Meet Neptune’s tiny new moon, beautifully named after a mythological seahorse

neptune's new moon
© ESA/Hubble, NASA, L. Calçada

For billions of years, a tiny moon has been slyly orbiting the ice giant – now the little lovely has a poetic name, as well as a surprisingly violent backstory.

A few billion years ago, a comet crashed into one of Neptune’s moon, Proteus – just another day in the life of a celestial body. While the impact was almost enough to shatter the moon in half, she remained intact – but not before sending off a little offspring into world.

That fragment of Proteus has been doing the rounds along with the other moons ever since, but had gone undetected by us voyeurs down here. Until 2013, that is, when she was discovered by some eagle-eyed astronomers combing through pictures from the Hubble Space Telescope.

“It was just incredibly difficult to detect,” says Mark Showalter from the SETI Institute, who first spotted the moon in 2013 and describes it in the journal Nature.

Imagine discovering a new moon – and then being tasked with naming it? The load of that honor might be a bit intimidating, but for Showalter, it wasn’t a problem. He named it Hippocamp.

“When it came time to pick a name out of Greek and Roman mythologies from the seas, it was like, Oh, that’s not a hard one,” he says.

The rules of the International Astronomical Union require that the names of Neptune’s moons be selected from Greek and Roman mythology figures of the undersea world. The mythological hippocampi had the upper body of a horse and the tail of a fish. They pulled Neptune’s aquatic chariot through the water and were often the mounts of nymphs and other various inhabitants of the sea. Their name has been given to both the modern seahorse (whose genus is Hippocampus) as well as the seahorse-shaped bit in the human brain, where it forms an important part of the limbic system, the region that regulates emotions.

The newly discovered moon has a wee diameter of just 21 miles and stays in close contact with its mother moon, whose orbit is about 7500 miles away. Hippocamp is the seventh inner moon of Neptune, and brings the total to 14.

The European Space Agency explains that Hippocamp is part of a long and violent history of Neptune’s satellite system. Even big Proteus was the result of a cataclysmic event involving Neptune’s satellites, writing: “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.” Given that Hippocamp was born from a later bombardment, she is 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.”

Nobody said it was easy being one of the ice giant's moons, but at least the newest member of the pack has a sweet new name and a host of new admirers five planets away.

To read more about the new moon and its discovery, visit Nature.

Meet Neptune’s tiny new moon, beautifully named after a mythological seahorse
For billions of years, a tiny moon has been slyly orbiting the ice giant – now the little lovely has a poetic name, as well as a surprisingly violent backstory.

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