A new study led by the SETI Institute's astronomers suggests that the newly discovered moon of Neptune, Hippocamp, is probably an ancient fragment of a much larger neighboring river, Proteus.
During 1989, NASA's Voyager 2 spacecraft depicted six small moons of Neptune (Naiad, Thalassa, Despina, Galatea, Larissa, and Proteus), all revolving in the interior of the great moon Triton.
Along with a set of nearby rings, these inner moons are probably younger than the Neptune itself. They formed shortly after the capture of Triton, and most of them have probably been fragmented several times by cometical consequences.
On July 1
Named Hippocamp is the smaller than the other six, with a diameter of 20 miles (34 km).
The small moon, also known as the S / 2004 N 1, is exceptionally close to the much larger Proteus (260 km or 418 km in diameter).
The two-month circuit is currently 7,500 km (12,070 km) apart.
Usually, a moon like Proteus must have gravitationally swept aside or turned off the smaller moon while removing its orbit.
So why does that little moon exist? According to the new study, Hippocamp is probably a cut-off piece of Proteus, due to a collision with a few billion years ago.
"The first thing we realized was that you wouldn't expect to find such a small moon right next to Neptune's largest inner moon," says Dr. Showalter, leader of the study.
"In the distant past, when the slow migration was out of the larger moon, Proteus was once where Hippocamp is now."
This scenario is supported by Voyager 2 photos from 1989, showing a large battleground on Proteus, almost large enough to have crushed 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 left behind and we see it today as Hippocamp," Dr. Showalter says.
"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 several times," says co-author Dr. Jack Lissauer, a researcher at NASA's Ames Research Center.  "This pair of satellites gives a dramatic illustration that moons are sometimes broken apart by comets. "The study was published in the journal February 21, 2019 Nature .
MR Showalter et al. 2019. Neptune's Seventh Inner Moon. Nature 566: 350-353; doi: 10.1038 / s41586-019-0909-9