Pasiphae group

Lore & Background

The Pasiphae group is believed to have originated when Jupiter captured an asteroid that later broke up after a collision. The original body is calculated to have been 60 km in diameter, about the same size as Pasiphae, which retains 99% of the original body's mass. If Sinope belongs to the group, the ratio is much smaller at 87%. The moons' orbital elements show wide variation, with semi-major axes averaging around 23.4 million km, inclinations around 148°, and eccentricities around 0.34. The general position of the group in orbital element space is subject to gravitational scattering by mean-motion resonances with Jupiter's orbital period, suggesting these resonances explain the orbital dispersion rather than happenstance.

Pasiphae and Sinope are significantly affected by secular resonances and are two of the largest irregular moons of Jupiter, which suggests their orbits were affected in the past by some dissipative process such as gas drag. This process acted fast enough for smaller objects to bypass the resonances, but slow enough for larger objects to become captured. A later study found that other moons—Cyllene, Jupiter LVI, and Hegemone—were also close to secular resonance. Unlike the Carme and Ananke groups, the theory of a single impact origin for the Pasiphae group is not accepted by all studies, partly because the group is more widely dispersed in inclination. Differences in color class between objects (grey for Pasiphae, light red for Callirrhoe and Megaclite) also suggest a more complex origin than a single collision.

Typically the Pasiphae group is visually identified in orbital element space, distinct from the Carme group by inclinations and from the Ananke group by semi-major axes. At other times only a handful of moons are selected to be grouped with Pasiphae (Megaclite, S/2003 J 4, and Cyllene) from calculations of orbital dispersion. Sometimes no distinction is made between the Pasiphae and Ananke groups, and they may be considered a single group. Sinope, due to its large size and relatively distinct inclination, is sometimes considered its own category, along with the small moon Aoede, and has dissimilar spectral features from Pasiphae, further supporting a different origin.

Reader's Guide

The Pasiphae group holds significance as one of the major retrograde irregular satellite families of Jupiter, yet its status as a true collisional family remains open to discussion. The article notes that while the moons follow somewhat similar orbits, their orbital dispersion is very large, and measured colors show significant diversity, making identification of the group and a common origin uncertain. This contrasts with the Carme and Ananke groups, where a single impact origin is more widely accepted. The group's legacy lies in illustrating the complexity of satellite system formation: the possibility that Sinope was captured independently, the role of secular resonances and gas drag in shaping orbits, and the color differences among members all point to a potentially multi-stage or multi-source origin. The International Astronomical Union reserves names ending in -e for all retrograde moons of Jupiter, including those in this group. The ongoing debate about the group's boundaries—sometimes including only a handful of moons, sometimes merging with the Ananke group—underscores how dynamic the classification of irregular satellites remains as new data emerge.

Did You Know?

• The Pasiphae group is named after its largest member, Pasiphae, which retains 99% of the original captured asteroid's mass.
• The original asteroid that formed the group is calculated to have been 60 km in diameter.
• The moons in the group have an average inclination of about 148° and an average semi-major axis of 23.4 million km.
• Sinope is sometimes considered separate from the group due to its size, distinct inclination, and dissimilar spectral features from Pasiphae.