Is there, or was there once, life on Mars? Debate about martian life remains unresolved, but over the past decade, unprecedented observations have enabled us to address key astrobiological questions in new ways. This lecture will examine the observations of ancient sedimentary rocks made by the NASA rover, Opportunity, at Meridiani Planum. Opportunity has provided both physical and chemical evidence that liquid water once existed at the martian surface. At the same time, however, Opportunity's chemical data suggest that brines percolating through accumulating Meridiani sediments grew salty enough to inhibit most known life, even the hardiest microorganisms. Chemical observations further suggest that the sites investigated by Opportunity and its identical twin, Spirit, have not seen much water since their minerals were precipitated billions of years ago. Remote sensing of martian landforms independently suggests that Mars has been cold and dry for most of its planetary history, sharply constraining continuing debate about martian life.