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Neptune's smallest moon may be a piece of a nearby moon that has been overturned by a comet.
The hippocamp, one of the seven moons that revolve around Neptune and closer to the largest moon on the planet, has a diameter of about 34 kilometers, researchers said February 21 Nature. The second largest moon, Proteus, is the nearest neighbor to Hippocamp. It is around 12,000 kilometers.
"Why would such a small moon be so close to such a big moon?", Is interviewed astronomer Mark Showalter of the SETI Institute in Mountain View, California. the new moon, reported for the first time in 2013 (SN online: 16/07/13). Large moons tend to engulf smaller ones as moons form. Even stranger, the pair was probably even closer in the past, as the physics of their orbits suggests that Proteus is slowly migrating from Neptune.
Light gray dot
Neptune has seven inner moons (pictured here by the rings of the planet), and the smaller one now has a name: Hippocamp.
Source: A.J. Verbiscer /Nature 2019, M.R. Showalter et al/Nature 2019
Showalter thinks that Hippocamp is a piece of Proteus that was destroyed by a massive collision with a comet. Proteus has a massive crater, called Pharos, which is so big that the impact that broke it almost broke Proteus. The collision could have created a cloud of debris that eventually fused to form a smaller moon.
"They are far now, but they were part of the same thing in the past," says Showalter.
The name Hippocamp refers to a mid-horse / half-fish creature of Greek mythology, who was pulling a chariot for the god Poseidon (the Roman Neptune). All moons of Neptune have names related to the sea in Greek and Roman mythology.
This suits Showalter, who is an autonomous diver. "The true story is that I really like sea horses," he says.
Editor's note: This story was updated on February 20, 2019 with a new illustration of Hippocamp by an artist.
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