Even by the wild standards of the outer solar system, the strange orbit that carries the two deepest moons of Neptune is unprecedented, according to newly published research.
Orbital dynamics experts call it the "avoidance dance" performed by the small months of Naiad and Thalassa. Both are true partners, who orbit apart only around 1,150 miles (1,850 kilometers). But they were never that close to each other; Naiad's orbit is tilted and timed perfectly. Each time it passes through the slower-moving Thalassa, the two are about 2,200 miles (3,540 kilometers) apart.
In this eternal choreography, Naiad swirls around the ice giant every seven hours, while Thalassa, on the outer track, takes seven and a half hours. An observer seated in Thalassa will see Naiad in highly variable orbits in a zig-zag pattern, passing twice from above and then twice from below. This up, up, down, down pattern repeats itself every time Naiad gets four rounds at Thalassa.
Although the dance may look strange, it makes its orbit stable, the researchers said.
"We call this repeated pattern resonance," said Marina Brozović, an expert in solar dynamics at NASA's Jet Propulsion Laboratory in Pasadena, California, and lead author of the new paper, which was published November 13 in Icarus. "There are many types of 'dances' that can be followed by planets, moons and asteroids, but this one has never been seen before."
Far from the pull of the Sun, the giant planets of the outer solar system are the dominant source of gravity, and collectively, they boast of dozens of dozen moons. Some of the moons formed with their planet and never went anywhere; others are captured later, then locked in the orbit determined by their planet. Some orbit in the opposite direction to their rotating planet; others exchange orbits with each other as if to avoid a collision.
Neptune has 14 months to be confirmed. Neso, the furthest from them, orbits in a wild elliptical circle that takes him nearly 46 million miles (74 million kilometers) away from the planet and takes 27 years to complete.
Naiad and Thalassa are small and shaped like Tic Tac, which is only about 60 miles (100 kilometers) long. They are two of the seven moons in Neptune, part of a crowded system that is intertwined with a dim ring.
So how do they end up together – but separate? It is thought that the original satellite system was disrupted when Neptune captured its giant moon, Triton, and that the inner moons and rings were formed from the remaining debris.
"We suspect that Naiad was kicked into his tilted orbit by previous interactions with one of the moons in another Neptune," Brozović said. "Only later, after the tilt of the orbit formed, Naiad was able to adjust to this unusual resonance with Thalassa."
Brozović and his colleagues discovered unusual orbital patterns using observational analysis by NASA's Hubble Space Telescope. This work also provides the first clue about the internal composition of the moon in Neptune. The researchers used observations to calculate their mass and, thus, their density – close to water ice.
"We are always happy to find this co-dependency between moons," said Mark Showalter, a planetary astronomer at SETI Institute in Mountain View, California, and co-author of the new paper. "Naiad and Thalassa may have been locked together in this configuration for a very long time, because it makes their orbits more stable. They maintain peace by never getting too close."
Neptune's newest and smallest moon is probably the bigger part
Marina Brozović et al. The orbit and resonance of the regular moons of Neptune, Icarus (2019). DOI: 10.1016 / j.icarus.2019.113462, https://arxiv.org/abs/1910.13612
NASA found the moons of Neptune locked in 'avoidance dance' (2019, November 15)
taken 15 November 2019
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