Himalia group

The Himalia group (or family or cluster; also referred to as the 28° inclination cluster or simply the prograde group) is a group of prograde irregular satellites of Jupiter, named after its largest member, Himalia. The group is thought to have formed from the fragmentation of a captured asteroid that was involved in a collision, making them a collisional family. Though they follow generally similar orbits, and the moons with measured colours appear compatible with a common origin, the dispersion of their orbital elements is too large to be conventionally explained, suggesting post-formation scattering of the satellites or a particular set of circumstances of their collisional event.

History and discovery

For most of the late 20th century, there were only eight known irregular satellites orbiting Jupiter, half of them prograde (Himalia, Elara, Lysithea, and Leda) and half of them retrograde (Pasiphae, Carme, Sinope, and Ananke). These eight are sometimes referred to as the "classical" irregular satellites of Jupiter. It was thought that the progrades and retrogrades each formed a collisional family, from two different parent objects. Some even suggested that all eight satellites were created together from a single collisional event (albeit still from two parent bodies). These proposals were hard to support and were replaced by alternative theories as new moons were discovered. While the retrograde moons were eventually determined to be composed of several different families, the concept of the prograde cluster remained intact and developed into what is known today as the Himalia group (among other names), though there are other prograde irregular moons now discovered that do not belong to the group. The International Astronomical Union (IAU) reserved names ending in -a (Leda, Himalia and so on) to indicate moons that orbit in prograde motion relative to Jupiter, their gravitationally central object. This later shifted to only apply to members of the Himalia group; other prograde moons with higher inclinations (presumably unrelated to the group) now receive names ending in -o. Seven moons of the family have names at present. Two possible satellites originally sighted by Sheppard in 2017 were identified to be likely part of the Himalia group, but were too faint (mag >24) to be tracked and confirmed as satellites. They were later officially reported as S/2011 J 3 and S/2018 J 2 in 2023, and confirmed to be part of the group.

Characteristics

Physical characteristics In physical appearance, the group is very homogeneous, all "classical" satellites displaying neutral colours (colour indices B−V = 0.66 and V−R = 0.36) similar to those of C-type asteroids. Given the evident clustering of the orbital parameters and the spectral homogeneity, it has been suggested that the group could be a remnant of the break-up of an asteroid from the outer part of the main asteroid belt. The radius of the parent asteroid was probably about 89 km, only slightly larger than that of Himalia, which retains approximately 87% of the mass of the original body. This indicates the asteroid was not heavily disturbed. The spectral characteristics of Himalia, Elara, and Lysithea (the three largest moons) are consistent with different levels of aqueous alteration, suggesting the progenitor asteroid may have been midway through this process when it fragmented. In this case, the progenitor may have been 300 km in diameter, with aqueous alteration occurring in the interior of the object. Himalia would have been the core of the parent body, Elara would be a piece of the transition area between the core and an overlying layer, and Lysithea would represent material near or at the surface. From studying its size distribution, it has been inferred the Himalia group probably formed from a moon–moon collision. This is unlike some other satellite families, which are thought to formed from an impact with a passing planetesimal, early in the Solar System's formation. The parent body split into relatively large fragments proportionally; only 78% of the parent body's mass stayed with the largest fragment. Combined with the large assumed original diameter of 150 km, this requires a relatively energetic impact, with an impactor of diameter ~13 km. Planetesimals large enough to meet this threshold were probably uncommon. However, moon–moon collisions, with both retrograde and other prograde satellites, were historically a frequent occurrence for objects in the Himalia group's current position, suggesting the family likely formed in this manner.

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