Orbiting around Neptune we can find one of the most peculiar moons in the solar system: Triton. This moon is unique not because of its shape, color or composition, but because of its origin. The satellite did not form in the place it occupies today, in the orbit of Neptune. Triton formed in the Kuiper Belt , a region that extends beyond the orbit of the icy giant and is made up of thousands of asteroids of very different sizes, but also some dwarf planets such as Haumea, Eris or Pluto. But before we talk about how they met, let’s talk about our two leads separately.
Neptune is the fourth largest planet orbiting the Sun , slightly smaller than Uranus but more massive than it. Its existence was predicted and discovered in 1846 , independently by John Couch Adams and Urbain Le Verrier, from irregularities in the orbit of Uranus, which had been discovered 65 years earlier. It is composed mainly of hydrogen and helium although it contains a higher proportion of ammonia, water and methane ice than Jupiter and Saturn. The fastest winds in the solar system have been observed in its atmosphere, exceeding speeds of 2000 km/h.
Around it orbits Triton, which accumulates 99.5% of the mass of all Neptune’s moons . The moon was discovered just 17 days after the planet. It got its name from Greek mythology, as Triton was the son of the god of the seas, Poseidon, the Greek equivalent of the Roman god Neptune. In addition to dominating against the other moons of Neptune, it is the largest satellite in the solar system with a retrograde orbit around its planet . That is, Triton orbits in the opposite direction to the rotation of Neptune itself. This, especially for such a large moon, clearly tells us that Triton did not form from the same material that formed the planet and that it must have formed in another region and was later captured.
In our solar system we have many examples of satellites with this same origin . The moons of Mars could have reached the planet by this mechanism (although there are also other possibilities) and most of the satellites of Jupiter and Saturn were formed in this way. For gas giants these moons may have one of several possible origins. Some of them will come from the asteroid belt located between the orbits of Mars and Jupiter. Others would form in the Kuiper belt, the one that gave rise to Triton. Still others could have emerged from the reservoir of asteroids known as Trojan asteroids, those that orbit the L4 and L5 Lagrange points of either of the two giant planets. However none of these captured satellites is as large as Triton, which is in fact the second largest satellite, in proportion to its planet (planet proper, not dwarf planet), in the solar system, behind only our own moon.
But of course, the gravitational capture of another body is not something trivial , it is not enough simply to get close enough to the planet. Only with an approach what we would achieve is that the object is shot in another direction and not a stable orbit. To achieve the latter, it is necessary to stop the asteroid (or dwarf planet) that we want to capture. This braking is usually achieved thanks to the friction of the object with the outermost layers of the planet’s atmosphere or with dust that may be in orbit at great distances. In the case of Triton, given its large size, we believe that something more extreme had to happen.
Today, the most feasible option for the gravitational capture of this moon is that, before being captured, it formed part of a double system, similar to the system formed by Pluto and its enormous moon Charon. Although it may seem that Pluto must be a special case, the truth is that we have observed satellites orbiting approximately 16% of objects called NEOs, objects that at some point in their orbit pass close to Earth’s orbit, and 11% of Kuiper belt objects. We therefore believe that Triton, before being a moon of Neptune, was the dominant partner of a double planet , the other member being about half of its 2700 km diameter. Size that, by the way, makes it about 330 km larger than Pluto. Triton would have been, had it continued in its original orbit, the largest of the known dwarf planets.
Thus, several million years after the formation of Neptune and most other bodies in the solar system, the double planet of Triton came too close to the ice giant . During the approach, instabilities introduced by Neptune’s large mass into Triton and its companion must have caused the latter to fly away, robbing Triton of enough speed to leave it orbiting Neptune. The appearance of such a large moon undoubtedly affected the original satellites , causing many of them to fall to Triton or Neptune or to be thrown to other regions of the solar system. The result is that today Triton easily dominates the surroundings of Neptune. The second largest moon, Proteus, is about six times smaller and about 500 times less massive.
In addition to an interesting origin, Triton has some notable features. It has a very thin atmosphere , which we think is capable of lasting over time because it is renewed by gaseous nitrogen that escapes from its surface. It also has a very young surface, a few million years old, indicative of geological activity inside the satellite . Other indicators of this activity could be the geysers observed emanating from its surface. We owe all the detail we know about Triton to the visit of the Voyager 2 probe in 1989. During this visit we were able to obtain the first detailed images of the surface of Neptune and Triton, as well as other hitherto unknown moons and rings. of Neptune. After its visit to Neptune, Voyager 2 continued its journey to the outer reaches of the solar system. Since then, no probe has passed close to Neptune or Triton. Perhaps it is time to pay a visit to this very interesting system.
Referencias:
Agnor, C., Hamilton, D. 2006. Neptune’s capture of its moon Triton in a binary–planet gravitational encounter. Nature 441. DOI: 10.1038/nature04792
