New observations of Neptune’s inner moons and dusty rings, captured by NASA’s James Webb Space Telescope, provide evidence of a celestial demolition derby caused by a Pluto-sized interloper billions of years ago. According to planetary scientist Ryleigh Davis, lead author of the study published in Science Advances, the findings reveal magnesium-rich clay minerals on Proteus, Larissa, and Galatea that point to the destruction of ancient, much larger worlds.
James Webb Space Telescope Detects Ancient Clays on Neptune’s Moons
Researchers examining Neptune’s small inner moons and dusty rings detected clay minerals typically found deep inside outer solar system worlds like the dwarf planet Ceres or specific meteorites, according to findings detailed in Science Advances. Ryleigh Davis, a postdoctoral researcher at the University of California, San Diego who worked on the study while at Caltech, notes that these minerals require prolonged contact between liquid water and rock to form.
Because the moons themselves are far too small and cold to support that chemistry in place, the clay had to originate deep inside a much larger ancient world. According to the research team, a catastrophic event destroyed those ancient worlds and exposed their deep interiors, allowing the small moons and rings observed today to re-form from the wreckage.
Did you know? Triton makes up more than 99% of the mass of Neptune’s entire satellite system, including all of its known moons and rings.
Triton’s Capture and the Destruction of Neptune’s Original Satellites
The culprit behind this ancient cataclysm appears to be Triton, which currently serves as Neptune’s largest moon but previously resided farther out in the solar system within the Kuiper Belt, according to the study authors. During the first billion years after the solar system formed roughly 4.5 billion years ago, as large gas planets migrated to their current orbits, Neptune’s gravitational pull captured Triton.
Once captured, Triton’s gravitational forces stirred up Neptune’s original moons, triggering collisions that destroyed most of them. Neptune’s current inner moons eventually formed from this debris. Triton is a bit larger than Pluto, though smaller than Earth’s moon, and stands out as the solar system’s only large moon orbiting in the opposite direction of its planet’s spin.
By contrast, Jupiter, Saturn, and Uranus each maintain an ordered system of large regular satellites orbiting in roughly circular paths in the direction of planetary rotation. “Neptune is the only giant planet in the solar system without an ordered system of large regular satellites,” Davis said, describing how Neptune ended up with a captured interloper rather than a standard family of moons.
The Sole Survivor: Clues from Nereid and Alternative Theories
Neptune currently boasts 16 known moons located about 30 times further from the sun than Earth. One outer moon, Nereid, displays among the most elliptical orbits in the solar system. According to the researchers, Nereid’s composition—revealed in a separate study—suggests it survived Triton’s wrath as the sole remnant among Neptune’s original moons.
While the team attributes the catastrophe to Triton’s arrival, they note another possible, though less likely, explanation: another Pluto-sized Kuiper Belt world may have passed too close to Neptune and been torn apart by its gravity. Triton itself features nitrogen ice, methane ice, and carbon dioxide on its surface, alongside a thin nitrogen atmosphere.
Frequently Asked Questions
How many moons does Neptune have?
Neptune has 16 known moons, with Triton serving as its largest satellite.
Why do Neptune’s inner moons contain clay minerals?
According to researchers, clay minerals on moons like Proteus, Larissa, and Galatea originated deep inside ancient worlds that were destroyed by a catastrophic collision event billions of years ago.
What makes Triton unique among the giant planets’ moons?
Triton is the solar system’s only large moon that orbits in the opposite direction of its planet’s spin, and it accounts for more than 99% of the mass of Neptune’s entire satellite system.
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