A long-lasting lake on ancient Mars provided stable environmental conditions that differed significantly from one part of the lake to another, according to a comprehensive look at findings from the first three-and-a-half years of NASA’s Curiosity rover mission. While previous work had revealed the presence of a lake more than three billion years ago in Mars’ Gale Crater, this study defines the lake’s chemical conditions and uses Curiosity’s powerful payload to determine that the lake was stratified.