3rd Largest Moon of Jupiter - Io (5th Moon outwards from Jupiter)

Io, the innermost of the four Galilean moons, is the third largest moon of Jupiter and the fourth-largest moon in the Solar System.

Discovery

Io's discovery is credited to Galileo Galilei, who was the first to observe it on January 7, 1610.

Together with Ganymede, Callisto and Europa, they are collectively known as Galilean satellites after the discover.


Naming

Io was named after the Greek mythological figure Io. Io, in Greek mythology, was a priestess of Hera in Argos, a nymph who was seduced by Zeus, who changed her into a heifer to escape detection.

Stats

Diameter: 4,821 km

Semi-major axis: 421,700 km

Orbital Period: 1.77 days

Orbit

Io is in a 2:1 mean-motion orbital resonance with Europa and a 4:1 mean-motion orbital resonance with Ganymede, completing two orbits of Jupiter for every one orbit completed by Europa, and four orbits for every one completed by Ganymede.

This resonance helps maintain Io's orbital eccentricity (0.0041), which in turn provides the primary heating source for its geologic activity.

Io takes as long to rotate on its axis as it does to make one orbit of Jupiter; and therefore always keeps the same hemisphere pointed to Jupiter.

Geological Activity

With over 400 active volcanoes, Io is the most geologically active object in the Solar System. This extreme geologic activity is the result of tidal heating from friction generated within Io's interior as it is pulled between Jupiter and the other Galilean satellites — Europa, Ganymede and Callisto.

Several volcanoes produce plumes of sulfur and sulfur dioxide that climb as high as 500 km above the surface. Io's surface is also dotted with more than 100 mountains that have been uplifted by extensive compression at the base of the moon's silicate crust. Some of these peaks are taller than Earth's Mount Everest.

Composition

Unlike most satellites in the outer Solar System, which are mostly composed of water ice, Io is primarily composed of silicate rock surrounding a molten iron or iron sulfide core. Most of Io's surface is characterized by extensive plains coated with sulfur and sulfur dioxide frost.

Io's volcanism is responsible for many of the satellite's unique features. Its volcanic plumes and lava flows produce large surface changes and paint the surface in various shades of yellow, red, white, black, and green, largely due to allotropes and compounds of sulfur.

Numerous extensive lava flows, several more than 500 km in length, also mark the surface.

Atmosphere

The materials produced by volcanism provide material for Io's thin, patchy atmosphere and Jupiter's extensive magnetosphere.

Io has an extremely thin atmosphere consisting mainly of sulfur dioxide (SO2), with minor constituents including sulfur monoxide (SO), sodium chloride (NaCl), and atomic sulfur and oxygen.

The atmosphere has significant variations in density and temperature with time of day, latitude, volcanic activity, and surface frost abundance.

Life

Life is thought to has less chance at Io, then at the other Galilean moons.