Earth and Mars are both so-called "Goldilocks" planets, writes Burkhard Bilger, each large, warm and atmospheric in a way seemingly conducive to life. But, to the best of our knowledge, only one of them thrived.
Still, we keep going back. Like a delinquent sibling, Mars is all we've got—the next Earth-like planet may be in the Tau Ceti system, seventy trillion miles away—and its virtues nearly redeem its vices. Mars has sunlight, water, carbon, and nitrogen. Its surface is no more unpleasant than the inside of a volcanic vent, where bacteria thrive. It may yet have life. On November 26, 2011, NASA sent the world's most sophisticated mobile science lab to explore it: the robotic rover Curiosity. The project's scientists were quick to lower expectations: they were just looking for places that might once have been habitable, they said. Yet Mars, even dead, may answer some very old questions about life: What sets its machinery in motion? Why here and not there? Why us and not them?
