Martian waters run deep

There may be no canals on Mars, but Nasa's latest findings suggest life-giving water may once have flowed on the surface. Matthew Genge reports

In 1906, an American astronomer called Percival Lowell captured the imagination of the world when he announced the discovery of networks of canals on Mars. Modern observations have shown that, although no such artificial waterways exist on the red planet, there is abundant evidence for flowing water on the surface in its past. Networks of channels lay testament to cataclysmic deluges of water during a wetter epoch of the planet's long history. Water is such an important requisite for life that, like Lowell's imagined canals, these natural channels have implications for whether living organisms could have evolved on Mars. When the floods occurred, and what has become of their water, are a crucial part of the puzzle of life on Mars – and they are being answered by the latest results from the Nasa Odyssey mission.

Identifying when water flowed on Mars is difficult, since it involves dating the channel-like features on the surface. Dating the surface is a particular problem. On Earth, we can simply collect rocks and determine their age from the radioactive elements they contain, but on Mars we have to rely largely on pictures of the surface taken from spacecraft.

Until now, dating features such as the channels on Mars has been all about counting holes. The surface of all the planets, including our own, are pockmarked with craters due to the collision of asteroids and comets. These scars provide the planetary equivalent of counting the rings in a tree. Since we know, roughly, how often a crater forms on the surface of a planet, and we know the surface must be older than the craters, then we can simply count the number of craters to get an age.

Crater-counting, however, fails when we want to know the age of one small part of the Martian surface, such as an area cut by channels. Unlike tree rings, craters do not form every year and there are not, therefore, enough of them in any one region to give away its age. The age of the surface at the landing site of Nasa's Mars Pathfinder at Chryse Planitia, for example, estimated from craters, is put somewhere between 1.8 billion and 3.5 billion years. Crater-counting is an imprecise science.

New results from the Infrared Themis spectrometer of the Odyssey spacecraft, however, suggest there may be a way of estimating the age of even a single crater from the mess that the collision leaves behind. Infrared pictures are all about seeing in heat rather than in visible light, and, like a hot little criminal ineffectively hiding in the bushes from the police, craters betray themselves on Mars due to their heat.

In the daytime, infrared pictures from Odyssey craters appear as circular depressions, much like they do in visible light images, because the sunlit slopes of the crater are warmed by the Sun and thus give off more infrared energy than those in shadow. However, surrounding the craters in the Odyssey images are dark halos, which become fainter on the outer edges. These are absent from the ordinary optical images. In the infrared images taken at night, the halos around craters are still visible but, strangely, brighter than everything else. To add even more complications, the halos around some craters are not as visible as those around others. It is this observation, according to Dr Philip Christensen, the chief scientist of the Themis spectrometer team, that may betray the age of a crater.

Christensen can explain why craters on Mars are surrounded by dark halos of cool materials during the day and bright halos of warm material at night. The reason is easy to understand if you've ever had to jump from boulder to boulder on a beach on a hot summer's day to avoid the baking-hot sand. Dust and sand heats up much more quickly in the sunshine than pebbles or boulders, simply because the dust and sand particles are smaller. They literally have much more surface compared to what's inside than larger rocks, and so they heat up quicker. When the Sun goes down, the opposite is true. The larger rocks stay warmer for longer.

The same goes for Mars. The dark halos surrounding craters in the daytime infrared images are caused by the presence of large boulders that were thrown out by the impacts. These stay warm at night and cool during the day, and thus give rise to the halos. The boulders in these ejecta blankets become smaller away from craters, simply because large boulders are not thrown as far by impacts as small ones. Thus, the halos become fainter with distance.

The difference in brightness between halos around craters of the same size, Christensen suggests, must simply be due to the size of boulders, pebbles and sand. It is this that will change with the age of the crater. Boulders on Mars, like on Earth, are not eternal – with time, they slowly break down into pebbles and then sand. The older the crater, the fainter the halo of debris surrounding it in infrared images. The high-resolution Odyssey infrared images may allow the craters, and thus the channelled surfaces, to be dated – if only we knew how quickly the boulders break down.

Calculating how quickly rocks break down on Mars is simpler than on the Earth, due to the dry Martian conditions. On Mars, rocks are worn down, mainly, by gradual sandblasting from the strong winds. The speed at which sandblasting will reduce the size of boulders surrounding craters could, therefore, be calculated and allow the craters, and the channels they cut, to be dated.

As well as telling us when life-giving water has flowed across the surface of Mars, data from the Odyssey spacecraft has also solved the riddle of where the water has gone. In a paper published in the current issue of the journal Science, Nasa scientists announced the discovery of large deposits of ice buried beneath the Martian surface. The ice was detected by the gamma-ray and neutron spectrometers on the Odyssey spacecraft, and has been found in enormous quantities – enough to fill Lake Michigan twice over – surrounding the southern polar region of Mars.

Although water ice was known to be present in the north polar cap among the frozen carbon dioxide, it has long been suspected that far more would occur as an icy permafrost below the surface. Despite evidence from channels that burst out from the ground that subsurface ice has been melted by rising magma in the past, there has been little evidence for permafrost until now. The new discovery has used measurements by Odyssey of gamma-rays and neutrons produced by the collision of cosmic rays that bathe and penetrate the surface of Mars.

The intensity of the gamma-rays produced by hydrogen atoms in water ice that are struck by cosmic rays, and the intensity of neutrons that are affected by hydrogen, has allowed the amount of water ice and its depth below the surface to be calculated. The results suggest that a layer containing up to 50 per cent ice lies underneath an ice-free soil in the upper metre of the Martian surface, and that the ice gets closer to the surface towards the south pole.

"It may be better to characterise this layer as dirty ice rather than as dirt containing ice," said William Boynton, principle investigator for the gamma-ray spectrometer suite. "This is the best evidence we have for subsurface water ice on Mars. We were hopeful that we could find evidence of ice, but what we have found is much more ice than we ever expected."

The signature of buried hydrogen is also seen in the north polar region of Mars. The Nasa team hope that, once the carbon-dioxide dry-ice that coats the ground in the north recedes, they will be able locate ice here as well.

The story of water on Mars has come a long way from the canals suggested by Lowell, but it is still one that is closely related to the possibility of life on Mars. The water that once flowed on the surface, which is now stored as permafrost, is a resource that would be essential to human exploration of the red planet. The results of the Mars Odyssey mission, therefore, have implications not only for past life on Mars but also for its future – and that life may well be us.

Dr Matthew Genge is a meteoritics researcher at the Natural History Museum in London