Planetary scientists sometimes joke that we know more about Mars than we do about the moon. NASA first landed a spacecraft on the surface of the fourth planet during the U.S. Bicentennial, five years before the first space shuttle ever lifted off. And we’ve learned plenty in the intervening 35 years: Viking 1 and 2 analyzed Mars rocks, Spirit and Opportunity found evidence of ancient water, and Phoenix saw the Martian snow. Yet the biggest question — whether Mars could ever be home to life — still eludes us.