Ananke group

Lore & Background

The Ananke group is believed to have formed when an asteroid was captured by Jupiter and later fragmented by a collision with a passing planetesimal, rather than a moon-moon impact. This is supported by the small dispersion of mean orbital parameters among core members, which can be accounted for by a velocity impulse between 15 and 80 m/s, consistent with a single breakup. The original asteroid may have been about 28 km in diameter, close to the size of Ananke itself, suggesting the parent body was not heavily disrupted.

Photometric studies cast some doubt on this origin, indicating that secular resonance may have mixed the Ananke and Pasiphae groups. Three moons of the Ananke group—Harpalyke, Praxidike, and Iocaste—display similar grey colors, while Ananke itself is on the boundary between grey and light red. Numerical integrations show that collisions among retrograde moons over 4.5 billion years are rare, with an expected total of about one, making a moon-moon origin less likely. However, if retrograde irregular moons were much more numerous in the past, a moon-moon collisional origin remains plausible.

The group is typically identified visually in orbital element space, differing in inclination from the Carme group and having smaller semi-major axes than the Pasiphae group. Some authors consider only the tight cluster around Ananke as part of the group, while others do not distinguish between the Ananke and Pasiphae groups.

Reader's Guide

The Ananke group is significant as a probable collisional family among Jupiter's irregular satellites, providing insight into the early solar system's dynamics and the capture and fragmentation of planetesimals. Its inferred origin from a single impact by a 0.53 km planetesimal, common early in solar system formation, suggests that such events were frequent enough to produce the observed satellite groupings. The group's orbital parameters—semi-major axes between 19.2 and 21.8 million km, inclinations from 144.3° to 155.5°, and eccentricities from 0.09 to 0.30—define its distinct region in orbital element space. The legacy of the Ananke group includes ongoing uncertainty about its relationship with the Pasiphae group, as photometric studies and secular resonance mixing blur the boundaries. The International Astronomical Union reserves names ending in -e for all retrograde moons of Jupiter, including members of this group, reflecting a naming convention that aids classification. The group's formation via a non-catastrophic collision, leaving 96% of the parent body's mass in the largest fragment, contrasts with other families like the Carme group, which is inferred to have a similar but distinct origin.

Did You Know?

• The Ananke group's orbital inclinations range from 144.3° to 155.5°.
• The impactor that fragmented the parent body was about 0.53 km in diameter.

Discovery and a Long Wait for a Name

In 1951, astronomer Seth Barnes Nicholson, working from the Mount Wilson Observatory, identified a faint new body circling Jupiter. For more than two decades, this satellite carried only a numerical designation—Jupiter XII—before briefly being referred to as Adrastea between 1955 and 1975, a name that would later be reassigned to a different Jovian moon. It was not until 1975 that the body finally received the name Ananke, drawn from Greek mythology, where Ananke personifies necessity and is described as the mother of the Moirai, the three Fates, by Zeus. The naming reflects a tradition of linking Jupiter's many small satellites to figures from classical lore. Nicholson's discovery was part of a broader effort to catalogue Jupiter's outer moons; he also identified Sinope, Lysithea, and Carme during the same era. The long gap between discovery and formal naming underscores how little was initially known about these distant, dim objects, and how the Jovian satellite system was still being mapped out during the mid-twentieth century.

Retrograde Orbit and the Ananke Family

Ananke traces a retrograde path around Jupiter, meaning it circles the gas giant in the opposite direction to the planet's own spin. Its mean orbital distance is roughly 21 million kilometres, and one full revolution takes about 624 days. The orbit is tilted at 149 degrees relative to the ecliptic plane and carries an eccentricity of 0.174, making it noticeably elongated rather than circular. Gravitational tugs from the Sun and neighbouring planets continuously nudge the orbit, so its exact parameters shift over time. Ananke is the largest member of the Ananke group, a family of irregular retrograde Jovian moons that share orbital distances between roughly 19 and 22 million kilometres, inclinations from 144 to 156 degrees, and eccentricities spanning 0.10 to 0.30. Scientists believe the entire group originated from a single larger body—an asteroid or possibly a trans-Neptunian object—that was gravitationally captured by Jupiter and subsequently shattered by a collision billions of years ago, scattering debris into the orbits we observe today.

A Small, Dark World with P-Type Traits

Despite being the largest moon in its group, Ananke remains poorly characterised. Thermal infrared data from the WISE space telescope yielded an albedo of roughly 3.8 percent, from which an estimated diameter of about 29 kilometres was derived. Its mass is calculated at approximately 1.68 × 10¹⁶ kilograms, assuming a density of 1.3 grams per cubic centimetre—a value consistent with P-type asteroids. In visible light the moon appears dark, with a colour that sits at the boundary between neutral grey and a faint reddish tint (colour indices B-V = 0.90, V-R = 0.38). Its infrared spectrum closely resembles that of P-type asteroids, though a possible hint of water has been noted. The exact shape is unknown, but given its small size, an irregular, lumpy form is expected. A rotation period of roughly 8 hours and 19 minutes was reported, with a brightness variation of about 0.4 magnitudes, though a later study flagged this value as uncertain because of the brevity of the observation windows used.

Still Beyond the Reach of Close-Up Probes

As of the most recent available information, no space probe has flown close enough to Ananke to capture detailed images of its surface. It remains a distant smudge in telescopic observations, known only through indirect measurements of its brightness, thermal emission, and orbital motion. However, the European Space Agency's Jupiter Icy Moons Explorer, commonly called JUICE, has plans to include all of Jupiter's irregular moons—including Ananke—as distant observation targets during its mission. The strategy involves monitoring each moon's brightness as it rotates, allowing scientists to infer rotation periods and approximate shapes without requiring a close flyby. This approach is particularly valuable for small, far-flung satellites where a dedicated close-up pass would be impractical. For Ananke specifically, such observations could help confirm or refine the uncertain rotation period and provide the first meaningful constraints on its three-dimensional form, adding to a physical picture that has remained largely incomplete since its discovery in 1951.