According to recently published studies, even by the wildest standards of the outer solar system, the strange orbits that have the two innermost moons of Neptune are unprecedented.
Orbital dynamics experts call this the “avoidance dance” performed by the tiny moons of Naiad and Thalassa. These two partners are true partners. They move at a distance of only about 1850 kilometers from each other. But they never fly so close to each other – the orbit of Naiad is inclined and perfectly calculated. Each time it passes the slow Talassa, the distance between them is about 3540 kilometers.
In this eternal choreography, Naiad revolves around the ice giant every seven hours, while Thalassa, in outer orbit, spends seven and a half hours. An observer sitting on Thalass will see Naiad in orbit, which changes wildly zigzag, passing twice from above and then twice from below.
According to the researchers, although the dance may seem strange, it maintains the stability of the orbits.
“We call this repeating pattern resonance,” said Marina Brozovi, an expert on solar system dynamics at NASA’s Jet Propulsion Laboratory in Pasadena, California, and lead author of a new article that was published November 13 in Icarus. “There are many different types of“ dances ”that can be followed by planets, moons and asteroids, but we have never seen anything like this before.”
Far from the attraction of the Sun, the giant planets of the outer solar system are the dominant sources of gravity, and together they can boast dozens and dozens of moons. Some of these moons formed near their planets and never went anywhere; others were later captured and then locked in orbits dictated by their planets. Some orbits of the moons are found in the opposite direction to the rotation of the planet itself; other moons exchange orbits with each other to avoid a collision.
Neptune has 14 confirmed moons. Neso, the farthest of them, rotates along a wild elliptical loop, which takes it almost 74 million kilometers from the planet, and it takes 27 years to complete it.
Naiad and Thalassa are small and have the shape of TikTak candy, their length is only about 100 kilometers. They are two of the seven inner moons of Neptune, part of a tightly packed system that is intertwined with faint rings.
So how did they end up together and far apart? It is believed that the original common satellite system was broken when Neptune captured its giant moon Triton, and that these inner moons and rings formed from the remnants of debris.
“We suspect that Naiad was launched into its inclined orbit as a result of an earlier interaction with one of Neptune’s other internal satellites,” Brozovi said. “Only later, after its orbital tilt was established,” Naiad “was able to come to this unusual resonance with Thalassa.”
Brozovi and her colleagues discovered an unusual orbital pattern using an analysis of NASA’s Hubble Space Telescope observations. The work also gives the first hint about the composition of Neptune’s internal satellites. The researchers used the observations to calculate their mass and, therefore, their density – which were close to the density of water ice.
“We are always happy to find these interdependencies between the moons,” said Mark Showalter, planetary astronomer at SETI at Mountain View, California, and co-author of a new article. “Naiad and Thalassa have probably been locked in this configuration for a very long time, because it makes their orbits more stable. They support the world, never getting too close. “