A new study, published in the journal Icarus, shows that Naiad and Thalassa, two innermost moons of Neptune, are locked in an unusual type of orbital resonance.
The odd orbits of Neptune’s inner moons Naiad and Thalassa enable them to avoid each other as they race around the giant planet. Image credit: Brozović et al, doi: 10.1016/j.icarus.2019.113462.
Far from the pull of the Sun, giant planets are the dominant sources of gravity, and collectively, they boast dozens upon dozens of moons.
Some of those moons formed alongside their planets and never went anywhere; others were captured later, then locked into orbits dictated by their planets. Some orbit in the opposite direction their planets rotate; others swap orbits with each other as if to avoid collision.
The Neptune system consists of seven regular inner moons, Triton, Nereid, and five irregular outer moons.
Inner moons Naiad and Thalassa were discovered by NASA’s Voyager 2 spacecraft during the 1989 flyby of Neptune.
They are about 60 miles (100 km) in length and are true partners, orbiting only about 1,150 miles (1,850 km) apart.
But they never get that close to each other; Naiad’s orbit is tilted and perfectly timed. Every time it passes the slower-moving Thalassa, the two are about 2,200 miles (3,540 km) apart.
In this perpetual choreography, Naiad swirls around Neptune every seven hours, while Thalassa, on the outside track, takes seven and a half hours.
An observer sitting on Thalassa would see Naiad in an orbit that varies wildly in a zigzag pattern, passing by twice from above and then twice from below.
This up, up, down, down pattern repeats every time Naiad gains four laps on Thalassa. Although the dance may appear odd, it keeps the orbits stable.
Dr. Marina Brozović, a researcher at NASA’s Jet Propulsion Laboratory, and colleagues discovered the unusual orbital pattern using analysis of observations by the NASA/ESA Hubble Space Telescope.
“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 been seen before,” Dr. Brozović said.
So how did Naiad and Thalassa end up together — but apart? It’s thought that the original satellite system was disrupted when Neptune captured its giant moon, Triton, and that these inner moons and rings formed from the leftover debris.
“We suspect that Naiad was kicked into its tilted orbit by an earlier interaction with one of Neptune’s other inner moons,” Dr. Brozović said.
“Only later, after its orbital tilt was established, could Naiad settle into this unusual resonance with Thalassa.”
The study also provides the first hint about the internal composition of Neptune’s inner moons.
The scientists used the Hubble observations to compute their mass and, thus, their densities — which were close to that of water ice.
“We are always excited to find these co-dependencies between moons,” said Dr. Mark Showalter, a planetary astronomer at the SETI Institute.
“Naiad and Thalassa have probably been locked together in this configuration for a very long time, because it makes their orbits more stable. They maintain the peace by never getting too close.”
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Marina Brozović et al. 2020. Orbits and resonances of the regular moons of Neptune. Icarus 338: 113462; doi: 10.1016/j.icarus.2019.113462
