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A new study by astronomers from the SETI Institute suggests that the newly discovered Neptune moon, Hippocamp, is probably an older fragment of a much larger moon, Proteus.
An artist hippocamp concept. Image credit: NASA / ESA / J. Olmsted, STScI.
During its flight in 1989, NASA's Voyager 2 spacecraft captured six small moons of Neptune (Naiad, Thalassa, Despina, Galatea, Larissa and Proteus), all with an orbit well inside that of the great Triton moon.
With a set of rings nearby, these inner moons are probably younger than Neptune herself. They formed shortly after the capture of Triton and most of them were probably fragmented several times by cometary impacts.
On July 1, 2013, Dr. Mark Showalter of the SETI Institute discovered a seventh inner moon until then unknown on images taken by the Hubble Space Telescope of NASA / ESA.
Named Hippocamp, it is smaller than the other six, with a diameter of 20 km (34 km).
The tiny moon, also known as S / 2004 N 1, is exceptionally close to the much larger Proteus (260 km, or 418 km in diameter).
The orbits of the two moons are currently separated by 12,500 km.
Normally, a moon like Proteus should have gravitationally swept or swallowed the smaller moon while clearing its orbital trajectory.
So why does the little moon exist? According to the new study, Hippocamp is probably a chunk of chipped Proteus resulting from a collision with a comet billions of years ago.
This diagram shows the orbital positions of Neptune's inner moons, ranging in size from 20 to 260 miles. The outer Triton moon was captured in the Kuiper Belt several billion years ago. It would have torn the original Neptune satellite system. Triton settled in a circular orbit and the debris of broken moons found themselves in a second generation of inner satellites, as we know it today. However, the comet bombardment continued to tear things apart, resulting in the birth of Hippocamp, a broken piece of Proteus. It is therefore a third generation satellite. Neptune's best-known satellite, Nereid, which is in an extremely eccentric orbit, may well be a survivor of the era of Triton capture. Image credit: NASA / ESA / A. Feild, STScI.
"The first thing we realized is that you would not expect to find such a small moon next to Neptune's biggest inner moon," said Dr. Showalter, lead author of the study.
"In the distant past, given the slow migration of the great moon to the outside, Proteus was once the place where Hippocamp is now located."
This scenario is corroborated by Voyager 2 images from 1989 that show a crater of significant impact on Proteus, large enough to have broken the moon.
"In 1989, we thought the crater was the end of history. With Hubble, we now know that a small piece of Proteus has been forgotten and we see it today as Hippocamp, "said Dr. Showalter.
"According to estimates of comet populations, we know that other moons of the outer solar system have been affected by comets, broken and repeatedly re-accrued," said the co-author, Dr. Jack Lissauer, researcher at Ames Research at NASA. Center.
"This pair of satellites provides a dramatic illustration of the fact that moons are sometimes separated by comets."
The study was published in the February 21, 2019 issue of the journal Nature.
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M.R. Showalter et al. 2019. The seventh inner moon of Neptune. Nature 566: 350-353; doi: 10.1038 / s41586-019-0909-9
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