Origin of Neptune's Smallest Moon Hippocamp Revealed

It turns out the tiny moon is really a piece of larger moon Proteus.

Neptune has a moon so small that it was missed by NASA's Voyager 2 cameras when the probe flew by the planet in 1989. Called Hippocamp, it measures no more than 12 miles across.


The origins of the tiny moon

Origin of Neptune's Smallest Moon Hippocamp Revealed
Source: NASA

Now, the NASA/ESA Hubble Space Telescope, with help from older data from Voyager 2, has revealed more about the origins of this tiny moon. First discovered in 2013, it is believed to be a piece of one of its larger neighbors Proteus.

"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 project lead Mark Showalter of the SETI Institute. 

Hippocamp also has some strange orbit chemistry with much bigger neighbor Proteus. They are very close, only 12,000 km apart.

Normally, at such a distance the objects would have either seen the bigger one kick the smaller out of orbit or have crashed into each other. However, in this case, they simply co-exist peacefully.

A comet collision

The astronomers are speculating that billions of years ago a comet collision took a piece out of Proteus and saw the creation of Hippocamp. This theory has been supported by images from the Voyager 2 probe from 1989 showing a large impact crater on Proteus.


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"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."

Neptune's satellite system has had a very turbulent history with celestial objects crashing in and out of orbit and existence all the time. Proteus, for instance, came about from the upheaval that resulted when Neptune captured an enormous body from the Kuiper belt billions of years ago.

That object is now Neptune's largest moon Triton. Triton's unexpected massive size tore apart all the other satellites in orbit at that time and saw the debris from shattered moons re-organized into the system we witness today.

"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 co-author of the new research.

"This pair of satellites provides a dramatic illustration that moons are sometimes broken apart by comets."

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