Ananke group

File:TheIrregulars JUPITER GROUPS.svg

This diagram illustrates the largest irregular satellites of Jupiter. The location of the Ananke group is illustrated by Ananke's presence near the bottom. An object's position on the horizontal axis indicates its distance from Jupiter. The vertical axis indicates its inclination. Eccentricity is indicated by yellow bars illustrating the object's maximum and minimum distances from Jupiter. Circles illustrate an object's size in comparison to the others.

The Ananke group is a group of retrograde irregular satellites of Jupiter that follow similar orbits to Ananke and are thought to have a common origin.

Their semi-major axes (distances from Jupiter) range between 19.2 and 21.8 million km, their orbital inclinations between 144.3° and 155.5°, and their orbital eccentricities between 0.09 and 0.30.

The members of the Ananke group are (in order of date announcement):

Name	Diameter (km)	Semi-Major Axis (km)	Period (days)Template:Efn
Ananke	28		–623.11
Iocaste	5		–624.55
Harpalyke	4		–616.78
Praxidike	7		–618.72
Thyone	4		–620.59
Hermippe	4		–626.38
Euanthe	3		–613.93
Orthosie	2		–617.23
Euporie	2		–546.18
S/2003 J 2	2		–621.47
Eupheme	2		–611.32
Helike	4		–617.86
S/2003 J 12	1		–619.96
S/2003 J 16	2		–622.88
S/2003 J 18	2		–592.33
Mneme	2		–613.61
Thelxinoe	2		–620.55
S/2010 J 2	1		–612.35
S/2016 J 1	1		–612.78
S/2017 J 3	2		–618.97
S/2017 J 7	2		–620.02
S/2017 J 9	3		–656.05
S/2021 J 1	1		–619.77
S/2021 J 2	1		–618.50
S/2021 J 3	2		–611.87
S/2022 J 3	1		–622.44
S/2017 J 10	2		–625.15

The International Astronomical Union (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 was captured by Jupiter and subsequently fragmented by a collision. This belief is founded on the fact that the dispersion of the mean orbital parametersTemplate:Efn 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.

File:TheIrregulars JUPITER Ananke CORE.svg

This diagram compares the orbital elements and relative sizes of the core members of the Ananke group. The horizontal axis illustrates their average distance from Jupiter, the vertical axis their orbital inclination, and the circles their relative sizes.

File:TheIrregulars JUPITER Ananke.svg

This diagram offers a wider field of view than the previous one, showing other small satellites clustered near the core Ananke group. (Philophrosyne and Eurydome were thought to be Ananke group members when this diagram was produced; they turned out to be Pasiphae group members.)