The four inner or terrestrial planets are characterised by their dense, rocky composition, few or no moons, and lack of ring systems. They are composed largely of minerals with high melting points, such as the silicates which form their solid crusts and semi-liquid mantles, and metals such as iron and nickel, which form their cores. Three of the four inner planets (Venus, Earth and Mars) have substantial atmospheres; all have impact craters and possess tectonic surface features such as rift valleys and volcanoes. The term inner planet should not be confused with inferior planet, which designates those planets which are closer to the Sun than the Earth is (i.e. Mercury and Venus). The four inner planets are: Mercury Mercury (0.4 AU), the closest planet to the Sun, is also the least massive of the planets, at only 0.055 Earth masses. Mercury is very different from the other terrestrial planets; it has no natural satellite, and its only known geological features besides impact craters are "wrinkle ridges" probably produced by a period of contraction early in its history. Its almost negligable atmosphere consists of atoms blasted off its surface by the solar wind.[25] Its relatively large iron core and thin mantle have not yet been adequately explained. Hypotheses include that its outer layers were stripped off by a giant impact, and that it was prevented from fully accreting by the Sun's gravity. Venus Venus (0.7 AU) is of comparable mass to the Earth (0.815 Earth masses), and, like Earth, possesses a thick silicate mantle around an iron core, as well as a substantial atmosphere and evidence of internal geological activity, such as volcanoes. However, it is much drier than Earth and its atmosphere is 90 times as dense. Venus has no natural satellite. It is the hottest planet, with surface temperatures over 400 °C, most likely due to the amount of greenhouse gases in the atmosphere. Although no definitive evidence of current geological activity has yet been detected on Venus, its substantial atmosphere and lack of a magnetic field to protect it from depletion by the solar wind suggest that it must be regularly replenished by volcanic eruptions. [26] Earth The largest and densest of the inner planets, Earth (1 AU) is also the only one to demonstrate unequivocal evidence of current geological activity. Earth is the only planet known to have life. Its liquid hydrosphere, unique among the terrestrial planets, is probably the reason Earth is also the only planet where plate tectonics has been observed, because water acts as a lubricant for subduction.[27] Its atmosphere is radically different from the other terrestrial planets, having been altered by the presence of life to contain 21 percent free oxygen. It has one satellite, the Moon; the only large satellite of a terrestrial planet in the Solar System. Mars Mars (1.5 AU), at only 0.107 Earth masses, is less massive than either Earth or Venus. It possesses a tenuous atmosphere of carbon dioxide. Its surface, peppered with vast volcanoes and rift valleys such as Valles Marineris, shows that it was once geologically active and recent evidence[28] suggests this may have been true until very recently. Mars possesses two tiny moons (Deimos and Phobos) thought to be captured asteroids.