Researchers from the University of Kerala have pinpointed places on Earth whose geology and chemistry resemble conditions on Mars — in Tamil Nadu and in Chile. They found melanterite at Neyveli and gypsum in the Tiruchirappalli Badlands of Tamil Nadu and in Chile's Atacama Desert. Both are hydrous sulphate minerals that shed light on how water interacted with rock and on the history of water on Mars. At Neyveli, melanterite forms when marcasite, a sulphide mineral linked to lignite, oxidises; the team saw it form within 23 days of exposure to air. Gypsum preserves a record of past water and could hold biosignatures, possible traces of earlier life.