Pasiphae group

The Pasiphae group is a collection of retrograde irregular moons of Jupiter, taking its name from its largest member, Pasiphae. The leading theory is that it originated as a captured asteroid that was later broken apart by a collision, forming a collisional family.

However, the moons' orbits are spread out quite widely, and those with measured colors show a range of different hues. This makes both the group's identification and the idea of a single common origin subjects of debate. The International Astronomical Union assigns names ending in "-e" to all of Jupiter's retrograde moons, which includes every member of this group.

Characteristics and origin

The original captured asteroid is thought to have been about 60 kilometers across, roughly the size of Pasiphae itself, which still holds about 99% of the original body's mass. If Sinope is included in the group, that figure drops to 87%. The moons' average distance from Jupiter is around 23.4 million kilometers, similar to the Carme group, with an average orbital inclination of about 148° and an average eccentricity of roughly 0.34. The wide spread in their orbits is likely caused by gravitational scattering from mean-motion resonances with Jupiter's orbit around the Sun, rather than being random.

Pasiphae and Sinope are strongly affected by secular resonances, and they are also two of Jupiter's largest irregular moons. This has led to the suggestion that their orbits were shaped by a size-dependent dissipative process, such as gas drag, which acted quickly on smaller objects, letting them pass through resonances, but slowly on larger ones, trapping them. Later research found that Cyllene, Jupiter LVI, and Hegemone are also close to such secular resonances.

Unlike the Carme and Ananke groups, the idea of a single impact origin for the Pasiphae group is not universally accepted. The group is more spread out in inclination than those others, even though its semi-major axes are similar. Some researchers suggest Sinope might have been captured independently, rather than being part of the same collision. The color differences among the moons—Pasiphae is grey, while Callirrhoe and Megaclite are light red—also hint at a more complex origin than a single smash-up.