Volcanoes on Mars. Mars Global Surveyor image of two volcanoes on Mars. These are Ceraunius Tholus (lower) and Uranius Tholus (upper). The caldera (crater) of Ceraunius Tholus is around 25 kilometres in diameter. The presence of numerous impact craters on the flanks of these volcanoes indicates that they are extinct, as lava flows would have otherwise covered the old impact craters. The image was taken by the Mars Orbiter Camera on the MGS spacecraft in orbit around Mars, in March 2002.