Galatea (moon)

For the asteroid, see 74 Galatea.

Galatea

Galatea as seen by Voyager 2 (elongation is due to smearing)
Discovery
Discovered by	Stephen P. Synnott[1] and Voyager Imaging Team
Discovery date	July 1989
Designations
Designation	Neptune VI
Pronunciation	/ɡæləˈtiːə/[2]
Named after	Γαλάτεια Galateia
Adjectives	Galatean[3]
Orbital characteristics[4][5]
Epoch 18 August 1989
Semi-major axis	61952.57 km
Eccentricity	0.00022 ± 0.00008
Orbital period (sidereal)	0.42874431 ± 0.00000001 d
Inclination	0.052 ± 0.011° (to Neptune equator) 0.06° (to local Laplace plane)
Satellite of	Neptune
Group	ring shepherd
Physical characteristics
Dimensions	(204±10) × (184±16) × (144±8) km[6]
Mean radius	88±4 km[6]
Volume	~2.8×10^6 km^3[a]
Mass	1.94×10^18 kg[8]
Mean density	~0.69 g/cm^3[b]
Surface gravity	~0.012–0.025 m/s^2[c]
Escape velocity	~0.05–0.06 km/s[d]
Synodic rotation period	synchronous
Axial tilt	zero
Albedo	0.08[6][9]
Temperature	~51 K mean (estimate)
Apparent magnitude	21.9[9]

Galatea /ɡæləˈtiːə/, also known as Neptune VI, is the fourth-closest inner moon of Neptune, and fifth-largest moon of Neptune. It is named after Galatea, one of the fifty Nereids of Greek legend, with whom Cyclops Polyphemus was vainly in love.

Discovery

Galatea inside of a faint ring arc near Neptune

Galatea was discovered in late July 1989 from the images taken by the Voyager 2 probe. It was given the temporary designation S/1989 N 4.^[10] The discovery was announced (IAUC 4824) on 2 August 1989, and mentions "10 frames taken over 5 days", implying a discovery date of sometime before July 28. The name was given on 16 September 1991.^[11]

Physical properties

Galatea is irregularly shaped and shows no sign of any geological modification. It is likely that it is a rubble pile re-accreted from fragments of Neptune's original satellites, which were smashed up by perturbations from Triton soon after that moon's capture into a very eccentric initial orbit.^[12]

Compositionally, Galatea appears to be similar to other small inner Neptunian satellites, with a deep 3.0 micron feature attributed to water ice or hydrated silicate minerals. It has a 0.08 albedo at 1.4 and 2.0 microns, dropping to 0.04 at 3.0 microns, and increasing to 0.12 at 4.6 microns.^[13]

Orbit

Galatea's orbit lies below Neptune's synchronous orbit radius, so it is slowly spiralling inward due to tidal deceleration and may eventually impact the planet or break up into a new planetary ring system upon passing its Roche limit due to tidal stretching.

Galatea appears to be a shepherd moon for the Adams ring that is 1,000 kilometres (620 mi) outside its orbit. Resonances with Galatea in the ratio 42:43 are also considered the most likely mechanism for confining the unique ring arcs that exist in this ring.^[14] Galatea's mass has been estimated based on the radial perturbations it induces on the ring.^[15][7]

Notes

1. Volume derived from the long axis A, the medium axis B and the short axis C: ${\displaystyle {\frac {\pi }{6}}ABC}$
2. Density derived from mass m and the volume V: ${\displaystyle {\frac {m}{V}}}$
3. Surface gravity derived from the mass m, the gravitational constant G and the radius r: ${\displaystyle {\frac {Gm}{r^{2}}}}$
4. Escape velocity derived from the mass m, the gravitational constant G and the radius r: ${\displaystyle {\sqrt {\frac {2Gm}{r}}}}$

Since Galatea is irregularly shaped, the actual surface gravity and escape velocity will vary significantly between different positions on the surface.

References

1. Planet Neptune Data http://www.princeton.edu/~willman/planetary_systems/Sol/Neptune/
2. "galatea". Oxford English Dictionary (Online ed.). Oxford University Press. (Subscription or participating institution membership required.)
3. AMIA (1999), Transforming health care through informatics
4. Jacobson, R. A.; Owen, W. M. Jr. (2004). "The orbits of the inner Neptunian satellites from Voyager, Earthbased, and Hubble Space Telescope observations". Astronomical Journal. 128 (3): 1412–1417. Bibcode:2004AJ....128.1412J. doi:10.1086/423037.
5. Showalter, M. R.; de Pater, I.; Lissauer, J. J.; French, R. S. (2019). "The seventh inner moon of Neptune" (PDF). Nature. 566 (7744): 350–353. Bibcode:2019Natur.566..350S. doi:10.1038/s41586-019-0909-9. PMC 6424524. PMID 30787452.
6. Karkoschka, Erich (2003). "Sizes, shapes, and albedos of the inner satellites of Neptune". Icarus. 162 (2): 400–407. Bibcode:2003Icar..162..400K. doi:10.1016/S0019-1035(03)00002-2.
7. Madeira, Gustavo; Winter, Silvia Maria Giuliatti (2022-04-21). "Numerical analysis of processes for the formation of moonlets confining the arcs of Neptune". Monthly Notices of the Royal Astronomical Society. 513 (1): 297–309. arXiv:2204.01063. doi:10.1093/mnras/stac944. ISSN 0035-8711.
8. Giuliatti Winter et al. (2020), as cited in Madeira & Giuliatti Winter (2022).^[7]
9. "Planetary Satellite Physical Parameters". JPL (Solar System Dynamics). 2008-10-24. Archived from the original on 2013-11-01. Retrieved 2008-12-13.
10. Marsden, Brian G. (August 2, 1989). "Satellites of Neptune". IAU Circular. 4824. Retrieved 2011-10-26.
11. Marsden, Brian G. (September 16, 1991). "Satellites of Saturn and Neptune". IAU Circular. 5347. Retrieved 2011-10-26.
12. Banfield, Don; Murray, Norm (October 1992). "A dynamical history of the inner Neptunian satellites". Icarus. 99 (2): 390–401. Bibcode:1992Icar...99..390B. doi:10.1016/0019-1035(92)90155-Z.
13. Belyakov, Matthew; Davis, M. Ryleigh; Milby, Zachariah; Wong, Ian; Brown, Michael E. (2024-05-01). "JWST Spectrophotometry of the Small Satellites of Uranus and Neptune". The Planetary Science Journal. 5 (5): 119. arXiv:2404.06660. Bibcode:2024PSJ.....5..119B. doi:10.3847/PSJ/ad3d55. ISSN 2632-3338.
14. Namouni, F.; Porco, C. (2002). "The confinement of Neptune's ring arcs by the moon Galatea". Nature. 417 (6884): 45–7. Bibcode:2002Natur.417...45N. doi:10.1038/417045a. PMID 11986660. S2CID 4430322.
15. Porco, C.C. (1991). "An Explanation for Neptune's Ring Arcs". Science. 253 (5023): 995–1001. Bibcode:1991Sci...253..995P. doi:10.1126/science.253.5023.995. PMID 17775342. S2CID 742763.

External links

• Galatea Profile by NASA's Solar System Exploration
• Neptune's Known Satellites (by Scott S. Sheppard)