Mounting evidence for a wet Mars

With five craft actively studying Mars, watery discoveries are coming in torrents. And the evidence points to the liquid kind, not just ice, which we've known was there for a long time. Most planetary scientists dream of finding liquid water on the Red Planet, and that may well happen in the next year or two.In the past 12 months, three significant discoveries have made the existence of water on Mars the new orthodoxy. They are the latest in a series of milestones stretching back a century that have shaped our understanding of water on Mars.

Imaginary canals (early 1900s)

In 1895 American astronomer Percival Lowell reported his observations of an intricate network of canals across the Red Planet. The canals were commonly portrayed as a last-ditch attempt by an advanced, intelligent species trying to sustain a drying world. But they turned out to be just tricks of the eye. The succession of spacecraft that visited Mars in the 1960s and 1970s revealed a world more parched than Earth's driest desert.

Tantalising tributaries (1970s-1990s)

No sooner had a succession of spacecraft revealed how dry Mars is today than evidence started flowing in of a very different past. The density of the modern Martian atmosphere is less than 1 per cent that of Earth's, far too thin to hold water droplets. But sinuous channels across the surface suggest a time when it was substantial enough to sustain a rainfall cycle. A variety of surface features revealed by ever sharper cameras seemed to indicate huge, catastrophic floods early in the planet's history, including basins that may have been lake beds and even signs that there may have been an ocean covering most of the northern hemisphere.

But all the signs indicating water were indirect, leading some scientists to propose alternative mechanisms for producing the observed features. Perhaps a fizzy slush of frozen carbon dioxide was responsible for the channels, or thin lava, or flowing ice. As long as the evidence was circumstantial there was room for alternative theories, and all references to water remained tentative.

Possibility of puddles (2002)

In 2002, the sharp-eyed Mars Global Surveyor spacecraft started spotting features suggestive of liquid water on the surface of Mars - not 4 billion years ago, but in the very recent past and maybe even today. Water seemed to be seeping from canyons and craters to form steep, dark gullies. And though the amounts were small, the potential significance was huge. It raised the possibility that there may once have been bodies of liquid water somewhere in the planet's crust. Maybe they provided moist, cosy habitats that lasted long enough for life to develop and even for it to persist today.

Again, all these signs were indirect. Though they seemed to show water, some suggested that the flows came from melting patches of snow and did not really amount to much liquid. Maybe they were flows of powdery dust with no liquid at all. From orbit, there was just no way to be sure.

Clearly mud (2004)

Last March direct and unambiguous evidence for water on Mars arrived. Measurements made by NASA's Opportunity rover on Meridiani Planum showed clearly that the material making up the rock had once been sopping wet.

The proof was chemical, morphological and mineralogical. Cavities in the rock showed places where salt crystals must have dissolved; concentrations of sulphur revealed where salty water had evaporated; perfect spheres of the iron oxide mineral haematite appear to have been built up layer by layer in water; and jarosite, a mineral that can only form in water, was clearly identified in the rock.

But Opportunity's evidence still only proved the presence of water briefly, at some point long ago in the planet's history - and only in the soil itself. There was still no proof that it had ever flowed or rippled across the surface.

A shallow sea (2004)

It took a deeper look into the layered Martian rock to find clear signs that water had once formed rivers, lakes and shallow seas. Wave patterns in layers of sand preserved in the stone show distinctive arcs. They are quite unlike any patterns on Earth produced by winds, but exactly like those due to the action of shifting currents or waves.The spot where Opportunity landed was once covered in water, perhaps a shallow salty ocean.

Seas that stayed (2004)

For the last few months Opportunity has been deep inside a crater called Endurance. Layer after layer has shown that there was water during the period when the 20 metres of rock studied by Opportunity were deposited. What's more, material filling the cracks that run through the layers suggests that it was deposited by water. For that to happen there must have been enough time for one set of sodden sediments to compact into stone brittle enough to crack before it became drenched in water once again. The random distribution of grain sizes shows the minerals were laid down by water, not wind.