Neptune’s odd moon Nereid may be sole survivor from ancient cosmic shake-up
Nereid may be an original Neptunian moon that was pushed outward when Triton entered the system billions of years ago.
New images from NASA’s James Webb Space Telescope show auroras around Neptune for the first time. The discovery will allow scientists to study aspects of the planet that have previously been out of reach.
Neptune’s strangest moon Nereid may not be a captured object after all, but a survivor from the planet’s original moon system, according to a new study that is casting doubt into a long-standing theory about the moon.
Researchers studying Nereid with NASA’s James Webb Space Telescope found that the distant moon appears to be unusually rich in water ice, more reflective and bluer than many Kuiper Belt objects, according to findings published in Science Advances. The results point to a surprising possibility: Nereid may have formed near Neptune and survived the disruptive arrival of Triton, Neptune’s largest moon.
A global color mosaic of the moon Triton, taken in 1989 by Voyager 2 during its flyby of the Neptune system. (Image credit: NASA/JPL/USGS)
Nereid has long puzzled scientists because of its extreme orbit. NASA says the moon is one of Neptune’s outermost known moons and among its largest. It takes 360 Earth days to complete one trip around Neptune, and its path is one of the most eccentric moon orbits in the solar system. That odd orbit has led scientists to consider whether Nereid was a captured asteroid or Kuiper Belt object or whether it was thrown into its unusual path when Neptune captured Triton.
The new research strengthens the second idea. Rather than being an icy wanderer captured from the outer solar system, Nereid may be an original Neptunian moon that was pushed outward when Triton entered the system billions of years ago. Triton is believed to have been captured from the Kuiper Belt, a distant region of icy bodies beyond Neptune, and its arrival would have disrupted Neptune’s earlier moon system.
This is a composite picture showing images of storms on Neptune from the Hubble Space Telescope (left) and the Voyager 2 spacecraft (right). The Hubble Wide Field Camera 3 image of Neptune, taken in Sept. and Nov. 2018, shows a new dark storm (top center). In the Voyager image, a storm known as the Great Dark Spot is seen at the center. It is about 13,000 km by 6,600 km (approximately 8,000 miles by 4,100 miles) in size -- as large along its longer dimension as the Earth. The white clouds seen hovering in the vicinity of the storms are higher in altitude than the dark material. (NASA/ESA/GSFC/JPL)
The study combined Webb observations with computer simulations of Neptune’s early history. Researchers found that Nereid’s surface does not closely resemble typical Kuiper Belt objects. Its abundant water ice and the apparent lack of common volatile organics makes it harder to explain as a captured object and easier to understand as a moon that formed around Neptune.
That makes Nereid an important clue in one of the solar system’s colder mysteries. Neptune is the eighth and farthest major planet from the sun, and its moon system is largely unexplored. NASA’s Voyager 2 is the only spacecraft to have visited Neptune, flying past the planet in 1989. Nereid itself was discovered decades earlier, on May 1, 1949, by Gerard P. Kuiper using a ground-based telescope.
Nereid was first seen by Gerard Kuiper in 1949 and was the last satellite of Neptune to be discovered before Voyager 2's discoveries in 1989. (Image: NASA/JPL)
NASA says Nereid was the last satellite of Neptune discovered before Voyager 2 revealed additional moons in 1989. Kuiper proposed the name Nereid after the sea nymphs of Greek mythology.
The finding could reshape how scientists understand Neptune’s early history. If Nereid is truly an original moon, it may be one of the few remaining pieces of a system that was largely destroyed or rearranged when Triton was captured. Other inner moons around Neptune may have formed later from debris left behind after that disruption.
For now, the distant moon remains difficult to study. Nereid is small, remote and faint, and no spacecraft mission to Neptune is currently planned.
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