Be careful bringing rocks back from Mars

It's impossible to overstate our ignorance of life, even life on Earth. We continue to find microbes living in places that we didn't think could support life, and many of these organisms get their energy in ways we never imagined were possible: in February, the journal Nature reported the discovery of bacteria that seem to live off electrons directly obtained from metallic iron. Our chances of recognizing Martians, whatever they are, will surely be greater when we know more about life here.