“The discovery of abundant siderite in Gale Crater represents both a surprising and important breakthrough in our understanding of the geologic and atmospheric evolution of Mars,” Benjamin Tutolo, associate professor at the University of Calgary, Canada, and lead author of the paper, said in a statement.

NASA's Curiosity rover has found iron-rich carbonates on Mars — a major clue that the Red Planet used to be habitable.

NASA’s Curiosity rover has uncovered a missing piece in the puzzle of Mars' past atmosphere. Scientists discovered siderite—an iron-rich carbonate mineral—in layered rock near the surface of Gale Crater,