The fourth planet from the Sun and the second smallest in the solar system, with a diameter about half that of Earth. Mars is the last of the terrestrial or inner planets and has notable similarities to Earth, including polar ice caps and a tilted axis that gives it seasons. However, it is significantly less dense than Earth and has no magnetic field, suggesting that it lacks a metallic core, and its atmosphere, made up mostly of carbon dioxide, is much thinner than Earth's. Mars has no surface water apart from a layer of permanent ice that underlies the seasonally changing caps of frozen carbon dioxide at its poles; there is, however, clear evidence of earlier water flows in the form of channels, outwashes, and canyons. Other surface features include numerous craters, especially in the southern hemisphere, along with very large volcanoes and extensive windblown dunes. Mar's reddish color is due to the abundance of hematite in its surface rocks. Its two small, irregular moons, Phobos and Deimos, may be asteroids captured earlier by gravitational attraction.