The fifth of Jupiter's known moons and the third largest. It is the innermost of the Galilean satellites. With a diameter of 3630 km, Io is slightly larger than Earth's Moon. It revolves at a mean distance of 422,000 km from Jupiter. Its mass is 8.93 x 10^22 kg (about 1.2 Earth Moons) and its orbital period 1.8 Earth days. The mean surface temperature of Io is -155 °C. Io's yellow color derive from sulfur and molten silicate rock. The unusual surface of Io is kept very young by its system of active volcanoes. The intense tidal force of Jupiter stretches Io and damps wobbles caused by Jupiter's other Galilean moons. The resulting friction greatly heats Io's interior, causing molten rock to explode through the surface. Io's volcanoes are so active that they are effectively turning the whole moon inside out. Some of Io's volcanic lava is so hot it glows in the dark.