Any molecule that contains Iron and Oxygen. Usually formed when Iron is exposed to liquid water. The surface of Mars is red due to Iron Oxide. Iron Oxides may be present in Mud Volcanoes, indicating they could have been exposed to water when underground.
Mars/Iron Oxides Web Pages
Mars/Iron Oxides In the News
Mud Volcanoes in Acidalia Planitia Could be Methane Source (Apr 09)