The Ananke group is a group of retrograde

irregular satellite In astronomy, an irregular moon, irregular satellite, or irregular natural satellite is a natural satellite following an orbit that is irregular in some of the following ways: Distant; inclined; highly elliptical; retrograde. They have often be ...

s of

Jupiter Jupiter is the fifth planet from the Sun and the List of Solar System objects by size, largest in the Solar System. It is a gas giant with a Jupiter mass, mass more than 2.5 times that of all the other planets in the Solar System combined a ...

that follow similar

orbit In celestial mechanics, an orbit (also known as orbital revolution) is the curved trajectory of an object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an artificial satellite around an ...

s to

Ananke In ancient Greek religion, Ananke (; ), from the common noun ("force, constraint, necessity"), is the Orphic personification of inevitability, compulsion, and necessity. She is customarily depicted as holding a spindle. One of the Greek pr ...

and are thought to have a common origin. Their semi-major axes (distances from Jupiter) range between 19.2 and 21.8 Gm, their

orbital inclination Orbital inclination measures the tilt of an object's orbit around a celestial body. It is expressed as the angle between a reference plane and the orbital plane or axis of direction of the orbiting object. For a satellite orbiting the Earth ...

s between 144.3° and 155.5°, and their orbital eccentricities between 0.09 and 0.30. The members of the Ananke group are: The

International Astronomical Union The International Astronomical Union (IAU; , UAI) is an international non-governmental organization (INGO) with the objective of advancing astronomy in all aspects, including promoting astronomical research, outreach, education, and developmen ...

(IAU) reserves names ending in -e for all retrograde moons, including this group's members.

Origin

The Ananke group is believed to have been formed when an

asteroid An asteroid is a minor planet—an object larger than a meteoroid that is neither a planet nor an identified comet—that orbits within the Solar System#Inner Solar System, inner Solar System or is co-orbital with Jupiter (Trojan asteroids). As ...

was captured by Jupiter and subsequently fragmented by a collision. This belief is founded on the fact that the dispersion of the mean orbital parameters of the core members is very small and can be accounted for by a small velocity impulse (15 < δV < 80 m/s), compatible with a single collision and breakup. Based on the sizes of the satellites, the original asteroid may have been about 28 km in diameter. Since this value is near the approximate diameter of Ananke itself, it is likely the parent body was not heavily disrupted. Available photometric studies put this in doubt, however, and suggest that secular resonance has mixed the Ananke and Pasiphae groups: three of the moons of the former family ( Harpalyke, Praxidike and Iocaste) display similar grey colours (average colour indices: B−V = 0.77 and V−R = 0.42) while Ananke itself is on the boundary between grey and light red.

Notes

References

External links

{{Moons of Jupiter Ananke group Moons of Jupiter Irregular satellites Moons with a retrograde orbit