Wikipedia

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 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.

This diagram compares the orbital elements and relative sizes of the known members of the Himalia group as of April 2026. The horizontal axis illustrates their average distance from Jupiter, the vertical axis their orbital inclination, and the circles their relative sizes.

History and discovery

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107 irregular moons of Jupiter plotted by semi-major axis and inclination as of April 2026. The Himalia group is shown as a tight cluster of blue-colored points on the right.

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] These eight are sometimes referred to as the "classical" irregular satellites of Jupiter.[5] 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.[6]

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.[7] 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.[6][7] 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.[8] They were later officially reported as S/2011 J 3 and S/2018 J 2 in 2023, and identified as part of the group.[9]

Characteristics

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Physical characteristics

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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.[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.[a] 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,[b] 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]

Orbital characteristics

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The Himalia family have semi-major axes (distances from Jupiter) around ~11.6 Gm and inclinations of ~28°,[7] and eccentricities in the range of 0.11−0.24. All orbit in a prograde direction. The orbital characteristics of the group, along with Himalia's large size, make them distinct from the other irregular satellites, and are indicative of a past evolution by gas drag from Jupiter's primordial circumplanetary disk.[11] The group orbits at closer distances, and have orbital eccentricities on the low end compared to other irregular moons. Their inclinations are also the lowest of the progrades. A study simulated the gas drag capture scenario for the parent asteroid and found that it was plausible.[11]

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.[12][13][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] Moon-moon impacts 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, indicating a more energetic impact, so there was unlikely to be impact by a planetesimal large enough. The Himalia group may have formed more recently from a moon-moon collision instead.[4]

While the membership of the Himalia group is well-defined,[11] the moons are relatively widely spread in their orbital elements, 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.[13] There may have been secondary collision events later in the group's history that contributed to the dispersion seen today.[8] 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.[12]

Origin

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One study concluded that the progenitor most likely originated from the Hilda group.[11] The progenitor asteroid probably started off in heliocentric orbit with a distance about 3.8–4.2 AU. This region is devoid of asteroids except those safe in orbital resonance with Jupiter, therefore the progenitor may have been a Hilda group member, before having its orbit destabilized in some way and captured by Jupiter. The Hilda group generally have semi-major axes about 4 AU, occupying the 3:2 resonance with Jupiter, but is dominated by P/D-type asteroids, as opposed to the Himalia group's C-type. However, the Hilda group exhibits an empirical size-dependent trend in spectral slope, causing the largest members (153 Hilda and 334 Chicago) to be classed as potentially C-type, while most other larger members fall into P-type and smaller members tend to be D-type.[1][11] Extrapolating this trend for an object the size of the Himalia group's progenitor (larger than both of the previously mentioned asteroids) places it squarely in the C-type, making it compatible with a Hilda group origin.[11]

It has also been proposed that the Himalia group originated from the Nysa asteroid family.[14] Himalia's spectrum was found to have possible signs of phyllosilicates, and it was classed as an F-type asteroid, a subclass of C-type.[14][15] As many known F-types are part of the Nysa family, the authors speculated that it too, may have originated there. As the family exists near the 3:1 Kirkwood gap (around 2.5 AU), this would have provided a dynamical pathway for the progenitor to move on a Jupiter-crossing orbit. The Nysa family is also older in age, providing time for a fragment of the collision that created the Nysa family to then be captured by Jupiter while its protoplanetary envelope still existed.[14] However, a later study asserted that the 3:1 Kirkwood gap was unlikely to be able to kick an object out to Jupiter's orbit.[16] Furthermore, Himalia and Elara are as big or bigger than the largest asteroids in the Nysa family, suggesting they are not related.[16] Another study supported the Nysa family origin, based on their modelling which was compatible with phyllosilicates.[15]

While multiple studies have supported the hypothesis that the Himalia family originates from the main asteroid belt,[15] the progenitor may have alternatively come from the same source as the Jupiter trojans, which are usually assumed to have formed in the primordial Kuiper belt before being scattered inward by planetary migration.[17] However, the presence of ammoniated materials on Himalia and the lack of it on Jupiter trojans may preclude a relationship between them, though Lysithea exhibits trojan-like material. The Himalia group and the trojans may still have a shared origin if the unseen core of a trojan body is characteristic of Himalia.[17]

List

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The known members of the group are (in order of date announcement):

NameDiameter
(km)		Semi-Major Axis
(km)		Period
(days)		Notes
Himalia139.6
(150 × 120)		11439000249.91largest member and group prototype
Elara79.911710700258.89
Lysithea42.211699100258.50
Leda21.511145200240.33
Dia412257900277.25
Pandia311479600251.23
Ersa311399400248.62
S/2018 J 2311419700249.28
S/2011 J 3311716800259.09
S/2011 J 4311104600239.05
S/2017 J 17111776100261.07

Notes

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  1. ↑ Obtained simply by summing the estimated volumes of the moons in the Himalia group discovered at the time.[10]
  2. ↑ Obtained simply by summing the estimated diameters of the three moons.[2]

References

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  1. 1 2 3 4 Grav, Tommy; Holman, Matthew J.; Gladman, Brett; Aksnes, Kaare (November 2003). "Photometric Survey of the Irregular Satellites". Icarus. 166: 33–45. arXiv:astro-ph/0301016. Bibcode:2003Icar..166...33G. doi:10.1016/j.icarus.2003.07.005.
  2. 1 2 3 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. Bibcode:2024PSJ.....5...34V. doi:10.3847/PSJ/ad150b. ISSN 2632-3338. S2CID 267531422.
  3. 1 2 3 4 5 6 7 8 Nesvorný, David; Alvarellos, Jose L. A.; Dones, Luke; Levison, Harold F. (July 2003). "Orbital and Collisional Evolution of the Irregular Satellites". The Astronomical Journal. 126 (1): 398–429. Bibcode:2003AJ....126..398N. doi:10.1086/375461. ISSN 0004-6256.
  4. 1 2 3 4 5 6 Nesvorný, David; Beaugé, Cristian; Dones, Luke (March 2004). "Collisional Origin of Families of Irregular Satellites". The Astronomical Journal. 127 (3): 1768–1783. Bibcode:2004AJ....127.1768N. doi:10.1086/382099. ISSN 0004-6256.
  5. ↑ Grav, T.; Bauer, J. M.; Mainzer, A. K.; Masiero, J. R.; Nugent, C. R.; Cutri, R. M.; et al. (August 2015). "NEOWISE: Observations of the Irregular Satellites of Jupiter and Saturn". The Astrophysical Journal. 809 (1): 9. arXiv:1505.07820. Bibcode:2015ApJ...809....3G. doi:10.1088/0004-637X/809/1/3. S2CID 5834661. 3.
  6. 1 2 Nicholson, Philip D.; Ćuk, Matija; Sheppard, Scott S.; Nesvorný, David; Johnson, Torrence V. (May 8, 2008). "Irregular Satellites of the Giant Planets" (PDF). In Barucci, M. Antonietta; Boehnhardt, Hermann; Cruikshank, Dale P.; Morbidelli, Alessandro (eds.). The Solar System Beyond Neptune. The University of Arizona Space Science. University of Arizona Press. Bibcode:2008ssbn.book..411N. ISBN 978-0816527557. Archived (PDF) from the original on 2025-11-14.
  7. 1 2 3 Denk, Tilmann; Williams, David A.; Tosi, Federico; Bell III, James F.; Mottola, Stefano; de Pater, Imke; et al. (5 March 2026). "Io and the Minor Jovian Moons – Prospects for JUICE". Space Science Reviews. 222 (2) 27. Bibcode:2026SSRv..222...27D. doi:10.1007/s11214-025-01263-6.
  8. 1 2 Sheppard, Scott; Williams, Gareth; Tholen, David; Trujillo, Chadwick; Brozovic, Marina; Thirouin, Audrey; et al. (August 2018). "New Jupiter Satellites and Moon-Moon Collisions". Research Notes of the American Astronomical Society. 2 (3): 155. arXiv:1809.00700. Bibcode:2018RNAAS...2..155S. doi:10.3847/2515-5172/aadd15. S2CID 55052745. 155.
  9. ↑ 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. Bibcode:2023RNAAS...7..100S. doi:10.3847/2515-5172/acd766. ISSN 2515-5172.
  10. 1 2 Sheppard, Scott S.; Jewitt, David C. (May 5, 2003). "An abundant population of small irregular satellites around Jupiter" (PDF). Nature. 423 (6937): 261–263. Bibcode:2003Natur.423..261S. doi:10.1038/nature01584. PMID 12748634. S2CID 4424447. Archived from the original (PDF) on 2006-08-13.
  11. 1 2 3 4 5 6 Ćuk, Matija; Burns, Joseph A. (February 2004). "Gas-drag-assisted capture of Himalia's family". Icarus. 167 (2): 369–381. Bibcode:2004Icar..167..369C. doi:10.1016/j.icarus.2003.09.026.
  12. 1 2 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.
  13. 1 2 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.
  14. 1 2 3 Jarvis, Kandy S.; Vilas, Faith; Larson, Stephen M.; Gaffey, Michael J. (June 2000). "JVI Himalia: New Compositional Evidence and Interpretations for the Origin of Jupiter's Small Satellites". Icarus. 145 (2): 445–453. Bibcode:2000Icar..145..445J. doi:10.1006/icar.2000.6344.
  15. 1 2 3 Bhatt, M.; Reddy, V.; Schindler, K.; Cloutis, E.; Bhardwaj, A.; Corre, L. L.; Mann, P. (December 2017). "Composition of Jupiter irregular satellites sheds light on their origin". Astronomy & Astrophysics. 608: A67. arXiv:1710.06200. Bibcode:2017A&A...608A..67B. doi:10.1051/0004-6361/201630361. ISSN 0004-6361. S2CID 73594513.
  16. 1 2 Vilas, Faith; Lederer, Susan M.; Gill, Sara L.; Jarvis, Kandy S.; Thomas-Osip, Joanna E. (February 2006). "Aqueous alteration affecting the irregular outer planets satellites: Evidence from spectral reflectance". Icarus. 180 (2): 453–463. Bibcode:2006Icar..180..453V. doi:10.1016/j.icarus.2005.10.004.
  17. 1 2 Sharkey, Benjamin N. L.; Rivkin, Andrew S.; Cartwright, Richard J.; Holler, Bryan J.; Emery, Joshua P.; Thomas, Cristina (2025-10-01). "JWST Reveals Varied Origins between Jupiter's Irregular Satellites". The Planetary Science Journal. 6 (10): 242. Bibcode:2025PSJ.....6..242S. doi:10.3847/PSJ/ae04dd. ISSN 2632-3338.
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