One of the Galilean satellites and the most geologically (and especially volcanically) active body in the solar system, its volcanoes being due to heat generated by tidal heating and eruption temperatures reaching more than 1000 K. Io radiates more heat than it receives from the Sun. The Voyager spacecraft observed nine volcanic eruptions. It has a metallic core, rock mantle, and a rocky surface covered with sulphur and sulphur compounds, including sulphur dioxide frost. Io was discovered in 1610 by Simon Marius and Galileo. Its equatorial radius is 1821.3 km; mass 8.93 × 1022 kg; mean density 3530 kg/m3; visual albedo 0.61; mean distance from Jupiter 421 600 km; orbital period 1.769138 days; rotational period 1.769138 days; surface temperature about -143 °C, but with one volcanic feature with a temperature measured as 17°C.
Subjects: Astronomy and Astrophysics — Earth Sciences and Geography.