Neptune's third-largest moon may be the only original one left. A new study argues Nereid is likely a survivor from the planet's original moon system, upending the long-standing idea that it was an interloper snatched from the distant Kuiper Belt, per Space.com. That capture scenario made sense because Nereid's orbit is wildly wide and elongated—traits usually linked to gravitational chaos, like the event that pulled in Neptune's giant moon Triton from the Kuiper Belt and was thought to wipe out any native moons.
Neptune's other original moons were likely shattered by Triton before coalescing into the 14 other moons we see today, per the AP. Nereid, however, found a route to survival, according to researchers, who used the James Webb Space Telescope to examine the moon for just over 10 minutes, then compared it with known Kuiper Belt objects. Nereid came out looking very different: richer in water ice, more reflective, and bluer, with no sign of the volatile organic compounds common on Kuiper Belt bodies, Space.com reports.
Computer simulations then showed how Triton's arrival could have pushed the 220-mile-wide moon farther away from Neptune without destroying it, resulting in its odd orbit—coming within 1 million miles of the planet at one end, and 6 million miles at the other, per the AP. The findings, published Wednesday in Science Advances, suggest Nereid is, indeed, "of more than routine interest" and a "clue" to Neptune's "unusual cosmogonic problem," as its 1949 discoverer, Gerard Kuiper, initially described.
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