Io

Overview

Io is the innermost and third-largest of the four Galilean moons of the planet Jupiter. Slightly larger than Earth’s moon, Io is the fourth-largest moon in the Solar System, has the highest density of any moon, the strongest surface gravity of any moon, and the lowest amount of water of any known astronomical object in the Solar System.
With over 400 active volcanoes, Io is the most geologically active object in the Solar System. Several volcanoes produce plumes of sulfur and sulfur dioxide that climb as high as 500 km above the surface. Io's volcanism is responsible for many of its unique features. Its volcanic plumes and lava flows produce large surface changes and paint the surface in various subtle shades of yellow, red, white, black, and green.
Like the other Galilean moons, Io serves an important economic role. Much of the raw hydrogen and helium gas harvested from Jupiter's atmosphere is processed and refined on Io due to its proximity. Aside from the gas refining industry, Io's volcanic activity sustains a strong mining economy with its uniquely easy access to rare minerals and useful compounds. This industry is so localized around lava flows and volcanic patera that many Ionians live in company towns built for the purpose of housing sulfur miners or helium refiners.

Geography

Like the other Galilean satellites and the Moon, Io rotates synchronously with its orbital period, keeping one face nearly pointed toward Jupiter. Io's prime meridian intersects the equator at the sub-Jovian point. The side of Io that always faces Jupiter is known as the subjovian hemisphere, whereas the side that always faces away is known as the antijovian hemisphere. The side of Io that always faces in the direction that Io travels in its orbit is known as the leading hemisphere, whereas the side that always faces in the opposite direction is known as the trailing hemisphere. From the surface of Io, Jupiter subtends an arc of 19.5°, making Jupiter appear 39 times larger than Earth's Moon.
Io is slightly larger than Luna being 5% wider at the diameter and 21% more massive. Among the Galilean satellites, in both mass and volume, Io ranks behind Ganymede and Callisto but ahead of Europa. Composed primarily of silicate rock and iron, Io and Europa are closer in bulk composition to the terrestrial planets than to other satellites in the outer Solar System, which are mostly composed of a mix of water ice and silicates. Io has a density of 3.5275 g/cm3, the highest of any regular moon in the Solar System. 
Io's colorful appearance is the result of materials deposited by its extensive volcanism, including silicates (such as orthopyroxene), sulfur, and sulfur dioxide. Sulfur dioxide frost is ubiquitous across the surface of Io, forming large regions covered in white or grey materials. Sulfur is also seen in many places across Io, forming yellow to yellow-green regions. Explosive volcanism, often taking the form of umbrella-shaped plumes, paints the surface with sulfurous and silicate materials. Plume deposits on Io are often colored red or white depending on the amount of sulfur and sulfur dioxide in the plume.
Io's surface is dotted with volcanic depressions known as paterae which generally have flat floors bounded by steep walls, the largest of which being Loki Patera at 202km across. The Loki volcano is also consistently the strongest volcano on Io, contributing on average 25% of Io's global heat output. Lava flows represent another major volcanic terrain on Io. Magma erupts onto the surface from vents on the floor of paterae or on the plains from fissures, producing inflated, compound lava flows that can cover hundreds or thousands of square kilometers.

Climate

As the innermost of the Galilean moons, Io is in a 2:1 mean-motion orbital resonance with Europa and a 4:1 mean-motion orbital resonance with Ganymede. This resonance helps maintain Io's orbital eccentricity, which in turn provides the primary heating source for its geologic activity. Without this forced eccentricity, Io's orbit would naturally circularize, leading to a less geologically active world.
Io plays a significant role in shaping Jupiter's magnetic field, acting as an electric generator to give Jupiter a magnetic field inflated to more than twice the size it would otherwise have. The magnetosphere of Jupiter sweeps up tons of gases and dust from Io's thin atmosphere every second. This material is mostly composed of ionized and atomic sulfur, oxygen and chlorine; atomic sodium and potassium; molecular sulfur dioxide and sulfur; and sodium chloride dust. These materials originate from Io's volcanic activity, with the material that escapes to Jupiter's magnetic field and into interplanetary space coming directly from Io's atmosphere.
Io's atmosphere is one of the most uncomfortable in the Solar System. Due to the moon's volcanic activity, the air is always thick with sulfur and sulfur dioxide. The air is breathable (if a little odorous) in the few areas untouched by volcanic activity, but anywhere near the many paterae or lava flows it is necessary to wear protection outdoors.

Type

Planetoid / Moon

Location under

Jupiter