Also of the wild norms of the outer solar system are the strange orbits that carry Neptune 's two innermost moons unparalleled, according to recent research.
Orbital dynamics experts call it an "evasive dance" performed by the small moons Naiad and Thalassa. The two are real partners and only circle about 1
In this constant choreography, Naiad swirls around the ice giant every seven hours, while Thalassa on the outer track takes seven and a half hours. An observer sitting on Thalassa would see Naiad in a path that varies white in a zigzag pattern, passing twice from above and then twice from below. This up, up, down, down pattern is repeated every time Naiad gets four turns on Thalassa.
Although the dance may seem odd, the trajectories remain stable, researchers said.
resonance, "said Marina Brozovic, an expert on solar system dynamics at NASA's Jet Propulsion Laboratory in Pasadena, California, and the lead author of the new magazine, published November 13 in Icarus. "There are many different types of" dances "that planets, moons and asteroids can follow, but this one has never seen before."
Far from the pull of the sun, the giant planets of the outer solar system are the dominant gravity, and together they boast dozens upon dozens of moons. Some of these moons formed alongside their planets and never went anywhere; others were later captured and then locked into orbits dictated by their planets. Some go in the opposite direction and their planets rotate; others swap lanes with each other as to avoid collision.
Neptune has 14 confirmed moons. Neso, the longest closed of them, walks in a wild elliptical trail that carries it nearly 46 million miles from the planet and takes 27 years to complete.
Naiad and Thalassa are small and shaped like Tic Tacs, which extend just over 100 miles. They are two of Neptune's seven inner moons, part of a tightly packed system woven with weak rings.
"We refer to this repeating pattern as a resonance. There are many different types of "dances" that planets, moons and asteroids can follow, but this one has never seen before. ”- Marina Brozovic, NASA's Jet Propulsion Laboratory
So how did they end up together – but apart? It was believed that the original satellite system was disturbed when Neptune captured its giant moon, Triton, and that these inner moons and rings were formed by remaining debris.
"We suspect that Naiad was kicked into his inclined orbit by a previous interaction with one of Neptune's other inner moons," Brozovic said. "Only later, after the track was tilted, was Naiad able to settle into this unusual resonance with Thalassa."
Brozovic and her colleagues discovered the unusual orbital pattern using analysis of NASA's Hubble Space Telescope . The work also gives the first hint of the inner composition of Neptune's inner moons. Scientists used the observations to calculate their mass and thus their density – which was close to the ice.
"We are always happy to find these co-dependencies between moons," said Mark Showalter, a planetary astronomer at the SETI Institute in Mountain View, California, and a co-author of the new magazine. “Naiad and Thalassa have probably been locked in this configuration for a very long time, as it makes their trajectories more stable. They maintain peace by never getting too close. "
Reference:" Orbit and resonance for the regular moons of Neptune "by Marina Brozović, Mark R. Showalter, Robert A. Jacobson, Robert S. French, Jack J. Lissauer and Imke de Pater , October 22, 2019, Icarus .
DOI: 10.1016 / j.icarus.2019.113462