Did Neptune's Tiny Moon Come from Proteus?
Author: A Millisecond of Eternity
“The Moon That Should Not Exist”
After several years of analysis, a team of planetary scientists has finally explained the mysterious moon orbiting Neptune that they discovered with the Hubble Space Telescope in 2013.
The small moon, named Hippocamp, lies unusually close to a much larger Neptunian moon, Proteus. Under normal circumstances, a moon like Proteus should use its gravity to sweep smaller moons aside or swallow them as it orbits.
Why, then, does such a small moon still exist? Hippocamp is probably a fragment of the larger moon, broken off in a collision with a comet billions of years ago. Only 20 miles (about 34 kilometers) in diameter, the tiny moon has just one-thousandth the mass of Proteus, which is about 260 miles (about 418 kilometers) across.
“The first thing we realized was that you would not expect to find such a tiny moon right next to Neptune’s largest inner moon,” said Mark Showalter of the SETI Institute in Mountain View, California. “In the distant past, as the larger moon slowly migrated outward, Proteus was once where Hippocamp is now.”
Images taken by Voyager 2 in 1989 support this idea. They show an enormous impact crater on Proteus, almost large enough to have shattered the moon. “In 1989, we thought the huge depression must simply be a crater,” Showalter said. “With Hubble, we now know that a small piece of Proteus was left behind—the object we know today as Hippocamp.” The moons’ orbits are now separated by 7,500 miles (about 12,070 kilometers).
The “developmental history” of Neptune’s moon system has been remarkably violent and convoluted. Billions of years ago, Neptune captured its large moon Triton from the Kuiper Belt—a densely populated annular region beyond Neptune’s orbit filled with icy and rocky bodies. Triton’s gravity disrupted Neptune’s original system of moons before Triton settled into a relatively stable circular orbit. Fragments of the shattered moons then recombined to form Neptune’s second-generation moon system. Continued bombardment by comets, however, broke apart those second-generation moons and may have produced Hippocamp, which could be regarded as a third-generation moon.
“Based on estimates of the enormous number of comets, it is fairly clear that other moons in the outer Solar System have been struck and disrupted by comets many times, then reassembled, in a cycle that repeats over and over,” said Jack Lissauer of NASA’s Ames Research Center in Silicon Valley, California, a coauthor of the new study. “Proteus and Hippocamp provide a somewhat dramatic example of how moons are sometimes broken into several pieces by comets.”
Hippocamp, originally designated S/2004 N 1, is a small moon of Neptune about 34 kilometers (20 miles) in diameter. It orbits the planet in less than one Earth day. Its discovery on July 1, 2013, brought Neptune’s number of known moons to 14. When the Voyager 2 space probe flew past Neptune and its moons in 1989, Hippocamp was too dim to be observed. Mark Showalter of the SETI Institute discovered the moon by analyzing archival Hubble Space Telescope images of Neptune taken between 2004 and 2009.
The moon was formally numbered Neptune XIV on September 25, 2018, and named Hippocamp in February 2019.
The word Hippocamp comes from the horse-headed, fish-tailed creature of Greek mythology. The scientific name of the seahorse is Hippocampus, which is also the name of the important brain region associated with memory. The International Astronomical Union requires Neptune’s moons to be named after creatures from the undersea worlds of Greek and Roman mythology.
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