Even by the wild standards of the outer solar system, the strange orbits of Neptune's two darkest moons are unprecedented, according to newly released research.
Experts in orbital dynamics call it the "dance of evasion" performed by the tiny moons Nayad and Talasa. The two are true partners, orbiting at only 1,150 miles (1,850 kilometers). But they never get close to each other; Najad's orbit is tilted and perfectly timed. Each time it passes slower than Talasa, the two are about 2,200 miles apart.
In this constant choreography, Nayad revolves around the ice giant every seven hours, while Talasa, on the outer path, lasts seven and a half hours. The Talas sitter will see Niyad in a digital zigzag-like orbit, passing twice from above and then twice below. This up, up, down, down is repeated every time Niyad wins four rounds at Talassa.
Although the dance may seem strange, it keeps the orbits stable, researchers say.
"We are referring to this repetitive pattern as a resonance," said Marina Bozovic, a solar system dynamics expert at NASA's Jet Propulsion Laboratory in Pasadena, California, and lead author of the new paper, which was released November 13 in Icarus. "There are many different types of 'dances' that can follow the planets, the moon and the asteroids, but this has never been seen."
Away from the Sun's retreat, the giant planets of the outer solar system are the dominant sources of gravity and, collectively, they boast dozens of moons. Some of those moons formed side by side with their planets and never went anywhere; others were captured later, then locked in orbits dictated by their planets. Some orbits in the opposite direction their planets rotate; others exchange orbits with each other as to avoid collision.
Neptune has 14 confirmed moons. Neso, the farthest from them, orbits in an elliptical loop that takes it nearly 46 million miles (74 million kilometers) away from the planet and lasts 27 years.
Nayad and Thalassa are small and Tic Tacs in shape, only 60 miles long. They are two of Neptune's seven inner moons, part of a tightly packed system intertwined with weak rings.
So how did they end up together – but apart? The original satellite system is thought to have been disrupted when Neptune captured its giant moon, Triton, and these inner moons and rings are formed from the remnants of debris.
"We suspect Najad was thrown into his tilted orbit by previously interacting with one of Neptune's other inner moons," Brozovic said. "Only later, after his orbital tilt was established, could Najad settle down in this unusual resonance with the Thalassa."
Brozovic and her colleagues have discovered an unusual orbital pattern using an analysis of the observations from NASA's Hubble Space Telescope. The work also gives the first hint of the inner composition of Neptune's inner moons. The researchers used the observations to calculate their mass and, therefore, their density – which was close to that of water ice.
"We're always excited to find these co-dependencies between moons," said Mark Fowler, a planetary astronomer at the SETI Institute for Mountain View, California and co-author of the new paper. "Nayad and Talasa are probably locked into this configuration for a very long time because it makes their orbits more stable. They keep the peace by never coming close. "
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Marina Bozovic et al. Orbits and resonances of the regular moons of Neptune, Icarus (2019). DOI: 10.1016 / j.icarus.2019.113462, https://arxiv.org/abs/1910.13612
NASA reveals Neptune's moon locked in "dance of evasion" (2019, November 15)
Retrieved November 15, 2019
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