Io is the innermost of the four Galilean moons of Jupiter. Io is 3642 kilometres in diameter and is the most geologically active place in the Solar System. Io experiences massive tidal forces from Jupiter and the large moons, Ganymede, Europa, and Callisto. There are lava flows and over 400 active volcanoes on Io. The surface is dotted with mountains taller than Mount Everest.
Using most Earth based optical telescopes Io appears as a point source. In 1610 the German astronomer Simon Marius and the great Italian astronomer Galileo Galilei discovered the giant moons of Jupiter, Io, Ganymede, Europa and Callisto within days of one another in 1610. Galileo is famously regarded as the discoverer because he published first.
Marius, with encouragement from Kepler, decided to name the moons based upon the lovers of Zeus, or his Roman equivalent Jupiter. Although not popular with Galileo his naming system is in use today. Io was the priestess of Hera who became one of the lovers of Zeus. Many on the features on Io have been named according to the Io myth, or from other figures of mythology that are associated with fire and thunder.
Early scientists studied the orbits of Io, Ganymede, Europa and Callisto. Io is in orbit around Jupiter in nearly the same plane as Jupiter is in orbit around the Sun. Io completes an orbit every 42.5 hours and is frequently eclipsed by, or eclipsing Jupiter. In the second half of the seventeenth century the Danish astronomer Rømer used the eclipse to make the first measurement of the speed of light.
Counting out from the surface of JUpiter, Io is the fifth Jovian moon. Jupiter is so large and close that it dominates the sky. Io is in nearly circular orbit 421,700 kilometres from the centre of Jupiter, or 350,000 kilometres above its weather system. Io rotates on its axis at the same pace as it rotates about Jupiter. One face is permanently turned towards Jupiter while the other is permanently in its shadow. Io has one fortieth of the volume of Earth and one seventieth of the mass. Io is slightly larger than the terrestrial moon. The surface gravity is one fifth of the strength on Earth. The surface temperature generally varies from 90 to 130 Kelivn but can reach 1500 Kelvin at geological hotspots.
The great French mathematician Pierre-Simon Laplace was interested in the stability of the Solar System. He exhaustively calculated the orbits of Io, Ganymede, Europa and Callisto. Io is in orbital resonance with Ganymede and Europa. Io completes two orbits of Jupiter for every one by Europa and four by Ganymede. Ordinarily, an orbital resonance is unstable. At each overtaking Io receives a gravitational pull from Ganymede and Europa. The cumulative nudges usually push an orbiting body out of an orbital resonance. In the case of Io the unique 1:2:4 resonance counterbalances the nudge from Europa with that of Ganymede. The orbit of Io is stabilised by the 12:4 resonance. Laplace realised that the orbital stability exposed Io to enormous tidal forces and hypothesises that the planet might be geologically active. The resonance maintains a slight eccentricity in the orbit. This contributes to the tidal forces.
Recent space missions have vastly increased our knowledge of Io. In 1973 and 1974 the Pioneer 10 and Pioneer 11 probes passed close to the satellite. The probes clarified basic information such as mass and density. They discovered that Io had a thin atmosphere almost entirely composed of sulphur dioxide and an intense radiation belt. The 1979 fly-by by Voyager I and Voyager II mapped the surface and showed that Io was geologically active. Scientists singled out seven volcanic plumes and identified volcanic features, large mountains and a young surface with no obvious impact craters. The Galileo spacecraft which flew past in the 1990s and 2000s supplied information on the lunar interior and chemical composition of the surface. Both missions identified an interaction between Io and the Jovian magnetic field. Other surface features have been identified from the Cassini-Huygens mission in 2000, the New Horizons probe in 2007, the Earth based WM.Keck Observatory and the Hubble Space Telescope.
In essence Pioneer established that Io was made of silicate rock. It is denser than the other large Jovian moons. Voyager established that Io was a volcanic place with a richly coloured surface composed of sulphur and sulphur dioxide frosts. The surface is said to resemble a pizza topping. The sulphur frost is yellow. The polar areas are the colour of red rust. Here the sulphur, exposed to radiation, is chemically different. Elsewhere the plume deposits are bright red. The images showed strange pox marked depressions called paterae which are thought to be the caldera of collapsed volcanoes The Galileo probe identified an iron core within Io and witnessed the eruption of Pillan Patera, a giant volcano. The eruption clarified the composition of the magma. In Ionian vulcanology sulphur and sulphur dioxide adopt roles played on Earth by water and carbon dioxide. The Cassini and New Horizon probes contributed to the knowledge of volcanic plumes. Some sulphur and sulphur dioxide plumes rise a staggering 500 kilometres above the surface. The missions show that the surface of Io is dynamic and constantly changing.
Although Io has a thin atmosphere it has an important influence on the Jovian magnetic field. The Jovian field extracts gas, dust and plume material from Ionain atmosphere. Some of this material resides in a “banana” shaped region of space in which the gravitational influence of Io is stronger than that of Jupiter. Within the “banana” particles are charged by collision. Some charged particles interact with the Ionian atmosphere and produce an aurora.
More space missions are intended. Juno I will launch in August 2011. It will provide passing information on Io en route to Jupiter. In 2020 a joint NASA/ESA mission will launch two probes to study Europa. The NASA probe intends o make four close approaches to Io in 2025 and 2026 en route to Europa. A dedicated mission to Io has been proposed for launch in 2015. An Ionian lander would face severe engineering challenges, particularly in coping with the intense surface radiation.
Io is a dynamic place whose mysteries require much further study.
