Three of Neptune’s moons might have been born from the remnants of ancient icy worlds shattered by Neptune’s largest moon, Triton, a new study finds.
These findings suggest further research of these moons might give researchers an unprecedented look at the innards of icy worlds.
“The interiors of large icy moons are normally permanently hidden from us, buried beneath thick shells of water ice,” study lead author Ryleigh Davis, a planetary scientist currently at the University of California, San Diego, told Space.com. “Neptune‘s inner moons may be the only place in the solar system where we can directly observe that material, because a catastrophic event essentially turned those ancient worlds inside out.”
Triton is by far the largest of Neptune’s 16 known moons. More than 99 percent of the mass of all of Neptune’s moons is concentrated in Triton, which is roughly the size of Pluto, Davis said.
Whereas the largest moons of the solar system’s other giant planets all orbit their worlds in the same direction as those planets spin, Neptune’s largest moon, Triton, orbits in the opposite direction. This unusual configuration suggests the Neptunian system evolved in a dramatically different way than those of its siblings. Instead of Triton forming around Neptune, as the largest moons of the other giant planets likely did around their worlds, Triton’s backward orbit has led researchers to suspect “it was captured from the outer solar system,” Davis said.
When Neptune captured Triton with its gravity, Triton may have acted like a giant wrecking ball, smashing many of the moons Neptune may have originally had, Davis said. However, much remained uncertain about what happened in the aftermath of this destruction.
To shed light on the mysteries of Neptune’s history, scientists investigated Neptune’s rings and three of its moons — Larissa, Galatea, and Proteus. Discovered in 1989 with the Voyager 2 flyby of Neptune, these moons are small and close to Neptune, orbiting just outside the planet’s main rings. Their locations and tiny sizes have made it difficult for astronomers to study them in detail from Earth.
In the new study, the researchers analyzed near-infrared light from those inner moons using the James Webb Space Telescope . This data helped reveal their chemical makeup for the first time.
The rings and two of the inner moons — Larissa and Galatea — possessed clay minerals previously unseen on the surfaces of bodies in the outer solar system beyond Jupiter. Specifically, they contain magnesium-rich phyllosilicates, commonly found in main belt asteroids and carbonaceous chondrite meteorites.
“The most surprising thing is simply that we found clay minerals at all,” said Davis, who conducted this research at the California Institute of Technology in Pasadena. “That was genuinely not on our list of things we expected to find.”
These minerals require major prolonged heating to form. One likely place they were created was the deep interior of a large icy body, where heat might come from radioactive material and other sources.
As such, the researchers suggest these minerals are evidence Triton may have demolished Neptune’s original moons. The rubble could then come together, forming Neptune’s inner moons, with the clay minerals exposed on their surfaces.
Another possibility is that these clay minerals came from a dwarf planet similar in size to Pluto that passed too close to Neptune. Neptune’s gravity might then have shredded this dwarf planet apart, Davis said.
These clay minerals only form in the presence of liquid water. Mysteriously, no water ice was detected in any of the three moons studied or the rings.
“That’s really surprising because everything out in this part of the solar system is really icy,” Davis said in a statement. “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.”
The scientists noted they did not see these clay minerals on Proteus, the biggest of the three moons they investigated. They suggested it might have formed from a clay-mineral-poor part of the debris, or experienced subsequent heating that destroyed any such minerals.
Curiously, the scientists found all three moons and the rings had a hydrated mineral the researchers were not able to identify. “Identifying it will likely require new laboratory measurements under outer solar system conditions, which is technically challenging but doable,” Davis said.
To learn more about Neptune, a spacecraft dedicated to visiting the planet “is the obvious next step,” Davis said. “We’ve had only a single flyby — Voyager 2 in 1989 — and these results make clear there’s much more to learn. The main obstacle is time and resources: an ice giant mission was identified as a high priority in the most recent Planetary Science Decadal Survey, but these missions take decades to develop.”
The scientists detailed their findings July 29 in the journal Science Advances.