Scans of the surface of Mars have turned up clues about the Red Planet’s atmosphere and suggest Mars has always been a cold, barren place, U.S. scientists said on Thursday. Using the Thermal Emission Spectrometer on NASA’s orbiting Mars Global Surveyor spacecraft, geologist Philip Christensen of Arizona State University and his colleagues looked for minerals known as carbonate compounds. The compounds provide clues about Mars’s past because they form when carbon dioxide gas comes in contact with minerals and water.