Neptune’s small inner moons may actually be the broken remains of a much older family of icy worlds that were destroyed billions of years ago, according to a new study using NASA’s James Webb Space Telescope (JWST).
Scientists believe the evidence points to a dramatic event in the distant past, when Neptune captured its largest moon, Triton.
The capture may have completely disrupted the planet’s original system of moons, smashing them apart and leaving behind debris that later formed the small moons and rings we see today.
The findings, published in Science Advances, offer fresh clues about one of the biggest mysteries surrounding the solar system’s most distant giant planet.
Unlike the other giant planets, Neptune does not have a regular system of large moons orbiting in neat patterns.
This has long puzzled astronomers and suggested that something unusual happened during the planet’s history.
To investigate, researchers used JWST to study three of Neptune’s small inner moons—Larissa, Galatea and Proteus—as well as the planet’s rings. The telescope allowed scientists to examine the chemical makeup of these objects in greater detail than ever before.
The team made a surprising discovery. They found minerals called magnesium-rich phyllosilicates, a type of clay that forms only when rock interacts with liquid water. These minerals have never before been detected on objects this far from the Sun beyond Jupiter.
Even more surprising was what they did not find. The three moons and Neptune’s rings showed no clear signs of water ice, even though icy bodies are expected to dominate this region of the solar system.
The researchers believe the clay minerals must have come from deep inside much larger icy worlds. Large moons can generate enough internal heat to melt some of their ice into liquid water, allowing clay minerals to form beneath the surface. If those moons were later shattered, their hidden interiors would have been exposed.
Scientists think this is exactly what may have happened when Triton was captured by Neptune’s gravity after forming elsewhere in the solar system, likely in the Kuiper Belt. Triton’s arrival could have triggered violent gravitational disturbances that destroyed Neptune’s original moons. A small fraction of the debris later gathered together again, creating the tiny moons and rings that exist today.
One moon, however, did not fit the pattern. Proteus, the largest of the three moons studied, did not contain the same clay minerals. Researchers think it may have formed from a different part of the debris or experienced heating that erased the mineral evidence.
The team also found another mysterious hydrated mineral on all three moons that they cannot yet identify. Its chemical signature does not match anything currently found in scientific databases, suggesting there may still be unknown materials waiting to be discovered.
Although the researchers consider the destroyed-moon theory the most likely explanation, they have not ruled out another possibility. A large icy object from the Kuiper Belt, perhaps similar in size to Pluto, may have passed too close to Neptune and been torn apart by the planet’s powerful gravity, scattering its interior material across the system.
Whatever the true explanation, the study suggests that Neptune’s inner moons preserve material that normally remains buried deep inside large icy worlds. By studying these fragments, scientists are getting a rare glimpse into the hidden interiors of ancient moons that disappeared billions of years ago.
The researchers hope future studies will reveal exactly how Neptune’s moon system was rebuilt after the catastrophe and help determine how large the planet’s original moons once were.
Source: California Institute of Technology.
