Banded, beautiful, and a bewitching sapphire blue, the ice-giant planet Neptune is the most distant major planet from our Star–and it is also circled by a bizarre large moon that may not have been born a moon at all. Neptune’s moon, Triton, may really be an uninvited guest–the alien changeling child, taken in from the cold, by its current parent-planet, Neptune. Triton shows features that hauntingly resemble those seen on the dwarf planet Pluto, an icy denizen of the remote Kuiper Belt. The Kuiper Belt is a reservoir of a multitude of sparkling icy comet nuclei, as well as other frozen bodies–some large, some small–situated in a ring around our Sun beyond Neptune’s orbit. Triton and Pluto share roughly the same density and bulk composition, as well as similar atmospheres–and they both travel in unusual orbits. Also, Neptune’s system of moons is not what astronomers expect for a gaseous ice-giant planet, inhabiting the outer Solar System. In November 2017, a team of astronomers announced that they have explored the possibility that Neptune was really born with an ordinary system of moons that was later destroyed when it captured its murderous giant moon, Triton.
Because of the many similarities between Triton and Pluto, it has long been thought that there is some sort of historical connection between them. Indeed, it was once proposed that Pluto is really an escaped moon of Neptune, but this is now thought to be improbable. It is now considered to be much more likely that Triton, like Pluto, orbited our Sun freely, but was unlucky enough to be snared by Neptune–whereas Pluto was left independent and free to wander at will within its distant birthplace.
Triton is unique among our Solar System’s moons of planetary mass. This is because its orbit is retrograde to Neptune’s rotation and inclined relative to Neptune’s equator. This suggests that Triton was not born in orbit around Neptune, but was instead snared by the giant planet.
Astronomers’ current understanding of giant-planet birth predicts an episode of gas accretion that ultimately builds up the enormous size of these gaseous behemoths. According to theory, circumplanetary gas disks, that surround the forming planets during this early period, eventually become the strange nurseries that produce a giant planet’s system of moons, thus creating systems of co-planar and prograde (orbiting in the same direction as the planet) natural satellites in a way similar to the many moons of Jupiter and Saturn.
However, Neptune is wacky. This giant gaseous world has only a small number of moons when compared to the other three gaseous giant planets in our Sun’s outer realm: Jupiter, Saturn, and Uranus. Of the quartet of giant planets that inhabit our Sun’s outer kingdom, Jupiter and Saturn are classified as gas-giants, while Uranus and Neptune are ice-giants. While all four planets are enormous in size, Jupiter and Saturn are much larger than Uranus and Neptune, and possess much more massive gaseous envelopes. The ice-giants, Uranus and Neptune, are smaller, contain larger solid cores, and sport less massive gaseous envelopes than their two gas-giant planet kin.
Neptune is circled by only 14 moons, a considerably smaller number than Jupiter’s 70. Also, most of Neptune’s moons are extremely small. Triton, however, is an exception. In fact, Triton harbors 99.7% of the mass of Neptune’s entire system of moons combined. The second-largest irregular moon in our Solar System, Phoebe of Saturn, sports only about 0.03% of Triton’s mass. It is thought that Triton was snared by its adopted parent-planet some time after Neptune had already formed a system of moons. This means that the capture of the wandering Triton was likely a catastrophic event for Neptune’s original moons, disrupting their orbits, and causing them to blast into each other–thus creating a rubble disc.
Looking inward from Triton, there are seven regular moons, all of which display prograde orbits in planes that are situated close to Neptune’s equitorial plane, and some of these small moons orbit within Neptune’s rings. It is thought that these seven small moons were re-accreted from the rubble ring that formed after Neptune snared Triton. This would have occurred some time after Triton’s orbit had become circular. In addition, Neptune is circled by six more outer irregular moons other than Triton, including Nereid, whose orbits are considerably farther from Neptune and at high inclination: A trio of these moons sport prograde orbits, while the remainder have retrograde orbits. Indeed, Nereid has an unusually close and eccentric orbit for an irregular moon. This suggests that Nereid may have once been a regular moon that was badly disturbed and nudged into its current position when Triton was snared by Neptune’s gravity. The duo of outermost Neptunian irregular moons, Psamathe and Neso, have the largest orbits of any natural satellites known in our Solar System to date.
Triton was the second moon in our Solar System that was found to have a substantial atmosphere, which is primarily composed of nitrogen–with smaller quantities of carbon monoxide and methane. Discovered by William Lassell in 1846, only seventeen days after the discovery of Neptune, Triton is one of the most frigid worlds in our Solar System, with a surface temperature of only about 38 Kelvin. Triton’s frozen surface is coated by nitrogen, carbon dioxide, methane, and water ices, and it has a high geometric albedo of more than 70%. Surface features include a large southern polar cap, ancient cratered planes that are cross-cut by scarps and graben, as well as much younger features thought to have been formed by endogenic processes like cryovolcanism (ice volcanoes).
Voyager 2 flew by Neptune back in 1989, and its observations revealed a number of active geysers, situated within the polar cap heated by the Sun. The geysers hurl out plumes to the impressive height of up to 8 kilometers. Triton has a relatively high density that indicates rocks account for approximately two-thirds of its mass, and ices (mostly water ice) compose the remaining one-third.
The mass distribution of the moons of Neptune is lopsided. In fact, it is the most lopsided satellite system of any of the giant planets dwelling in our Solar System. Triton accounts for nearly all the mass of the system, with all of the other moons together accounting for only one-third of 1%. This is very similar to the system of moons that circle the ringed-planet Saturn, where the large, smoggy, orange moon Titan–the second-largest moon in our Sun’s family (after Ganymede of Jupiter)–accounts for over 95% of the total mass of Saturn’s system of moons.
Soon after it had been snared by Neptune, Triton’s orbit was highly eccentric. This indicates that Triton’s capture would have caused a mess of chaotic perturbations in the orbits of the original inner moons of Neptune–thus causing them to crash into one another, producing a disk of rubble created from these catastrophic smash-ups. For this reason, it is probable that Neptune’s current inner moons are not the original generation of moons that formed along with Neptune.
Only after Triton’s orbit became circular, around its adopted parent-planet, could some of the rubble re-accrete to form the moons of Neptune that astronomers observe today. Triton’s highly disruptive invasion of the Neptune system may be the reason why the moons of Neptune do not conform to the 10,000:1 ratio of mass between parent-planet and moon-offspring that literally all of the moons observed in the satellite systems of the other giant planets in our Solar System conform to.
Over the years, several theories have been proposed to explain the mechanism involved in Triton’s capture by Neptune. One theory suggests that Triton was snared in a three-body encounter. According to this scenario, Triton is the lone survivor of a binary Kuiper Belt Object (KBO) that had been fatally jostled during a destructive close encounter with Neptune.
Numerical simulations have been conducted that show that there is a 0.41 probability that Neptune’s moon Halimede blasted into Nereid in the past. Even though it is not known if this collision really did occur, both moons display similar grey colors. This implies that Halimede could be a chunk of Nereid that broke off during the collision.
Clearly, Triton is a bizarre moon-world, circling its giant parent-planet in the wrong direction. As Triton wandered away from its birthplace in the Kuiper Belt, during its journey through the darkness of interplanetary space, it at last ventured close enough to Neptune to feel the powerful lure of its gravitational embrace. As Neptune drew its adopted moon-child closer and closer, the frigid wanderer from afar experienced a sea-change from a comet-like inhabitant of the Kuiper Belt, to a moon of one of the major planets in our Solar System. So, now, Triton inhabits its new home, orbiting the planet Neptune, but orbiting it backwards. And like all moons, wherever they may be, it is now a dependent of its parent-planet. Indeed, Triton was given its name as an allusion to the demigod Triton’s dependence on the sea-god Neptune in Greek mythology.
Neptune’s Ruthless Guest
How did Triton acquire so many strange properties, and why is Neptune’s system of satellites so different from those predicted for a gaseous giant planet? Two planetary scientists, Dr. Raluca Rufu (Weizmann Institute of Science, Israel) and Dr. Robin Canup (Southwest Research Institute, US) demonstrate how Triton wreaked catastrophic havoc on Neptune’s first generation of very unfortunate moons.
Dr. Rufu and Dr. Canup studied the scenario in which Neptune once, when it was young, had a typical prograde system of moons orbiting it that was similar to those of the other gas giants in our Solar System. The two authors propose that Triton may have originally been a KBO that wandered away from its home in the Kuiper Belt, only to be snared by Neptune. The interactions that resulted from the capture of Triton between retrograde moons and Neptune’s original, prograde moons may have then resulted in the destruction of this orderly system, leaving in its wake only the ruthless Triton and Neptune’s moons that are still around today.
Using N-body simulations that model a recently snared Triton, and a likely primordial prograde system of moons, Dr. Rufu and Dr. Canup show that if the moons sport a mass ratio comparable to that of Uranus’s system of moons or smaller, Triton’s destructive dance with them has the tragic likelihood of reproducing the system that astronomers now observe. The simulations even demonstrate that the interactions decrease Triton’s original semi-major axis rapidly enough to to stop smaller, outer moons like Nereid from being unceremoniously evicted from the system.
If the authors’ proposal is correct, then it successfully explains why Neptune’s moon system looks so strange compared to Jupiter’s or Saturn’s–which means that astronomers’ models of how these primordial systems form around gaseous giant worlds still hold strong.