Sao (moon)

Sao (), also known as Neptune XI and previously as S/2002 N 2, is an irregular moon of Neptune. It was discovered on 14 August 2002 by Tommy Grav, Matthew Holman, JJ Kavelaars, Wesley Fraser, and Dan Milisavljevic at Cerro Tololo Inter-American Observatory in Chile. Named after one of the Nereids from Greek mythology, Sao follows a distant, highly eccentric, and highly inclined orbit around Neptune with an average orbital period of about 8 Earth years. Its orbit is prograde, meaning it revolves around Neptune in the same direction as the planet's orbit around the Sun.

Due to Sao's great distance from Neptune, its orbit is significantly influenced by the Sun's gravity. As a result, the moon experiences the Kozai resonance, which causes large, synchronized variations in its orbital eccentricity and inclination over a 3,000-year period. Sao shares similar orbital characteristics with two other Neptunian irregular moons, Laomedeia and S/2002 N 5, which suggests they originated from the collisional breakup of a once-larger moon of Neptune. Sao is estimated to have a diameter between , though most of its physical properties are unknown.

Discovery Sao was discovered by astronomers Tommy Grav, Matthew Holman, JJ Kavelaars, Wesley Fraser, and Dan Milisavljevic during a search for distant moons of Neptune. Led by Holman, the search began in 2001 and employed large optical telescopes on Earth to obtain numerous long-exposure images of the sky around Neptune. The team used the shift-and-add technique to align and combine the images according to Neptune's motion across the sky, which enhanced the faint moons as points of light. Through this technique, they discovered Sao alongside two other Neptunian moons—Halimede and Neso—in images taken on 14 August 2002 by the 4-meter Víctor M. Blanco Telescope at Cerro Tololo Inter-American Observatory, Chile. Sao was among the first Neptunian moons discovered from a ground-based telescope since Nereid in 1949.

To determine Sao's orbit around Neptune, astronomers conducted follow-up observations at several observatories, with the help of orbit predictions by Brian G. Marsden and Robert A. Jacobson. A team led by Brett Gladman observed Sao on 3 and 5 September 2002 with the 8.2-m Very Large Telescope at Cerro Paranal Observatory, while other teams observed it with the 2.5-m Nordic Optical Telescope at La Palma Observatory and the 5-m Hale Telescope at Palomar Observatory. Earlier observations of Sao were found by Holman in Canada–France–Hawaii Telescope images from 23 July 2001. The discovery of Sao, alongside the Neptunian moons Halimede and Laomedeia, was announced by the Minor Planet Center on 13 January 2003. The announcement raised Neptune's number of known moons to 11.

Name When the discovery of Sao was announced, it was given the temporary provisional designation "S/2002 N 2" by the Minor Planet Center. It was later named and given the Roman numeral designation Neptune XI by the International Astronomical Union's (IAU's) Working Group for Planetary System Nomenclature on 3 February 2007. In accordance with the IAU's naming convention for Neptunian irregular moons, it was named after Sao (Σαώ), one of the fifty Nereids or daughters of Nereus and Doris from Greek mythology.

Orbit Sao is an irregular moon of Neptune, meaning it follows a distant, highly eccentric, and highly inclined orbit around the planet. Sao's orbit is prograde, meaning it revolves around Neptune in the same direction as the planet's orbit around the Sun. Due to its great distance from Neptune, the moon is strongly affected by the Sun's gravity, which causes substantial long-term variations in its orbit. For this reason, Sao's orbit is better described by proper orbital elements, which are calculated by averaging out its perturbed orbit over an extended period of time.

Over a 10,000-year time span, Sao's semi-major axis from Neptune varies from , averaging about . On average, Sao takes about to complete one orbit around Neptune, though perturbations by the Sun can vary the orbital period from . While Sao's orbit has an average eccentricity of 0.30 and an average inclination of 50° with respect to the ecliptic, both values vary substantially due to the Sun's perturbations. The moon's eccentricity ranges from 0.07 to 0.63, while its inclination ranges from 39° to 55°. Among Neptune's known prograde moons, Sao has the highest average inclination.

Because of its varying eccentricity, Sao can occasionally pass close to Nereid, one of Neptune's largest irregular moons. Over a period of 4.5 billion years, these close encounters could result in gravitational scattering, or even a 1.6% chance of collision. A 2022 study led by Jet Propulsion Laboratory astrophysicists Marina Brozović and Robert A. Jacobson found that Sao and Laomedeia can approach within of each other during a 30,000-year time span.

Kozai resonance

Sao's high inclination places it in the Kozai resonance, a dynamical phenomenon in which perturbations from the Sun cause periodic exchanges between orbital eccentricity and inclination. First recognized by Holman and colleagues in 2004, the Kozai resonance causes Sao's eccentricity and inclination to oscillate out of phase over a period of about 3,000 years, with eccentricity reaching maximum at minimum inclination and vice versa. While Sao's orbit exhibits nodal precession with a period of 4,454 years, the Kozai resonance prevents it from apsidally precessing. Instead of circulating from 0° to 360°, Sao's argument of pericenter librates around 90° with a period of about 3,000 years. Sao is the only known prograde irregular moon of Neptune that is in the Kozai resonance.

Group and origin

Sao shares similar orbital characteristics with two other Neptunian irregular moons, Laomedeia and S/2002 N 5. Together, they form the Sao group, a family of prograde irregular moons of which Sao is the largest member. The Sao group was first suspected by astronomers in 2004 based on the similar orbits of Sao and Laomedeia, but it was not confirmed until the discovery of S/2002 N 5 as a third member in 2024. The group is predicted to contain many more smaller members, though they are too faint to be detected with current technology.

Because of their similar orbits, the moons of the Sao group have low mutual dispersion velocities between 100 and . The dispersion velocity is the speed needed to eject a fragment from a parent body onto its current orbit. For fragments produced in collisions, their dispersion velocities are typically below , comparable to the parent body's escape velocity. The minimum dispersion velocities of the Sao group are low enough to suggest that it originated from a collision, meaning it may be a collisional family. The Sao group is believed to have originated from the destruction of a once-larger irregular moon by a collision with a passing asteroid or comet. This former moon was likely gravitationally captured by Neptune during the Solar System's early history, during or shortly after the formation of the planets.

If the Sao group is a collisional family, then its members are expected to are expected to share similar surface characteristics, such as color. Compared to other collisional families, the Sao group is unusual in that two of its largest members—Sao and Laomedeia—have similar brightness and estimated sizes.

Physical characteristics Little is known about Sao's physical characteristics. Like most of Neptune's irregular moons, Sao is extremely faint and difficult to observe due to its small size and great distance from the Sun. In red light (R band), Sao has an apparent magnitude of 25.4, making it about 100 million times fainter than what can be seen with the naked eye. Because of this, Sao and most of Neptune's irregular moons can only be observed through long-exposure imaging with large optical telescopes.

The diameter of Sao has not been directly measured, although it can be indirectly estimated from its brightness and an assumed geometric albedo. Assuming a geometric albedo of 0.04, Sao's diameter would be about . For a higher albedo of 0.06, Sao would have a diameter of . Sao appears to be roughly the same size as Laomedeia, if both moons have the same albedo.

Sao (moon)

Bron en licentie