Science & Technology

There's a Tiny Moon Around Neptune with a Mysterious Origin Story

A moon at Neptune is literally a chip off the old block. Scientists discovered that a tiny world — whimsically named "Hippocamp" — must have been formed after a comet crashed into a much larger moon billions of years ago.

Related Video: Previously Unknown Moon of Neptune Detected

Flea Moon

Scientists say it's unlikely that Hippocamp should even exist. First discovered with the Hubble Space Telescope in 2013, Hippocamp orbits weirdly close to a much larger Neptune moon called Proteus. It's a strange situation because, under most circumstances, Proteus should have eaten up Hippocamp or at least shoved it aside after eons of gravitational interactions.

Compared to Proteus, Hippocamp is like a flea. It's only 20 miles (34 kilometers) across and about one-one-thousandth the mass of Proteus (which itself is 260 miles or 420 kilometers across.) So Hippocamp must be very young indeed, at least in cosmic terms. Remember that the solar system is 4.5 billion years old, so even millions of years is a short time in this region.

This is an artist's concept of the tiny moon Hippocamp that was discovered by the Hubble Space Telescope in 2013. Only 20 miles across, it may actually be a broken-off fragment from a much larger neighboring moon, Proteus, seen as a smaller crescent in the upper right. This is the first evidence for a moon being an offshoot from a comet collision with a much larger parent body.

"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 the lead author of the study, Mark Showalter of the SETI Institute in Mountain View, California, in a statement. "In the distant past, given the slow migration outward of the larger moon, Proteus was once where Hippocamp is now."

He said the new study is backed up by observations, especially Voyager 2 pictures in 1989, when the spacecraft was on its way past Neptune to an eventual path out of the solar system. Voyager 2 images revealed a huge impact crater on Proteus, demonstrating it was hit by something so big that it almost 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."

Fly Into the Danger Zone

It shouldn't be too big of a surprise that Hippocamp has a violent history. Neptune is a known neighborhood for weird interactions. Billions of years ago, Neptune snagged a huge moon called Triton from the Kuiper Belt, which is a zone of icy and rocky objects beyond Neptune's orbit. (It also happens to be the zone where dwarf planet Pluto orbits.)

When Triton came into Neptune's system, it changed everything. This moon eventually moved into a circular orbit around Neptune, but in the process, it destroyed other moons in the region. Ultimately, the shards of these old moons collected into a new generation of moons. But because the solar system is a chaotic place, things would change from time to time as comets or small worlds would smash into moons in the Neptune system. That's how Hippocamp came to be.

"Based on estimates of comet populations, we know that other moons in the outer solar system have been hit by comets, smashed apart, and reaccreted multiple times," said co-author Jack Lissauer of NASA's Ames Research Center in the same statement. "This pair of satellites provides a dramatic illustration that moons are sometimes broken apart by comets."

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For more wonders of our solar neighborhood, check out "Solar System: An Exploration of the Bodies that Orbit the Sun" by Marcus Chown. We handpick reading recommendations we think you may like. If you choose to make a purchase, Curiosity will get a share of the sale.

Written by Elizabeth Howell February 21, 2019

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