Discovered by Galileo Galilei in January 1610, Io is the fourth largest moon in the Solar System.
Unlike other moons of the outer Solar System which are composed primarily of rock and ice, Io is believed to have a molten iron, or iron sulfide, core and a silicate crust.
Io's mottled surface is due to the presence of various silicates, sulfer and sulfer dioxide ejected from its interior.
Due to the tidal forces between Io and Jupiter, the moon's core is kept molten, and Io is the most volcanically active body in the Solar System. It has over 400 volcanoes and mountains that are higher than Earth's Mount Everest.
The image shown below right is a montage of Jupiter and Io, created from photos captured by NASA's New Horizon's spacecraft in February 2007.
In the right place at the right time, New Horizons recorded an eruption of Io's Tvashtar Volcano, the sunlit plume of which appears as a faint blue arc above the moon.
Io has a very thin atmosphere composed mainly of sulfur dioxide, but it is constantly being stripped away by the flow of charged particles from Jupiter's magnetic field.
As Io orbits Jupiter, its thin atmosphere interacts with the planet’s magnetic field. This interaction produces a high level of radiation, with Io receiving 3,600 rem, a level 1,000 times stronger than that which is leathal to humans.
Diameter: 3,643.2 km
Distance from Jupiter: 421,800 km
Orbital period: 1.77 days
Surface temperature: -163°C
Image credit: NASA/JPL/University of Arizona
Image credit: NASA, JHU-APL, Southwest Research Institute