NASA鈥檚 Mars rovers Spirit and Opportunity, just halfway through their 90-day missions, have both found what they were sent to look for: hard evidence that there was once liquid water on the Red Planet. The discovery is a turning point in a controversy that has raged for a century.
We now know for certain that the parched soil of Mars was once wet and habitable, at least by the kind of simple microbes that dominated the first 3 billion years of life on Earth. Whether such organisms ever arose on Mars we still don鈥檛 know, but rover chief scientist Steven Squyres told a NASA press conference in Washington DC last week that it would certainly have been possible at Opportunity鈥檚 landing site. At a second conference three days later, his colleagues announced that there are also signs of water where Spirit landed, on the opposite side of the planet.
Until now, the only evidence for water on Mars had come from images and some chemical clues obtained from orbit. The images suggested water had carved valleys in the landscape. Now Spirit and Opportunity have found evidence for water in the rocks鈥� structures and chemistry.
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Opportunity鈥檚 analysis of the crater where it landed has shown that the bedrock contains small randomly distributed spheres. Their composition is as yet unknown but they lie scattered throughout the rock layers, suggesting they formed by precipitation in water. The rock also contains a high concentration of sulphates, just like terrestrial sedimentary rocks that formed in salty water.
There are also empty pockets that appear once to have contained salt crystals, now dissolved away. Perhaps most convincing of all, Opportunity also found the mineral jarosite, which can only form in water. Squyres says the evidence has converted even the sceptics. 鈥淚t was interesting to watch all of my colleagues go over that cliff,鈥� he says. 鈥淎ll of us have come around.鈥�
Meanwhile, there is clear evidence that water also once seeped through cracks in the rocks at Spirit鈥檚 landing site. People have been claiming on and off for nearly a century that Mars may have been wet, but this is the first time direct observations from the surface support that view. 鈥淭alk is one thing, but to actually see it is completely different,鈥� says James Head, a Mars expert at Brown University in Providence, Rhode Island. 鈥淚t鈥檚 very significant.鈥�
This week, Spirit and Opportunity will carry out close-up inspections of some of the rock layers. This should reveal whether the water was in lakes on the Martian surface, or simply mixed into the soil. If the rocks were deposited in a lake or sea, its moving currents should have left their mark as characteristic curves called cross-bedding in the layers of rock.
The implications of finding that could be enormous. The Gusev crater, where Spirit landed, is a clearly defined basin that may have contained a lake 100 kilometres wide. But at Opportunity鈥檚 site, the Meridiani Planum, there is no shore that could have contained a lake, so evidence of moving currents there would hint that Mars once had a vast northern ocean.
Later this month, the rovers will have some help in the search for water from the European Space Agency鈥檚 Mars Express orbiter, when it activates its radar to look for ice and water beneath the planet鈥檚 surface.