Actaea (moon)

Actaea
Salacia and its moon Actaea, imaged by the Keck telescope on 3 August 2010. Actaea is the fainter object to the left of Salacia.
Discovery
Discovered byKeith S. Noll, Harold F. Levison, Denise C. Stephen, William M. Grundy
Discovery date21 July 2006
Designations
Designation
Salacia I
Pronunciation/ækˈtiːə/
S/2006 (120347) 1
AdjectivesActaean /ækˈtiːən/
Orbital characteristics[1]
5700+30
−29 km
Eccentricity0.008±0.003
5.49389±0.00001 days
Inclination17.2±0.5°
108.9±1.6°
41+33
−22°
Satellite ofSalacia
Physical characteristics
Mean diameter
393±33 km[2]
Mass~4.4% of Salacia[2][a]
Mean density
~0.7 g/cm3 (assumed)[2]
5.49389±0.00001 d (synchronous)[1]
Albedo0.021±0.004[2]
Spectral type
V–I = 0.89±0.02[3]
6.850±0.053[3]

Actaea, formal designation (120347) Salacia I, is the only known moon of the classical Kuiper belt object 120347 Salacia.

History

Discovery

  • The Hubble Space Telescope, the telescope that discovered Actaea back in 2006.
    The Hubble Space Telescope, the telescope that discovered Actaea back in 2006.

Actaea was discovered on 21 July 2006 by Keith S. Noll, Harold F. Levison, Denise C. Stephen and William M. Grundy with the Hubble Space Telescope.[4]

Naming

On 18 February 2011, the moon was officially named Actaea after the Nereid of the same name. In Greek mythology, Actaea (/ækˈtiːə/; Ancient Greek: Ἀκταία, romanized: Aktaía, lit. 'the dweller on coasts',[5] from Ancient Greek: ἀκτή, romanized: akté, lit. 'seashore' or 'rocky shore') was one of the 50 Nereid sea-nymphs. These 50 sea-nymphs are daughters of the "Old Man of the Sea" Nereus and the Oceanid Doris.[6][7] Actaea and her other sisters appeared to Thetis when she cried out in sympathy for the grief of Achilles for his slain friend Patroclus.[8]

Orbit

Schematic of Actaea's orbit relative to Salacia

Orbital characteristics

Actaea follows a nearly circular orbit around Salacia at a distance of 5700+30
−29 km every 5.49389±0.00001 days, with an inclination of approximately 17.2° and a low eccentricity of 0.008±0.003.[1] The low eccentricity of Actaea's orbit suggests that the system has undergone tidal evolution since its formation.[3]

The ratio of its semi-major axis to its primary's Hill radius is 0.0023, it is the tightest trans-Neptunian binary with a known orbit.[3]

Tidal evolution

It has been calculated that the Salacia system should have undergone enough tidal evolution to circularize their orbits, which is consistent with the low measured eccentricity, but that the primary need not be tidally locked.[3] Salacia and Actaea will next occult each other in 2067.[3]

Physical characteristics

Size

Comparison of sizes, albedos, and colors of various large trans-Neptunian objects with diameters greater than 700 km (430 mi). Its parent body Salacia is shown on the middle row, third from the right, and Actaea is shown next to it. The dark colored arcs represent uncertainties of the object's size.

Actaea is approximately 2.372±0.060 magnitudes fainter than Salacia, implying a diameter ratio of 2.98 assuming equal albedos.[3] Early estimates assumed equal albedos for them, which have corresponded to a diameter of 284±10 km for Actaea.[9] Since Actaea has almost the same neutral (grey) color as Salacia that strongly supports the assumption of equal albedos.[3]

However, more accurate measurements in 2025 that was done by Kiss et al., measured an extremely low albedo of 0.021±0.004 for Actaea, which is approximately half the albedo of Salacia (0.041±0.004),[2] yielding a larger diameter of 393±33 km,[2] that is approximately one-half the diameter of Salacia; thus, Salacia and Actaea are viewed by William Grundy et al. to be a binary system.

Assuming that the following size estimates are correct, Actaea is about the fifth-biggest known moon of a trans-Neptunian object, after Charon (1212±1 km),[10] Dysnomia (615+60
−50 km),[11]: 7  Vanth (442.5±10.2 km),[12] and Ilmarë (403±40 km).[2]

Surface and spectrum

The surface of Actaea's color is dark grey,[2][3] with an extremely low geometric albedo of 0.021±0.004,[2] which is only half of Salacia's, and a measured V–I color index of 0.89±0.02, which is considered a neutral color.[3]

Mass and density

The mass of the system is 4.861+0.076
−0.074×1020 kg,[1] assuming equal densities of around ≈1.45 g/cm3, the satellite-to-primary mass ratio is would be 0:10:1.[2] However, this would be an unrealistically high density for the satellite.[2] Under the assumption that Actaea has a density of 0.7 g/cm3, which is thought to be more typical that of trans-Neptunian objects' moons,[2] The mass ratio between Actaea and Salacia will be approximately 0.044:1.[2]

See also

Notes

  1. ^ assuming that Actaea has a typical satellite density of 0.7 g/cm3[2]

References

  1. ^ a b c d Collyer, Cameron; Fernandez-Valenzuela, Estela; Jose Luis Ortiz; Holler, Bryan J.; Proudfoot, Benjamin; Morales, Nicolas; Morales, Rafael; Benecchi, Susan; Rommel, Flavia L.; Grundy, Will; Ragozzine, Darin (2025). "Synchronous Rotation in the (120347) Salacia-Actaea System". The Planetary Science Journal. 6 (11): 270. arXiv:2509.02734. Bibcode:2025PSJ.....6..270C. doi:10.3847/PSJ/ae0b6a.
  2. ^ a b c d e f g h i j k l m n Kiss, Csaba; Gabányi, Krisztina; Moór, Attila; Müller, Thomas; Fernandez-Valenzuela, Estela; Moullet, Arielle; Borkovits, Tamás; Kalup, Csilla (2025-07-09). ALMA submm measurements of the trans-Neptunian binary system satellites Ilmarë, Actaea, Hi'iaka and Namaka (Report). Copernicus Meetings.
  3. ^ a b c d e f g h i j Stansberry, J.A.; Grundy, W.M.; Mueller, M.; et al. (2012). "Physical Properties of Trans-Neptunian Binaries (120347) Salacia–Actaea and (42355) Typhon–Echidna". Icarus. 219 (2): 676–688. Bibcode:2012Icar..219..676S. CiteSeerX 10.1.1.398.6675. doi:10.1016/j.icarus.2012.03.029.
  4. ^ "IAUC 8751: (120347) 2004 SB_60; 2006gi, 2006gj; V733 Cep". Cbat.eps.harvard.edu. Archived from the original on 2013-12-03. Retrieved 2014-06-14.
  5. ^ Kerényi, Carl (1951). The Gods of the Greeks. London: Thames and Hudson. p. 64.
  6. ^ Homer, Iliad 18.41; Apollodorus, 1.2.7; Hesiod, Theogony 240
  7. ^ Bane, Theresa (2013). Encyclopedia of Fairies in World Folklore and Mythology. McFarland, Incorporated, Publishers. p. 11, 12. ISBN 978-0-7864-7111-9.
  8. ^ Homer, Iliad 18.39-51
  9. ^ Grundy, W. M.; Noll, K. S.; Roe, H. G.; Buie, M. W.; Porter, S. B.; Parker, A. H.; et al. (2019). "Mutual Orbit Orientations of Transneptunian Binaries" (PDF). Icarus. 334: 62–78. Bibcode:2019Icar..334...62G. doi:10.1016/j.icarus.2019.03.035. ISSN 0019-1035. S2CID 133585837. Retrieved 2019-10-26.
  10. ^ Stern, S.A.; Bagenal, F.; Ennico, K.; Gladstone, G.R.; Grundy, W.M.; McKinnon, W.B.; Moore, J.M.; Olkin, C.B.; Spencer, J.R. (2015-10-16). "The Pluto system: Initial results from its exploration by New Horizons". Science. 350 (6258) aad1815. arXiv:1510.07704. Bibcode:2015Sci...350.1815S. doi:10.1126/science.aad1815. PMID 26472913. S2CID 1220226.
  11. ^ Brown, Michael E.; Butler, Bryan J. (2023-10-01). "Masses and Densities of Dwarf Planet Satellites Measured with ALMA". The Planetary Science Journal. 4 (10): 193. arXiv:2307.04848. Bibcode:2023PSJ.....4..193B. doi:10.3847/PSJ/ace52a.
  12. ^ Sickafoose, A. A.; Bosh, A. S.; Levine, S. E.; Zuluaga, C. A.; Genade, A.; Schindler, K.; Lister, T. A.; Person, M. J. (2019-02-01). "A stellar occultation by Vanth, a satellite of (90482) Orcus". Icarus. 319: 657–668. arXiv:1810.08977. Bibcode:2019Icar..319..657S. doi:10.1016/j.icarus.2018.10.016. S2CID 119099266.
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