Scientists have found signs of water in rock layers. In 2006, the scientists saw new deposits of sediment on the surface of Mars. These sediments had not been there six years earlier. According to scientists at NASA, this is the strongest evidence so far that water still flows occasionally on the surface of Mars, though other scientists say the sediments could have been deposited by carbon dioxide frost or movement of dust. Samples of the minerals haematite and goethite have also been found in Mars. These minerals are sometimes formed in the presence of water.