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In 1989, the Voyager 2 mission discovered six new moons orbiting Neptune, including five tiny ones that orbit just outside the planet's main rings. Because of their small sizes and locations, these satellites have been difficult to study from Earth. Now, using NASA's James Webb Space Telescope (JWST), a team of Caltech researchers has observed Neptune's rings and three of those moons—Larissa, Galatea, and Proteus—and found that their composition is unique among outer solar system bodies.
These findings point to the existence of an original moon system that was demolished when Triton, Neptune's biggest moon, was likely captured by the planet's gravity after it formed elsewhere in the solar system
"If Neptune once had a system of moons that looked something like what we see at Uranus today, we expect it would've been completely destroyed by the process of Triton getting captured," says Ryleigh Davis. "This is exciting new evidence that something catastrophic happened at Neptune that completely destroyed its original satellites, and we're getting to see the fingerprints left behind by that process."
Neptune is a curious planet, as it is the only one in the solar system that lacks a "typical" moon system of large, ordered satellites, meaning its evolutionary pathway might also be unique. Another recent study from the same JWST research program, and led by Matthew Belyakov, provides independent evidence for the existence of the original satellite system, suggesting that Neptune's moon Nereid may be the sole surviving intact member.
"Phyllosilicates had never been detected anywhere in the outer solar system beyond Jupiter, so that was not on our list of things to look for," says Davis. "We were shocked to find the observed clays, which had to come from objects that were much, much bigger than Neptune's small inner ring moons."
One interesting result, Davis says, is that signatures of magnesium-rich phyllosilicates were found in the spectra of Larissa, Galatea, and the rings—minerals that only form in the presence of liquid water. And yet, no water ice is indicated by the spectra of any of the three moons studied or their rings.
"That's really surprising because everything out in this part of the solar system is really icy," she says. "So, we're fairly confident that they had to come from deep inside something that was big enough to generate enough heat that it melted its water ice. We think the most likely place would be an original system of icy moons, although it's a bit of a mystery where the ice may have gone."
Furthermore, the phyllosilicates were not present on Proteus, the largest of the small moons investigated, suggesting it may have reaccreted from a different region of the debris disk or undergone subsequent heating that destroyed any clay minerals present. In addition, the team found that each of the three moons has the same hydrated mineral that the team has not been able to identify.
"We see something that doesn't really look like anything else we've identified in the solar system; it doesn't match anything we have in our spectral libraries." Davis says. "We assume it's some form of hydrated rock from the moons as well, but there's still a lot of mystery."
While the team favors the primordial satellite destruction scenario, they note that an alternative explanation cannot be ruled out: the tidal shredding of a large, differentiated Kuiper Belt object, similar in size to Pluto, that passed too close to Neptune and was torn apart by the planet's gravity.
