Io

The Galileo spacecraft captured these views of Jupiter's moon Io. The large frame to the left is an enhanced colour composite, made to boost natural hues. The composites on the right were made using different combinations of image filters: the upper right in order to study the type of volcanism across Io and the lower right to enhance very subtle colour variations.

In astronomy, third-largest moon of the planet Jupiter, 3,630 km/2,260 mi in diameter, orbiting in 1.77 days at a distance of 422,000 km/262,000 mi. It is the most volcanically active body in the Solar System, covered by hundreds of vents that erupt sulphur (rather than lava), giving Io an orange-coloured surface. Io and Earth are the only two planetary bodies that are undergoing known high-temperature volcanism. Io's internal heating is caused by the continual stressing of its interior as the satellite travels through changing strengths of the gravitational fields of Jupiter and other Jovian satellites.

Data gathered by the spacecraft Galileo in 1996 indicated that Io has a large metallic core. The Galileo probe also detected a 10-megawatt beam of electrons flowing between Jupiter and Io.

In 1997, instruments aboard the spacecraft Galileo measured the temperature of Io's volcanoes and detected a minimum temperature of 1,800 K/1,500°C/2,732°F (in comparison, Earth's hottest volcanoes only reach about 1,600 K/1,300°C/2,372°F).

Io

In Greek mythology, the daughter of Inachus, first king of Argos; loved under the cover of dark clouds by Zeus, who transformed the princess into a white heifer to hide her from his jealous wife Hera.

Aware of the metamorphosis, Hera set the 100-eyed Argus to watch her. When Zeus ordered Hermes to kill the monster, Hera tormented Io with a gadfly which drove the heifer across many lands until she reached the banks of the River Nile in Egypt. Here she recovered her human form and bore Zeus' son Epaphus who became king of Egypt and built Memphis. She was sometimes identified with the principal Egyptian goddess Isis.

The names of the Ionian Sea and Bosphorus, or ‘ox ford’, were said to have derived from her swimming their waters.