Neptune: Puzzles of the "impossible" moon solved

Newly discovered Moon Hippocamp emerged from the wreckage of its neighbor

The tiny Neptune Moon Hippocampus (front) is a Lazarus: It was created several times anew from the rubble of impacts in the Neptune system. © NASA, ESA and J. Olmsted (STScI)
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Actually, it should not exist at all: The Neptune's only 14th moon, which was discovered in 2016, barely circles within the path of the 4, 000-fold larger satellite Proteus. Why he was still not attracted to this and destroyed, now researchers have found out. Accordingly, the little moon Hippocampus emerged from the rubble of a huge impact on Proteus - and could have since been destroyed several times and re-aggregated, as the scientists report in the journal "Nature".

The Neptune and its moons have a rich history behind them. Already in the early days of the solar system the ice planet exchanged the place with its neighbor Uranus and caused turbulences in the Kuiper belt. A little later, he caught a 2700-kilometer Kuipergürtel object - it became the largest Neptunmond Triton. However, much of the original moons of Neptune was destroyed, new satellites emerged from the rubble, including the small inner moons Naiad, Thalassa, Despina, Galatea and Larissa.

Orbits of the inner Neptune moons and the Triton. © NASA / ESA, A. Feild (STScI)

A new moon of Neptune

In 2016, however, another representative of the inner Neptune moons was discovered: In images of the Hubble Space Telescope, astronomers had discovered a faint point of light that circled Neptune just inside Proteus. The object baptized S / 2004 N 1 was too small and far away to find out more.

Now, Mark Showalter of the SETI Institute in California and his colleagues have used new observation data and a modern method of image processing to learn more about the newly discovered moon. Using an algorithm, they superimposed several images of the Hubble Space Telescope so that the moon and its orbit could be clearly seen despite the limited exposure time.

At the same time, the researchers now propose Hippocampus as a new name for S / 2004 N 1 after a "seahorse" from Greek mythology. display

"Impossible" orbit

The result: Hippocampus is actually the smallest moon of Neptune - its diameter is only 34 kilometers. But the really amazing thing is its orbit: The tiny creature circles only 12, 000 kilometers within Proteus - an object with around 4, 000 times its size and around 1, 000 times the mass. "You would not expect to find such a tiny moon right next to Neptune's largest inner moon, " says Showalter.

The reason: Normally, the gravity of the great Proteus would have long to attract and destroy the little hippocampus. Instead, calculations showed that the little Trabant was even closer to its big neighbor. However, because Proteus drifts outward a little faster than Hippocampus, their distance has increased, as the researchers report.

Size comparison of Proteus and Hippocamp. Mark R. Showalter / SETI Institute

Made of debris from Proteus

But how is this strange behavior explained? An indication of this is the fact that the Pharos crater on Proteus provides a sink that is almost half the diameter of the moon. "This crater is unusually large in terms of the size of the moon. That might indicate that Proteus barely escaped destruction in this impact, "say Showalter and his colleagues. They suspect that this cosmic collision ejected large amounts of rock streams several billion years ago, and possibly the "raw material" for the small hippocampus.

"Some of these bodies could have accumulated in a stable orbit around 1, 000 to 2, 000 kilometers within the Proteus, " the researchers said. Some of these chunks clenched into Hippocamp. That could explain the great proximity of both moons, the Pharos crater and the small size of Hippocampus. Should this scenario be confirmed, Hippocampus would be the "child" of Proteus.

Hippocampus as a cosmic "Lazarus"

But that did not end the turbulent history of Hippocamp. Because from the orbit of the little moon, the researchers conclude that he, like his inner neighbors, was repeatedly hit by icy comets in the course of time, but repeatedly emerged from the wreckage like in little Lazarus. The researchers estimate that Hippocampus may have been "resurrected" nine times in the last four billion years.

"Based on estimates of the comet population in this area, we know that other moons of the outer solar system were also hit by comets and then collapsed, " said co-author Jack Lissauer of NASA's Ames Research Center. (Nature, 2019; doi: 10.1038 / s41586-019-0909-9)

Source: NASA, SETI Institute

- Nadja Podbregar