Life is not always easy for astrophysicists: just when they understood another aspect of the movement patterns in our Solar System, two of Neptune’s moons arrive to ruin everything.
The two moons in question are Naiad and Thalassa, both about 100 kilometers wide or 62 miles wide, which run around their planet in what NASA researchers call “dance of circumvention”.
Their strange orbit was first detected by NASA researchers in November 2019.
Compared to Thalassa, Naiad’s orbit is inclined about five degrees: it spends half of its time above Thalassa and half below, in a connected orbit that is different from anything else recorded.
“We refer to this repeated pattern as resonance,”
“There are many different types of dances that can follow planets, moons and asteroids, but this has never been seen before.”
The orbits of the two small moons are only about 1,850 kilometers (1,150 miles) apart, but are perfectly synchronized and choreographed to avoid each other. Naiad takes seven hours to tour Neptune, while Thalassa takes seven and a half on the outside track.
If you were stationed on Thalassa, you would see Naiad pass up and down in a pattern that would repeat itself every four rings, while Naiad repeatedly turns his neighbor. The researchers say these maneuvers keep the orbits stable.
To understand this, the team used data collected between 1981 and 2016 from telescopes on Earth, Voyager 2 and the Hubble Space Telescope to determine how Naiad and Thalassa are moving around the ice giant they call home.
These moons are two of the 14 confirmed satellites for Neptune and two of the seven so-called internal moons, a very narrow system intertwined with weak rings.
According to the researchers, the capture of the great moon by Neptune Triton could explain where Naiad and Thalassa come from and how they came to turn around their planet in such an unusual way.
The internal moons may represent Triton’s leftovers, suggests the team, with Naiad eventually kicked into his slanted orbit through an interaction with another of these nearby neighbors.
In addition to tracing the orbits of Naiad and Thalassa, the new study was also able to take the first steps to determine the composition of the internal moons of Neptune, which appear to be made up of something similar to water ice.
“We are always excited to find these addictions in the moons,” said planetary astronomer Mark Showalter of the SETI Institute.
“Naiad and Thalassa have probably been stuck together in this configuration for a long time, because it makes their orbits more stable. They maintain peace by never getting too close.”
The research was published in Icarus.
A version of this article was first published in November 2019.