Even by the wild patterns of the outer solar system, the strange orbits that carry Neptune's two most intimate moons are unprecedented, according to a recently published survey.
Orbital dynamics experts are calling it the "dance of evasion" performed by the tiny moons Naiad and Thalassa. Both are true partners, orbiting only about 1,150 miles (1,850 kilometers) away. But they are never so close to each other; Naiad's orbit is tilted and perfectly synchronized. Every time you pass the slower moving Thalassa, the two are about 3,540 kilometers apart.
In this perpetual choreography, Naiad revolves 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 very zigzag orbit, twice from above and then twice from below. This up, up, down, and down pattern repeats every time Naiad wins four turns in Thalassa.
Although the dance may seem strange, it keeps the orbits stable, the researchers said.
"We refer to this repetitive pattern as resonance," said Marina Brozović, a solar system dynamics expert at NASA's Jet Propulsion Laboratory in Pasadena, California, and lead author of the new article, published Nov. 13 in Icarus. "There are many different types of" dances "that planets, moons and asteroids can follow, but this has never been seen before."
Far from the attraction of the sun, the giant planets of the outer solar system are the dominant sources of gravity and collectively boast dozens and dozens of moons. Some of these moons formed alongside their planets and never went anywhere; others were later captured, then locked in orbits dictated by their planets. Some orbit in the opposite direction in which their planets rotate; others swap orbits to avoid collisions.
Neptune has 14 confirmed moons. Neso, the furthest from them, orbits an elliptical loop that takes him 74 million kilometers from the planet and takes 27 years to complete.
Naiad and Thalassa are small and shaped like Tic Tacs, only 100 kilometers long. They are two of Neptune's seven inner moons, part of a tightly packed system that is intertwined with weak rings.
So how did they end up together – but apart? It is thought that the original satellite system was interrupted when Neptune captured its giant moon, Triton, and that these moons and inner rings formed from the debris remains.
"We suspect that Naiad was pushed into his inclined orbit by previous interaction with one of Neptune's other inner moons," Brozović said. "Only later, after his orbital inclination was established, could Naiad settle into this unusual resonance with Thalassa."
Brozović and his colleagues discovered the unusual orbital pattern using observation analysis from NASA's Hubble Space Telescope. The work also provides the first tip on the internal composition of Neptune's inner moons. The researchers used the observations to calculate their mass and therefore their densities – close to that of water ice.
"We are always excited to find these co-dependencies between the moons," said Mark Showalter, planetary astronomer at the SETI Institute in Mountain View, California, and co-author of the new article. "Naiad and Thalassa are probably stuck in this configuration for a long time because it makes their orbits more stable. They keep the peace by never getting too close."
Neptune's newest tiny moon is probably the largest
Marina Brozović and other Orbits and resonances of Neptune's regular moons, Icarus (2019). DOI: 10.1016 / j.icarus.2019.113462, https://arxiv.org/abs/1910.13612
NASA discovers that Neptune's moons are locked in the & # 39; s dance of evasion & # 39; (2019, November 15)
consulted November 15, 2019
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