The jovian satellite with the closest orbit to Jupiter. Its surface colour is reddish, apparently because of sulphur emitted from Io. Its diameter is 189 km (262 × 146 × 134 km), the irregular shape suggesting a rigid body. Its mass is 7.17 × 1018 kg; mean distance from Jupiter 181 000 km. It radiates more heat than it receives from the Sun.
Subjects: Astronomy and Astrophysics — Earth Sciences and Geography.