🔭 Nereid, the moon that survived a cataclysm

Long considered an object captured from the Kuiper belt, the third largest moon of the planet Neptune, Nereid, may actually be a primordial moon that survived a cosmic catastrophe.

For decades, the prevailing theory held that Nereid came from the Kuiper belt, that vast region of icy bodies beyond Neptune. The main argument rested on its highly eccentric and irregular orbit, rare among original moons. Furthermore, the capture of Triton, Neptune’s largest moon, by the planet’s gravity should have scattered or destroyed any preexisting satellite. Yet Nereid remains, intact.

The James Webb Space Telescope was used by a team of researchers to determine Nereid’s origins. With just about ten minutes of observation, they gathered enough data to show that Nereid is very different from known Kuiper belt objects. Richer in water ice, much brighter and more reflective, and bluer in color, it also lacks the typical volatile organic compounds of those distant objects.

Nereid’s composition therefore contradicts the hypothesis of a capture from the Kuiper belt. To support this idea, scientists ran simulations of the Neptunian system’s dynamics in its early days, at the time when Triton was captured. These models show that Triton’s violent arrival could have thrown Nereid onto its current orbit, very inclined and elongated, without destroying it.

The history of this moon dates back to 1949, when it was discovered by astronomer Gerard Kuiper, who named it and sensed its importance for understanding Neptune’s strange system.