Volcanoes and impact craters are major features of Martian geology. A. Plate motion on Mars, once considered to have played no role in shaping the planet's surface, is now seen as being directly associated with the planet's earliest volcanoes. B. Mars has shield volcanoes, some of which are extremely tall because of the planet's low surface gravity. C. Although the erosive power of the Martian atmosphere ensures that Mars has fewer craters than the Moon does, impact craters are prominent on Mars's surface. D. Scientists cannot yet reliably estimate the age of the Martian surface because there has been too much erosion of it. E. Scientists have been surprised to discover that conditions just below the surface of Mars are very similar to conditions just below the surface of the Moon F. Studies of crater ejecta have revealed the possibility of a layer of permafrost below the surface of Mars.