The Moons of Jupiter

"Jupiter family" by NASA/JPL - PIA01481This image or video was catalogued by Jet Propulsion Laboratory of the United States National Aeronautics and Space Administration (NASA) under Photo ID: PIA01481. Licensed under Public domain via Wikimedia Commons

 

Jupiter has 67 known natural satellites, with 51 of them having diameters of 20 km or less. Many of these moons are probably captured asteroids that got caught in Jupiter's massive gravitational field. Most of these moons have only been discovered since 1975 with improvements in telescopes and the Pioneer and Voyager missions.

Jupiter's moons are generally divided into groups based on proximity to Jupiter, composition of the moon, and other orbital characteristics.

These groups are:

• The Inner Group: as the name implies, these are the inner most moons of Jupiter. There are four of these moons are they are all less than 200 km in diameter with semi-major axes less than 200,000 km from Jupiter. They have generally low eccentricities and have inclinations close to 0°, i.e. their orbits are almost directly above the equator of Jupiter.
• The Galilean moons: These are probably the most well-known of all of Jupiter's moons. These are the four moons discovered by Galileo in 1609 or 1610 after Galileo pointed his telescope towards Jupiter. He noticed these small objects moving along with Jupiter as Jupiter went around the Sun. These four moons will be discussed in more detail later on.
• Themisto is a single moon that does not belong to any group. It is farther away from Jupiter than the Galilean moons but closer to Jupiter than the next group, the Himalia group. Themisto is located approximately halfway between Callisto and the innermost Himalia moon, Leda. It has a semi-major axis of 7.39 million km, an eccentricity of 0.2006, and an inclination to Jupiter's rotation of 47.48°.
• The Himalia group: This group is named after the largest member, Himalia. There are five known satellites in this group with orbits ranging from 11.15 million km to 11.75 million km. They orbit at an inclination of 26.6° to 28.3° and have eccentricities of 0.11 to 0.25. These moons have compositions similar to C-type asteroids, which lead astronomers to believe that this group is made up of a captured asteroid that was ripped apart by tidal forces from Jupiter's gravity. Any moons that are found in this group will have a name ending with "-a".
• Carpo is another group made up of a single moon. It has an orbit at the inner edge of the next group, the Ananke group, but its orbital parameters are different. It has an inclination of 55°, a semi-major axis of 17.15 million km, and an eccentricity of 0.4316.
• The Ananke group: This group is named after the largest satellite in the group, Ananke. Theses asteroids range in eccentricities from 0.02 to 0.28 (much less than Carpo), semi-major axes from 19.3 million km to 22.7 million km, and inclinations from 145.7° to 154.8°. From the inclination, and if you remember from the post about a day on Venus, the inclinations of these satellites tell you that these satellites orbit retrograde. Looking down on Jupiter from the north, Jupiter rotates counter-clockwise (or for you Europeans, anti-clockwise), but these satellites orbit clockwise around Jupiter. Any satellites found in this group will have a name ending in "-e".
  
• The Carme group: Named after the largest satellite, Carme. They have semi-major axes of 22.9 million km to 24.1 million km and eccentricities between 0.23 to 0.27. The inclinations range from 164.9° to 165.5°, which means these also orbit retrograde around Jupiter. It is believed that these moons formed after a D-type asteroid was captured by Jupiter and broke up. Like the Ananke group, any future Carme group moons will end in an "e". Two exceptions to these moons are Kalyke is much redder than the other asteroids, and Taygete has a much higher eccentricity (e=0.3678).
• The Pasiphae group: These satellites share similar orbital distances but at a slightly lower inclination (144.5° to 158.3°). The largest moon in this group is Pasiphae.