As the home to the gas giants Jupiter and Saturn and the ice giants Neptune and Uranus, the outer reaches of our solar system can be a pretty weird place. NASA’s Jet Propulsion Laboratory reinforced that notion today by sharing the strange orbits of two of Neptune’s moons, Naiad and Thalassa. NASA JPL says that these moons are locked in a “dance of avoidance” that’s “unprecedented.”
Naiad and Thalassa are the two innermost moons of the planet, which has 14 in total. They’re also tiny – NASA says that both moons have a length that tops out around 60 miles. The orbits of both moons are separated by only about 1,150 miles, with Naiad’s orbit taking about seven hours to complete and Thalassa’s taking slightly longer at seven and a half hours.
Compared to Thalassa’s orbit, Naiad’s orbit is tilted by about five degrees. This tilted orbit means that Naiad orbits around Neptune in a wave-like pattern, which in turn keeps the orbit of both moons stable. “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,” NASA JPL wrote on its website. “This up, up, down, down pattern repeats every time Naiad gains four laps on Thalassa.”
NASA refers to this dance as a “resonance,” and describes the interaction between these two moons in a new paper called “Orbits and resonances of the regular moons of Neptune,” which was published on November 13th in Icarus (DOI: 10.1016/j.icarus.2019.113462). The lead writer on the paper, Marina Brozović, works at NASA’s Jet Propulsion Laboratory researching solar system dynamics.
Brozović explains that there are many different types of resonances that planets and moons can fall into, but this one between Naiad and Thalassa has never been seen before. Brozović also says that the team responsible for this paper thinks that Naiad’s tilted orbit may be the product of an encounter with one of NASA’s other inner moons, allowing it to later lock into this resonance with Thalassa.
Beyond detailing this fascinating interaction between two of Neptune’s moons, the work NASA JPL conducted for this paper also allowed researchers to determine the mass and density of these moons, discovering that the densities of both are close to that of water ice.