The violent past of Neptune's newest moon © ESA/Hubble, NASA, L. Calçada

Hippocamp, Neptune’s newest moon, had a violent past

Discovered in 2013, the Neptune's smallest-known may be a chunk of its larger neighbour moon, Proteus.

Astronomers studying Neptune’s most recently-discovered moon have uncovered the story of its birth. The moon, named Hippocamp, was discovered in 2013 from archived photos from the Hubble Space Telescope.

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Hippocamp is Neptune’s smallest moon at only 34 kilometres across, and hovers surprisingly close to Proteus, the largest of Neptune’s inner moons. Proteus is a giant compared to Hippocamp, with a diameter of around 400km, and the two are only 12,000km apart.

For this reason, the team of astronomers led by Mark Showalter of the SETI Institute believe Hippocamp must have formed from debris blasted from Proteus after a collision with a comet. If it had simply wandered into Proteus’s orbit from elsewhere in the Solar System, the larger moon would have either booted it away or swallowed it.

Adding credence to this theory, the Voyager 2 probe, which studied the outer planets of the Solar System, revealed a crater on Proteus from an impact nearly large enough to blow the moon to pieces.

©  Roberto Molar Candanosa/Carnegie Institution for Science

Proteus itself is a second-generation moon of Neptune, formed from the reconstituted remains of Neptune’s earliest system of moons, which were destroyed when the planet captured its moon Triton from the Kuiper belt, an area of rocky debris that lies beyond Uranus – making Hippocamp a third-generation moon.

Showalter and his team were also the ones to discover Hippocamp in 2013 from Hubble’s images of the Neptune system from 2004 to 2009, bringing the number of Neptune’s moons up to 14. The moon’s diminutive size is the reason it took so long to be discovered. “This is the smallest known moon around the farthest known planet in the Solar System,” said Showalter.

The discovery of Hippocamp pushed the limits of detection: it is so small and faint that it is impossible to distinguish from images alone. However, the team developed new specialised image analysis techniques to compensate for the blurriness of the images by stacking several exposures on top of one another, revealing Hippocamp as a bright spot.

The name Hippocamp comes from Greek mythology: hippocamps, or sea-horses, were the half-horse, half-fish creatures which pulled Neptune’s chariot. This name is in line with the International Astronomical Union’s guideline that Neptune’s moons must be named for sea deities from Greek mythology.


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