When NASA's Voyager 2 spacecraft flew past Neptune in 1989, it spotted something strange: six tiny moons hiding near the planet's rings. Decades later, scientists have finally figured out where these moons came from — and the answer involves a cosmic smashup billions of years ago.
Using the powerful James Webb Space Telescope, a team of researchers at Caltech studied three of Neptune's inner moons: Larissa, Galatea, and Proteus. What they found surprised everyone. These moons contain clay-like minerals called phyllosilicates — the first time such minerals have been detected anywhere in the outer solar system beyond Jupiter. The clays form only when rock and water mix together over long periods, which requires a decent-sized world with enough internal heat to melt ice. Yet none of these moons are large enough to have done that on their own.
The answer, the researchers believe, lies with Triton — Neptune's largest moon. Triton is unusual because it orbits in the wrong direction, leading scientists to think it didn't form alongside Neptune. Instead, it was probably wandering through space before Neptune's gravity snagged it. When Triton arrived, it crashed through whatever moon system Neptune already had, destroying those ancient worlds. The wreckage then clumped back together to form the small inner moons we see today.
"If Neptune once had a system of moons that looked something like what we see on Uranus today, we expect it would've been completely destroyed by the process of Triton getting captured," said Ryleigh Davis, the study's lead author, who is now a postdoctoral researcher at UC San Diego.
Even more curious, the spectra showed no water ice on these moons — which is strange for this region of the solar system, where ice is everywhere. The clays must have come from deep inside much larger ancient worlds that have since been broken apart and reshaped. One surviving moon, Nereid, may be the last intact piece of that original system.
Davis called the findings "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." The research was published in the journal Science Advances.