New observations of some of Neptune's moons and rings are providing evidence of a catastrophe that occurred billions of years ago around the solar system's outermost planet in which a Pluto-sized interloper caused a celestial demolition derby.
Researchers used NASA's James Webb Space Telescope to examine the composition of three of Neptune's small inner moons – Proteus, Larissa and Galatea – and its inner dusty rings and detected clay minerals that one might expect to find deep inside certain worlds in the outer reaches of the solar system. These magnesium-rich minerals are known to exist on the dwarf planet Ceres, which orbits in the main asteroid belt between Mars and Jupiter and in some meteorites. "These minerals require prolonged contact between liquid water and rock to form," said planetary scientist Ryleigh Davis, a postdoctoral researcher at the University of California, San Diego and lead author of the study published in the journal Science Advances.
"The moons themselves are far too small and cold for that chemistry to have occurred in place, so the clay had to come from somewhere else – the deep interior of a much larger ancient world. Finding this material on Neptune's inner moons points to a catastrophic event in Neptune's past that destroyed those ancient worlds and exposed their deep interiors, with the small moons and rings we see today re-forming from that wreckage," said Davis, who worked on the study while at Caltech.
The researchers said the culprit behind this cataclysm appears to be Triton, which currently is Neptune's largest moon but previously resided farther out in the solar system in a region called the Kuiper Belt. They said that sometime during the first billion or so years after the solar system's formation roughly 4.5 billion years ago – a chaotic time when the large gas planets were still migrating to their current orbits – Neptune's gravitational pull captured Triton.
Triton is a bit larger than Pluto, another Kuiper Belt resident, though smaller than Earth's moon.
After being captured, Triton's own gravitational forces stirred up Neptune's original moons and caused most of them to collide with each other. Neptune's current inner moons eventually formed from some of the resultant debris.
"The interiors of large icy moons are normally permanently hidden from us, buried beneath thick shells of water ice and accessible only through models and inference. Neptune's inner moons may be the only place in the solar system where we can directly observe that material, because this catastrophic event essentially turned those ancient worlds inside out," Davis said.
Triton is the solar system's only large moon that orbits in the opposite direction of its planet's spin. This fact provides further evidence that Triton was captured after forming elsewhere rather than forming as one of Neptune's original moons.
The other three large gas planets – Jupiter, Saturn and Uranus – each has a system of large moons that formed around the planet, orbiting in a roughly circular path in the same direction that the planet rotates.
"Neptune is the only giant planet in the solar system without an ordered system of large regular satellites," Davis said. "So, Neptune ended up with a captured interloper dominating its satellite system rather than the orderly family of moons we see at the other giant planets."
Triton makes up more than 99% of the mass of Neptune's entire satellite system – its moons and rings.
Neptune, located about 30 times further from the sun than Earth is, has 16 known moons. Nereid, one of Neptune's outermost moons, displays among the most elliptical orbits of any moon in the solar system. The researchers said Nereid's composition, as revealed in another study, suggests it was the sole survivor among Neptune's original moons, somehow avoiding Triton's wrath.
Triton, thought to be similar in composition to Pluto, has nitrogen ice, methane ice and carbon dioxide on its surface, as well as a thin nitrogen atmosphere.
The researchers said another possible, though less likely, explanation for their observations is that another Pluto-sized Kuiper Belt world passed too close to Neptune and was ripped apart by its gravity.
The story of Neptune's moons, Davis said, is a reminder that the solar system has a violent history.
"It's a vivid example of how a single random event can fundamentally alter a planetary system's architecture," Davis said.
