NASA’s 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’t know, but rover chief scientist Steven…
