Iocaste (Jupiter XXIV) — a small irregular moon of Jupiter

Physical characteristics

The body is small and non-spherical, implying a shape governed by material strength rather than self-gravity; this is common among minor irregular moons. Photometric and color measurements show a neutral to slightly grey spectrum similar to carbonaceous objects. Such characteristics are often compared to C-type asteroids, which are dark and carbon-rich. Physical knowledge of Iocaste — rotation period, detailed composition and surface geology — remains limited because of its diminutive size and distance from Earth.

Orbit and dynamical group

Iocaste orbits far from Jupiter at an average distance near 20.7 million kilometres and completes one orbit in about 609 days. Its path is strongly inclined and retrograde relative to Jupiter’s equator, with an inclination around 147° to the ecliptic (roughly 146° to Jupiter’s equator). The orbit is noticeably elliptical, with an eccentricity of about 0.2874. These orbital properties place Iocaste within the Ananke group, a cluster of retrograde, similarly distant moons that share comparable inclinations and eccentricities and are thought to form a collisional family.

Discovery and name

Iocaste was discovered in 2000 by a team from the University of Hawaii led by astronomer Scott S. Sheppard during ground-based searches for faint Jovian satellites; it was provisionally designated S/2000 J 3 at the time of discovery. In October 2002 the satellite received the name Iocaste, drawn from classical myth: she is the figure Jocasta, known from Greek legend as the mother and later wife of Oedipus. For more on the mythic source see Jocasta in Greek mythology.

Origin and significance

Like other members of the Ananke group, Iocaste is believed to be a fragment of a larger captured asteroid that underwent a collisional breakup after becoming bound to Jupiter. This capture-and-fragmentation scenario explains the shared orbital elements and similar spectral characteristics across the group. Studying such small irregular moons helps astronomers reconstruct the collisional history of the Jovian system and provides constraints on the processes of capture, disruption, and long-term dynamical evolution of satellites around giant planets.

Observational notes and notable facts

• Iocaste is faint and requires large ground-based telescopes and sensitive detectors to measure its motion and brightness.
• Its classification as an irregular, retrograde satellite distinguishes it from Jupiter’s larger, regular moons (such as the Galilean satellites) that orbit much closer and in the planet’s equatorial plane.
• Because of its size and distance, detailed physical studies (surface mapping, spectroscopy at high signal-to-noise) are challenging; most information comes from orbital tracking and broadband photometry.
• References and additional data are recorded in astronomical catalogs and survey publications; related resources can be explored via general astronomical databases and mission archives (satellite catalogues, observatory releases, researcher pages, discovery announcements, orbital element listings, eccentricity summaries, nomenclature histories, mythology notes, group descriptions, spectral comparisons).

Because Iocaste is representative of a collisional family rather than an isolated formation, it remains a useful object for studies that aim to link small body populations, capture processes, and the long-term dynamics of satellites in the outer Solar System.