink.
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Despite the excitement, the new data doesn鈥檛 say anything directly about water. What it does tell researchers is where hydrogen atoms are present in the Martian soil. Mars Odyssey carries two instruments sensitive to hydrogen: one camera for detecting gamma rays and another for high-energy neutrons. Neutrons are ejected when incoming cosmic rays strike atomic nuclei in the Martian soil. These are absorbed by hydrogen atoms, which then emit gamma rays. So soil that contains hydrogen emits fewer high-energy neutrons and more gamma rays than soil which doesn鈥檛.
Both sets of data show huge bands of hydrogen around the poles of Mars, and smaller amounts at the equator. The researchers have interpreted the data in two ways. Near the poles, they say, the hydrogen signal comes from the hydrogen of water molecules. But near the equator, the signal comes from hydroxyl groups鈥攈ydrogen bound up in clay-like minerals.
So how come the same signal means different things in different places? Jim Bell, an astronomer at Cornell University, says the evidence for water is circumstantial. 鈥淭he position of the signal tips you off,鈥� he says. Although the temperature on Mars averages 鈭�50 掳C, sunlight falling on the equator would create enough heat to evaporate any water ice near the surface. So the researchers reason that the hydrogen near the equator must be bound up in minerals.
If all the hydrogen on Mars were bound up in minerals, however, there鈥檇 be no reason for its concentration to fall off so quickly towards the equator. Mineral deposits would vary due to local volcanic activity and weathering, not in large bands across the entire planet. Near the poles, on the other hand, where it鈥檚 colder and the sunlight is much weaker, water ice is expected to be stable. So the large hydrogen signal at the poles is likely to come from water.
The amount of water has exceeded all expectations. Previous measurements of infrared radiation emitted from the Martian surface showed only a tiny amount of hydrogen, but infrared only penetrates a few dozen micrometres into the soil. Cosmic rays go deeper, up to a metre into the rocky debris built up over billions of years of impacts from meteorites and comets. Ice crystals would fill the gaps in the rock. 鈥淭his is something like the Siberian permafrost,鈥� says Bell. The Martian permafrost may contain even more water than terrestrial tundra鈥攗p to 40 per cent water by weight and 50 per cent by volume.
At the moment, no one knows just how far down the fragmented rock reaches before turning to solid bedrock. Bell says it could extend down a kilometre or more, in which case the water seen so far may represent only the tip of the underground iceberg. Even a few metres of ice would in theory provide an ample supply of water for a crewed mission鈥攅nough to make hydrogen fuel for a return journey or simply to drink.
But it wouldn鈥檛 be as simple as just melting the permafrost. Robert Reedy, who worked with the neutron data at Los Alamos National Laboratory in Albuquerque, New Mexico, points out that Martian soil contains high levels of chlorine and sulphur. 鈥淚t鈥檚 probably very, very salty water,鈥� he says. 鈥淵ou would have to distil it.鈥�
