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Jupiter has 79 known moons, the largest number of moons for a planet in the solar system, that vary in size and characteristics. Researchers have grouped the moons into two broad categories namely regular satellites and irregular satellites. The regular satellites have almost circular orbits and are prograde while irregular satellites are smaller with eccentric orbits. There are two sub-groups of regular satellites – inner satellites and main satellites. The inner satellites (the Amalthea Group) orbit nearest to Jupiter and are responsible for replenishing and maintaining Jupiter’s ring system. NASA observes that main group (Galilean Moons) consists of some of the largest bodies in the solar system besides the sun and planets. On the other hand, there are two sub-groups of irregular satellites namely prograde and retrograde.

Examples of Some of the Moons of Jupiter

Since their discoveries, scientists have been able to conduct in-depth analysis on some of Jupiter’s moons than of any other solar system moons excluding the earth’s. However, some of Jupiter’s moons have only been observed few times because of their unknown orbits. This article analyses some of the well-researched moons orbiting around Jupiter.

Amalthea (Jupiter V)

Amalthea is the number three moon by distance from the planet and the largest of the inner satellites. Observers believe that this moon consists of porous water ice below the large craters and ridges making up its surface. Discovered in 1892, this planet orbits Jupiter at a distance of 2.54 Jupiter radii, an eccentricity of 0.003, and an inclination of 0.37° relative to Jupiter’s equator. Photographs show Amalthea to be red in color in what may be sulfur originating from the nearby moon, Io. Observations also show that the leading hemisphere appears to be 1.3 times brighter than the trailing hemisphere probably due to the high velocity and friction. Amalthea’s surface measures between 33976.98 and 65,637 square miles and radiating more than it receives from the sun. This radiation may be due to the influence of the Jovian heat flux, sunlight reflected from Jupiter, and bombardment of charged particles.

Callisto (Jupiter IV)

Callisto is the second-largest moon of Jupiter and third-largest moon in the Solar system. This Galilean moon has a diameter of 2,996 miles and an orbital radius of about 1,170,042 miles. Callisto is tidally locked during rotation meaning that it orbits Jupiter with the same hemisphere facing inwards making Jupiter seemingly stands still in Callisto's sky. This planet has fairly equal rock and ice composition with confirmed compounds like water ice, carbon dioxide, silicates, and organic compounds. Investigations also reveal a possible underground ocean of liquid water and surface features like ring structures and craters of different shapes. Other characteristics include a thin atmosphere of carbon dioxide and low radiation levels leaving speculations of possibilities that the moon can support some form of life. Evidence shows that Callisto orbits at a distance of approximately 1,168,178 miles and has an orbital period of 16.7 Earth days. The eccentricity is between 0.0072 and 0.0076 and inclination of 0.20°–0.60° that causes the axial tilt to vary between 0.4° and 1.6°.

Io (Jupiter I)

Io is the innermost Galilean moon with the highest density of all the moons and over 400 active volcanoes (producing sulfur and sulfur dioxide) resulting from tidal heating from within its interior. There are over 100 mountains, some taller than Mount Everest. Critical analysis reveals that the volcanic plumes and lava flows are responsible for the surface color of different shades of yellow, red, white, black, and green or, as people call it rotten orange or pizza colors. Atmospheric studies indicate that Io has a thin atmosphere consisting of sulfur dioxide, sulfur monoxide, sodium chloride, and atomic sulfur and oxygen. Made up of silicate rocks and iron, Io has a radius of 1,131.7 miles and orbits at a distance of 217,000 miles from Jupiter’s cloud tops. With a slight ellipsoid in shape, this moon has an orbital eccentricity of 0.0041 and rotates synchronously with its orbital period ensuring that one side always facing Jupiter.

Discovery and Naming

Galileo Galilei and Simon Marius hold the credits for being the first people to sport the four Galilean moons in 1610 with up to 69 moons discovered to date. The International Astronomical Union (IAU) outlined the system of naming Jupiter’s moons after lovers, favorites, and descendants of god Jupiter (Zeus). Highly inclined satellites (prograde irregulars) have names ending with "a" or "o" whereas names ending with "e" belong to retrograde irregulars.