Neptune's satellites

Neptune’s family of fourteen known satellites has challenged astronomers to push the limits of their observational technology and theoretical knowledge. While similar in some respects to the satellite systems of the other three Jovian planets, Neptune’s satellites, especially Triton, have proven to have unique characteristics, which allow astronomers to increase their understanding of conditions in the early solar system.

Overview

Eight days after Neptune’s discovery on September 23, 1846, astronomer John Herschel sent a letter to colleague William Lassell, suggesting that he might turn his telescope toward the new planet to search for satellites. Nine days later, on October 10, Lassell discovered what would become known as Triton, the largest satellite of Neptune and the seventh largest satellite in the solar system, with a diameter of 2,706 kilometers. The satellite’s name is credited to French astronomer Camille Flammarion and was used unofficially for decades before being officially adopted. Mythologically, Triton was the son of the Greek sea god Poseidon (Roman name, Neptune).

Triton proved to be puzzling to astronomers; unlike most large satellites, it orbits its planet retrograde, or “backward” (opposite to the direction of Neptune’s rotation). Its orbit is nearly circular but is highly inclined to Neptune’s equator (23°). These anomalous features led astronomers to hypothesize that Triton was captured by Neptune’s gravity during a close approach to the planet, despite the fact that other planetary satellites suggested to be likewise captured, such as Saturn’s Phoebe, are much smaller.

Despite several searches, no other satellites were discovered around Neptune until the Dutch-born astronomer Gerard Kuiper found a faint (magnitude 19.5) satellite using the 82-inch telescope at McDonald Observatory in 1949. The name Nereid was suggested after the fifty sea-nymph daughters of Nereus and Doris, attendants of Poseidon in Greek mythology. As strange as Triton’s orbit was, Nereid’s was no less peculiar. Although it orbits Neptune in a prograde direction at an average distance some fifteen times greater than that of Triton, its orbit is highly elongated (eccentric), with its most distant orbital point nearly seven times farther than its closest approach to the planet. Such an orbit is more similar to that of a comet than the orbits of most satellites, so—although at a diameter of 340 kilometers, Nereid is larger than most asteroids—some astronomers suggested that it also was captured by Neptune’s gravity. Others pointed an accusatory finger at Triton, suggesting that the capture of Triton might have disrupted Nereid’s original orbit. The discovery of Nereid thus led to more questions than answers, some of which awaited the flyby of the Voyager 2 Spacecraft in late August 1989.

Nereid was too distant (4.7 million kilometers) to be effectively imaged by Voyager 2, which has proven to be a lingering source of frustration to astronomers. The brightness of the satellite is known to change on both short and long timescales, which astronomers have interpreted as due to the rotation of the satellite (although no definitive period can be established). The changes in brightness may be due to differences in the surface material on different sides of the satellite (as in the case of Saturn’s satellite Iapetus) or a nonspherical shape. Interestingly, water ice has been spectroscopically found on the surface, leading to speculations that the satellite’s surface might be a mixture of ice and some dark material, similar to the satellites of the other Jovian planets (and dissimilar to known Kuiper Belt objects). In addition, Halimede, the innermost of the outer five irregular satellites, has spectroscopic similarities to Nereid, and their orbits have a 0.41 in 1 probability of collision during the history of the solar system. This has led to the suggestion that Halimede is a splinter of a nonspherical Nereid.

The other three Jovian planets were found to have many satellites, which could be divided into two broad classes: regular moons, usually larger in size with prograde, nearly circular orbits nearer the planet, and more distant, irregular satellites, usually smaller and with greater orbital eccentricities and inclinations. Irregular satellites appear to be captured objects, while regular satellites were considered to be the “original” satellites of the planet, dating from the formation of the planet itself. Since Voyager 2 had discovered three new satellites around Jupiter, four at Saturn, and ten orbiting Uranus, it was not surprising when the Voyager 2 imaging team announced the discovery of a new Neptunian satellite in June 1989, two months before the probe’s closest approach to Neptune. Now named Proteus for a Greek sea god, it is actually larger than Nereid (416 kilometers in diameter), although it is exceedingly difficult to observe from earthbound telescopes because of its close orbit (roughly a third of Triton’s distance from Neptune).

Three more satellites, all roughly one-third the size of Proteus and orbiting closer to Neptune, now named Larissa, Galatea, and Despina, were discovered in late July 1989. It was later determined that Larissa had been the cause of a brief dip in brightness seen in a star that Neptune had nearly occulted in May 1981, an observation that had initially been explained as due to the planet’s then undiscovered rings. Thalassa and Naiad, each a fraction of the size of Proteus, were discovered orbiting even closer to Neptune only a few days before Voyager’s August 25, 1989, closest approach. The names were selected from mythological characters associated with Poseidon/Neptune, according to the International Astronomical Union’s (IAU’s) convention for Neptunian satellites and based on the historical precedence of Triton and Nereid. For example, Despina was the daughter of Poseidon and Demeter, and Galatea was one of the Nereids. All six of the new satellites were “regular” in the sense that they had prograde orbits that were nearly circular and, with the exception of Naiad, were well aligned with Neptune’s equator.

Following the Voyager 2 flyby, numerous attempts were made to discover additional satellites orbiting farther from Neptune than Nereid, which led to the discovery of five “irregular” satellites in 2002 and 2003: in order out from Nereid, these were named Halimede, Sao, Laomedeia, Psamathe, and Neso, all named after individual Nereids in Greek mythology. Sao and Laomedeia have prograde orbits, the other three satellites orbit retrograde, and all five satellites have significant orbital eccentricities (0.3–0.6). Each is between approximately forty and sixty kilometers wide, comparable to Naiad or smaller, and are largely presumed to have been captured by Neptune.

Knowledge Gained

Even though Neptune was found to have both “regular” and “irregular” satellites, significant mysteries remained after their discovery. The five innermost satellites were found to be potato-shaped (not unexpected for small satellites whose self-gravity is small). However, some astronomers felt this was more evidence of violence in the Neptunian system. The large size of Triton and its surprising orbit suggested that it had played an important role in the past and future dynamics of the entire satellite system. In 1966, Thomas McCord calculated these effects and found that Triton is in an unstable orbit that eventually will lead to its destruction in possibly tens of millions of years (or more). When it passes inside Neptune’s Roche limit, tidal effects will tear it to shreds. McCord also suggested that Nereid’s peculiar orbit was caused by the effects of Triton’s capture and later orbital evolution from an initially parabolic or nearly parabolic orbit to a nearly circular one.

Later, researchers continued to use various computer simulations and computational techniques to model the evolution of the Neptunian system in an attempt to explain both the gross and subtle characteristics of each moon’s orbit. Many astronomers presume that Neptune had an original generation of satellites, possibly similar to the satellites of Uranus, which was destroyed in the aftermath of Triton’s capture, either through mutual collisions between the satellites or through satellites being gravitationally slingshot out of the Neptunian system. The debris is posited to have formed a disk, some of which was accreted by Triton, increasing its diameter and mass, while some of the material accreted to form the currently observed six regular (inner) satellites. The irregular (outer) satellites are suggested to be survivors off this catastrophic event. An unknown number of other satellites collided with each other or one of the larger satellites or were slingshot out of the system. In the process, the orbits of some of the satellites that were originally in retrograde-type orbits (normally seen in irregular satellites) were changed into prograde orbits. The orbits of all fourteen satellites have slowly evolved since then through mutual gravitational effects, and these orbits continue to evolve today.

The detailed images taken of Triton by Voyager 2 strengthened the assumption that it was originally a Kuiper Belt object (similar to Pluto). One of the other important discoveries of the Voyager 2 Neptune flyby was Neptune’s five rings, named (in order out from Neptune) Galle, Le Verrier, Lassell, Arago, and Adams. As in the case of the other Jovian planets, astronomers expected the ring system and the inner moons to have gravitational interactions. It is suggested that Galatea affects the particles of the Adams ring, which orbit just beyond the orbit of Galatea, although the exact nature of the gravitational resonance is still being debated. Interestingly, it is the Adams ring that has the famous “arcs” (which led to its nickname, the “sausage ring”). Despina, Thalassa, and Naiad orbit between the Le Verrier and Galle rings. Despina has been suggested to be acting as a shepherding moon.

Astronomers continue to explore the details of the satellites’ orbital dynamics through cutting-edge computer simulations and theoretical models and, in so doing, test our assumptions about the evolution of the solar system in general and the Jovian planets in particular.

In July 2013, a previously undiscovered moon was found in Hubble Space Telescope images by research scientist Mark Showalter from the SETI Institute. The satellite—a small moonlet—was designated S/2004 N 1. It maintains a near-circular orbit close to the planets equatorial plane. It orbits between the inner moons Larissa and Proteus, keeping an average of 65,420 miles (105,283 km) from Neptune's center; each orbit takes approximately twenty-three hours.

Context

The Voyager 2 mission led to an explosion of knowledge about the Neptunian system, as it did for the Jovian, Saturnian, and Uranian systems before it. However, given the late discovery of the regular satellites, Voyager 2 was able to target only the outermost four of the eight then-known satellites: Triton, Nereid, Proteus, and Larissa.

Little is known about the composition of the other satellites, although they are presumed to have low densities (0.4–0.8) similar to that of Saturn’s small satellites Janus, Epimetheus, and Prometheus. Since the exact orbital interactions between the satellites depend on their densities, further research into those dynamics will allow astronomers to put further constraints on the satellites’ densities and hence compositions, and any future observational evidence gathered on the satellites’ compositions will lead to further refinements in models of the Neptunian system’s dynamic history. Therefore, although Voyager 2 has undoubtedly been a boon to Neptune researchers, further significant breakthroughs certainly await any future spacecraft visiting Neptune and its fourteen satellites. Launched in 2021, the James T. Webb telescope sent scientists pictures of fourteen of Neptune’s moons in September 2022. This breakthrough technology allowed for further study of Neptune’s satellites and provided the clearest images of Neptune’s satellites ever sent back to Earth.

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