Hubble Uncovers Origin of Neptune’s Smallest Moon Hippocamp

This artist’s impression shows the outermost planet of the Solar System, Neptune, and its small moon Hippocamp. Hippocamp was discovered in images taken with the NASA/ESA Hubble Space Telescope. Whilst the images taken with Hubble allowed astronomers to discover the moon and also to measure its diameter, about 34 kilometers, these images do not allow us to see surface structures. Credit:
ESA/Hubble, NASA, L. Calçada

Astronomers using the NASA/ESA Hubble Space Telescope, along with older data from the Voyager 2 probe, have revealed more about the origin of Neptune’s smallest moon. The moon, which was discovered in 2013 and has now received the official name Hippocamp, is believed to be a fragment of its larger neighbor Proteus.

A team of astronomers, led by Mark Showalter of the SETI Institute, have used the NASA/ESA Hubble Space Telescope to study the origin of the smallest known moon orbiting the planet Neptune, discovered in 2013.

“The first thing we realized was that you wouldn’t expect to find such a tiny moon right next to Neptune’s biggest inner moon,” said Mark Showalter. The tiny moon, with an estimated diameter of only about 34 km, was named Hippocamp and is likely to be a fragment from Proteus, Neptune’s second-largest moon and the outermost of the inner moons. Hippocamp, formerly known as S/2004 N 1, is named after the sea creatures of the same name from Greek and Roman mythology.

This composite image shows the location of Neptune’s moon Hippocamp, formerly known just as S/2004 N 1, orbiting the giant planet Neptune, about 4.8 billion kilometers from Earth. The moon is only about 34 kilometers in diameter and dim, and was therefore missed by NASA’s Voyager 2 spacecraft cameras when the probe flew by Neptune in 1989. Several other moons that were discovered by Voyager appear in this 2009 image, along with a circumplanetary structure known as ring arcs. Mark Showalter of the SETI Institute discovered Hippocamp in July 2013 when analyzing over 150 archival images of Neptune taken by Hubble from 2004 to 2009. The black-and-white image was taken in 2009 with Hubble’s Wide Field Camera 3 in visible light. Hubble took the color inset of Neptune on August 19, 2009. Credit: NASA, ESA, and M. Showalter (SETI Institute)

The orbits of Proteus and its tiny neighbor are incredibly close, at only 12,000 km apart. Ordinarily, if two satellites of such different sizes coexisted in such close proximity, either the larger would have kicked the smaller out of orbit or the smaller would crash into the larger one.

Instead, it appears that billions of years ago a comet collision chipped off a chunk of Proteus. Images from the Voyager 2 probe from 1989 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.”

This artist’s impression shows how the smallest known moon of Neptune, now named Hippocamp, might look close up. In the animation the camera rotates once around the tiny moon, showing first the distant Sun and at the end the planet Neptune, which the moon is orbiting. While the NASA/ESA Hubble Space Telescope allowed researchers to discover the moon and to measure its size, the images from Hubble do not allow us to see surface structures. Credit: ESA/Hubble, L.Calçada

Hippocamp is only the most recent result of the turbulent and violent history of Neptune’s satellite system. Proteus itself formed billions of years ago after a cataclysmic event involving Neptune’s satellites. The planet captured an enormous body from the Kuiper belt, now known to be Neptune’s largest moon, Triton. The sudden presence of such a massive object in orbit tore apart all the other satellites in orbit at that time. The debris from shattered moons re-coalesced into the second generation of natural satellites that we see today.

Later bombardment by comets led to the birth of Hippocamp, which can therefore 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,” noted Jack Lissauer of NASA’s Ames Research Center, California, USA, a coauthor of the new research. “This pair of satellites provides a dramatic illustration that moons are sometimes broken apart by comets.”

Paper: Discovery of Neptune’s Inner Moon Hippocamp with the Hubble Space Telescope