Neptune's tiny, mysterious moon may be a 'chipped-off piece' from a larger Moon as a result of collision with a comet that took place billions of years ago, scientists say.
NASA's Hubble Space Telescope had discovered the diminutive moon in 2013. Astronomers call it "the moon that shouldn't be there."
The tiny moon, named Hippocamp, is unusually close to a much larger Neptunian moon called Proteus. Normally, a moon like Proteus should have gravitationally swept aside or swallowed the smaller moon while clearing out its orbital path, researchers said.
The research, published in the journal Nature, shows that Hippocamp is likely a chipped-off piece of the larger moon that resulted from a collision with a comet billions of years ago.
The moon, only about 34 kilometers across, is 1/1000th the mass of Proteus.
"The first thing we realised was that you wouldn't expect to find such a tiny moon right next to Neptune's biggest inner moon," said Mark Showalter of the SETI Institute in the US.
"In the distant past, given the slow migration outward of the larger moon, Proteus was once where Hippocamp is now," said Showalter.
This scenario is supported by Voyager 2 images from 1989 that show a large impact crater on Proteus, almost large enough to have shattered the moon.
"In 1989, we thought the crater was the end of the story," said Showalter.
"With Hubble, now we know that a little piece of Proteus got left behind and we see it today as Hippocamp," he said.
The orbits of the two moons are now about 12,070 kilometers apart.
Neptune's satellite system has a violent and tortured history. Many billions of years ago, Neptune captured the large moon Triton from the Kuiper Belt, a large region of icy and rocky objects beyond the orbit of Neptune.
Triton's gravity would have torn up Neptune's original satellite system. Triton settled into a circular orbit and the debris from shattered Neptunian moons re-coalesced into a second generation of natural satellites.
However, comet bombardment continued to tear things up, leading to the birth of Hippocamp, which might be considered a third-generation satellite.
"Based on estimates of comet populations, we know that other moons in the outer solar system have been hit by comets, smashed apart, and re-accreted multiple times," said Jack Lissauer of NASA's Ames Research Center in the US.
"This pair of satellites provides a dramatic illustration that moons are sometimes broken apart by comets," said Lissauer.
Hippocamp is a half-horse half-fish from Greek mythology. The scientific name for the seahorse is Hippocampus, also the name of an important part of the human brain.
The rules of the International Astronomical Union require that the moons of Neptune are named after Greek and Roman mythology of the undersea world.