Astronomy:Himalia group
The Himalia group (or family or cluster; also referred to as the 28° inclination cluster[1][2] or simply the prograde group[1][3][4]) 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 probable collisional family. Though the moons 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).[4] It was thought that the progrades and retrogrades were their separate groups and were associated with their own collisional histories, or even that all eight satellites all shared a single collisional origin.[3] These proposals were hard to support and were replaced by alternative theories as new moons were discovered.[4] 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, though there are other prograde irregular moons now discovered that do not belong to the group.[5]
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.[6] 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.[5][6] 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.[7] They were later officially reported as S/2011 J 3 and S/2018 J 2 in 2023, and identified as part of the group.[8]
Characteristics
The Himalia family have semi-major axes (distances from Jupiter) around ~11.6 Gm and inclinations of ~28°,[6] and mean eccentricities in the range of 0.1−0.2.[9] All orbit in a prograde direction. In physical appearance, the group is very homogeneous, all 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, possibly from the Hilda group or the Nysa family.[1] 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.[lower-alpha 1] This indicates the asteroid was not heavily disturbed.[10]
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,[lower-alpha 2] 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.[2]
Origin and evolution
Numerical integrations show a high probability of collisions among the members of the prograde moons of Jupiter during the lifespan of the Solar System (e.g. on average 1.5 collisions between Himalia and Elara).[3][4] In addition, the same simulations have shown fairly high probabilities of collisions between prograde and retrograde satellites (e.g. Pasiphae and Himalia have a 27% probability of collision within 4.5 gigayears).[3] However, due to their longer orbital periods and greater distances from Jupiter, collisions are rare among the retrograde satellites, the expected number of collisions in the past 4.5 billion years among all retrograde moons combined being around 1.[3] Many of the smaller members of the group that once existed are predicted to have been removed in collisions with Himalia itself over the past 4 billion years.[11][12][3]
Consequently, it has been suggested that the current Himalia group could be a result of a more recent, rich collisional history among the prograde and retrograde satellites as opposed to the single break-up shortly after the planet formation that has been inferred for the Carme and Ananke groups.[4] Satellite-to-satellite interactions are unlikely to be common enough to have formed the Carme and Ananke groups, suggesting that their progenitors may have collided with passing planetesimals instead, early in the Solar System's formation. On the other hand, the parent asteroid for the Himalia group split into much more massive fragments proportionally, so there was unlikely to be impact by a planetesimal large enough. The Himalia group may have formed more recently from a collision with another irregular moon instead.[4]
Gas drag from Jupiter's primordial circumplanetary disk has been proposed as a mechanism that could have caused the capture of the progenitor asteroid.[9]
While the membership of the Himalia group is well-defined, the moons are relatively widely spread in their orbital elements,[9] more so than collisional dispersion can conventionally explain. More specifically, there is significant spread in semi-major axis and eccentricity, but the inclination is narrowly distributed.[3] This may have been caused by the parent asteroid colliding in a specific way, either colliding while far away from an orbital node, or being hit from a direction roughly coplanar with its orbit.[3]
Alternatively, the group may have been scattered in some way after the collision. Gravitational interactions such as secular resonances between Himalia and the other moons over time could be a factor, but while it is the most dominant of all potential perturbation sources, it is not enough by itself to explain all of the scattering.[12] There may have been secondary collision events later in the group's history that contributed to the dispersion seen today.[7] Another option is that the parent asteroid may have been already captured and the collisional family already created while planetary migration was ongoing, and gravitational interactions between Jupiter and other planet-sized objects may have displaced their orbits.[11]
List
The known members of the group are (in order of date announcement):
| Name | Diameter (km) |
Semi-Major Axis (km) |
Period (days) |
Notes |
|---|---|---|---|---|
| Himalia | 139.6 (150 × 120) |
11439000 | 249.91 | largest member and group prototype |
| Elara | 79.9 | 11710700 | 258.89 | |
| Lysithea | 42.2 | 11699100 | 258.50 | |
| Leda | 21.5 | 11145200 | 240.33 | |
| Dia | 4 | 12257900 | 277.25 | |
| Pandia | 3 | 11479600 | 251.23 | |
| Ersa | 3 | 11399400 | 248.62 | |
| S/2018 J 2 | 3 | 11419700 | 249.28 | |
| S/2011 J 3 | 3 | 11716800 | 259.09 | |
| S/2011 J 4 | 3 | 11104600 | 239.05 | |
| S/2017 J 17 | 1 | 11776100 | 261.07 |
Notes
References
- ↑ 1.0 1.1 1.2 Grav, Tommy; Holman, Matthew J.; Gladman, Brett; Aksnes, Kaare (November 2003). "Photometric Survey of the Irregular Satellites" (in en). Icarus 166: 33-45. doi:10.1016/j.icarus.2003.07.005. Bibcode: 2003Icar..166...33G.
- ↑ 2.0 2.1 2.2 Vilas, Faith; Hendrix, Amanda R. (2024-02-01). "Clues to the Origin of Jovian Outer Irregular Satellites from Reflectance Spectra". The Planetary Science Journal 5 (2): 34. doi:10.3847/PSJ/ad150b. ISSN 2632-3338. Bibcode: 2024PSJ.....5...34V. https://iopscience.iop.org/article/10.3847/PSJ/ad150b.
- ↑ 3.0 3.1 3.2 3.3 3.4 3.5 3.6 3.7 Nesvorný, David; Alvarellos, Jose L. A.; Dones, Luke; Levison, Harold F. (July 2003). "Orbital and Collisional Evolution of the Irregular Satellites" (in en). The Astronomical Journal 126 (1): 398–429. doi:10.1086/375461. ISSN 0004-6256. Bibcode: 2003AJ....126..398N.
- ↑ 4.0 4.1 4.2 4.3 4.4 4.5 Nesvorný, David; Beaugé, Cristian; Dones, Luke (March 2004). "Collisional Origin of Families of Irregular Satellites" (in en). The Astronomical Journal 127 (3): 1768–1783. doi:10.1086/382099. ISSN 0004-6256. Bibcode: 2004AJ....127.1768N.
- ↑ 5.0 5.1 Nicholson, Philip D.; Ćuk, Matija; Sheppard, Scott S.; Nesvorný, David; Johnson, Torrence V. (May 8, 2008). "Irregular Satellites of the Giant Planets". in Barucci, M. Antonietta. The Solar System Beyond Neptune. The University of Arizona Space Science. University of Arizona Press. ISBN 978-0816527557. Bibcode: 2008ssbn.book..411N. https://web.gps.caltech.edu/~mbrown/out/kbbook/Chapters/Nicholson_IrregSat.pdf.
- ↑ 6.0 6.1 6.2 Denk, TilmannExpression error: Unrecognized word "etal". (5 March 2026). "Io and the Minor Jovian Moons – Prospects for JUICE". Space Science Reviews 222 (2). doi:10.1007/s11214-025-01263-6. Bibcode: 2026SSRv..222...27D.
- ↑ 7.0 7.1 Sheppard, ScottExpression error: Unrecognized word "etal". (August 2018). "New Jupiter Satellites and Moon-Moon Collisions". Research Notes of the American Astronomical Society 2 (3): 155. doi:10.3847/2515-5172/aadd15. 155. Bibcode: 2018RNAAS...2..155S.
- ↑ Sheppard, Scott S.; Tholen, David J.; Alexandersen, Mike; Trujillo, Chadwick A. (2023-05-24). "New Jupiter and Saturn Satellites Reveal New Moon Dynamical Families". Research Notes of the AAS 7 (5): 100. doi:10.3847/2515-5172/acd766. ISSN 2515-5172. Bibcode: 2023RNAAS...7..100S. https://iopscience.iop.org/article/10.3847/2515-5172/acd766.
- ↑ 9.0 9.1 9.2 Ćuk, Matija; Burns, Joseph A. (February 2004). "Gas-drag-assisted capture of Himalia's family" (in en). Icarus 167 (2): 369–381. doi:10.1016/j.icarus.2003.09.026. Bibcode: 2004Icar..167..369C.
- ↑ 10.0 10.1 Sheppard, Scott S.; Jewitt, David C. (May 5, 2003). "An abundant population of small irregular satellites around Jupiter". Nature 423 (6937): 261–263. doi:10.1038/nature01584. PMID 12748634. Bibcode: 2003Natur.423..261S. http://www.ifa.hawaii.edu/~jewitt/papers/JSATS/SJ2003.pdf.
- ↑ 11.0 11.1 Li 李, Daohai 道海; Christou, Apostolos A. (2017-11-01). "Orbital Modification of the Himalia Family during an Early Solar System Dynamical Instability". The Astronomical Journal 154 (5): 209. doi:10.3847/1538-3881/aa8fc9. ISSN 0004-6256.
- ↑ 12.0 12.1 Li, Daohai; Christou, Apostolos A. (2018-08-01). "Long-term self-modification of irregular satellite groups". Icarus 310: 77–88. doi:10.1016/j.icarus.2017.12.004. ISSN 0019-1035.
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