Eyewitness: The Surface of Mars

This image, provided by NASA, shows volcanic cones on Mars that are similar in size and shape to cones found in Iceland where hot lava has run over wet ground. The heat from the lava boils the water which then bursts up through the lava flow in a steam-driven explosion. A long series of such explosions is needed to build up one of the large cones. The image was taken by the HiRISE camera onboard the Mars Reconnaissance Orbiter Illustration: AP