Io is one of the four Galilean moons of Jupiter, so called because they were discovered by Galileo in 1610.
Io has the closest orbit to Jupiter of the four Galileans, about 262,000 miles (422,000 km) from its host planet, which is similar to the distance between our Moon and Earth. However, it takes only 1.77 Earth days to complete an orbit. It is slightly larger than our Moon, but – due to the intense gravity exerted by Jupiter – it is not a perfect sphere but slightly ellipsoid in shape with the “bulge” pointing towards its giant neighbor. Not surprisingly it is tidally locked – like our own Moon – which means that the same face always points towards Jupiter.
The closeness to Jupiter means that Io is subjected to intense bombardment from the radiation emanating from Jupiter’s magnetic field.
(Image taken by the Galileo Orbiter and made available for general release by NASA)
The gravitational pull not only of Jupiter but of Io’s neighboring moons Europa and Ganymede generate huge amounts of heat in the sulphur-rich rocks of which Io is largely composed. The net result is that Io is the most volcanically active world in the entire solar system, with huge plumes of molten sulphur and sulphur dioxide rising more than 300 miles (500 km) above the surface before crashing down again to blanket the surface in colourful sulphur compounds – red, yellow, orange and white.
Sulphur also reaches the surface through volcanic vents.
The weight of all the material landing on the surface has led to mountain ranges being formed as the crust buckles, but none of these features last long. Within a few decades, Io’s pockmarked face will look completely different.
Io is a violent, inhospitable world that no human is ever likely to visit. The moon therefore bears little resemblance to the beautiful priestess, after whom the moon was named, who was lusted after by Zeus in the Greek myth!