Ananke (moon)

Ananke (/əˈnæŋki/) also known as Jupiter XII, is a one of larger retrograde irregular moon of Jupiter.

Ananke

Ananke photographed by the Haute-Provence Observatory in August 1998
Discovery[1]
Discovered by	Seth B. Nicholson
Discovery site	Mt. Wilson Observatory
Discovery date	28 September 1951
Designations
Designation	Jupiter XII
Pronunciation	/əˈnæŋkiː/[2]
Named after	Ἀνάγκη Anagkē
Adjectives	Anankean /ænəŋˈkiːən/[3]
Orbital characteristics[4]
Epoch 17 December 2020 (JD 2459200.5)
Observation arc	69.05 yr (24,338 days)
Semi-major axis	0.1406602 AU (21,042,470 km)
Eccentricity	0.1747248
Orbital period (sidereal)	–623.59 d
Mean anomaly	339.61045°
Mean motion	0° 34m 38.281s / day
Inclination	148.67482° (to ecliptic)
Longitude of ascending node	86.44368°
Argument of perihelion	135.63033°
Satellite of	Jupiter
Group	Ananke group
Physical characteristics
Mean diameter	29.1±0.6 km[5]
Mass	1.68×1016 kg (calculated)
Mean density	1.30 g/cm3 (assumed)[6]
Synodic rotation period	8.31±0.15 h[7]
Albedo	0.038±0.006[5]
Spectral type	P[5]
Apparent magnitude	18.9[8]
Absolute magnitude (H)	11.7[4]

Discovery and Naming

It was discovered by Seth Barnes Nicholson at Mount Wilson Observatory in 1951.^[1]

It is named after the Greek mythological Ananke, the personification of necessity, and the mother of the Moirai (Fates) by Zeus. Ananke did not receive its present name^[9] until 1975;^[10] before then, it was simply known as Jupiter XII. It was sometimes called "Adrastea"^[11] between 1955 and 1975 (Adrastea is now the name of another satellite of Jupiter).

Orbit

The diagram illustrates Ananke's orbit in relation to other retrograde irregular satellites of Jupiter. The eccentricity of selected orbits is represented by the yellow segments (extending from the pericentre to the apocentre). The outermost regular satellite Callisto is located for reference.

Ananke orbits Jupiter (on average 21,042,470 km) on a high-eccentricity (0,174) and high-inclination (149° to ecliptic) retrograde orbit. They are continuously changing due to solar and planetary perturbations.

Ananke gives its name to the Ananke group, of irregular retrograde moons orbiting Jupiter at a distance ranging between 19–22 million km, inclinations between 144 and 156°, and eccentricities between 0.10 and 0.30.

Physical characteristics

Single-exposure image of Ananke by the Wide-field Infrared Survey Explorer (WISE) spacecraft in 2010

From infrared thermal measurements by the WISE spacecraft, Ananke's albedo is measured at 3,8 %, corresponding to a diameter of 29,1 kilometres. ^[5]

In the visible spectrum, Ananke appears neutral to light-red (colour indices B-V=0.90 V-R=0.38).^[12] The infrared spectrum is similar to P-type asteroids but with a possible indication of water.^[13]

The rotation period is approximately 8 hours and 18,6 minutes with a peak amplitude of about ∼0.4 magnitudes.^[7]

Origin

Ananke probably did not form near Jupiter but was captured by Jupiter later.Given these orbital elements and the physical characteristics known so far, Ananke is thought to be the largest remnant^[14] of an original break-up, forming the Ananke group.^[15][12]

See also

• Astronomy portal

• Irregular satellites