Moons of Neptune: Frigid worlds.
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| Image of Neptune and its moons by JWST in 2022. Credit: NASA/ESA/CSA/STScl |
Neptune is the farthest planet from the Sun, so you may expect the planet and its surrounding moons to be cold and boring. However, Neptune has the fastest winds in the solar system, and its moon Triton has been an interesting curiosity since the Voyager 2 flybys. While there was an intruiging proposal to send a flyby mission to Triton, called Trident, it wasn't selected, and there are currently no missions at Neptune as of now. This article will take a look at Neptune's known moons, and explain what makes them unique.
Triton
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| Mosaic of Triton's subNeptunian hemisphere. Credit: NASA/JPL/USGS |
- Date Discovered: October 10, 1846 by William Lassell
- Radius: 235.8 Kilometres
- Surface Pressure: 0.0000163 Atmospheres
- Atmospheric Composition: Nitrogen, Methane, and Others
- Density: 2.061 g/cm^3
- Mass: 2.1395x10^22 Kilograms
- Surface Gravity: 0.779 m/sec^2
- Rotational Period: -5.877 Days*
- Orbital Period: -5.877 Days*
Triton was the first moon of Neptune to discovered. William Lassel discovered the moon 17 days after John Herschel discovered Neptune, and named it after the Greek son of Poseidon (Neptune). Triton is Neptune's largest moon, and is bigger than Pluto. Unsually for such a massive moon, Triton orbits retrograde to Neptune. This means that Triton orbits backwards relative to Neptunes rotation. This means that the moon couldn't have formed around Neptune, and must be a captured body. In order to lose enough momentum to get captured in Neptune's orbit, Triton might have been a binary dwarf planet similar to Pluto and Charon. When it approached Neptune, the binary partner was flung into the kuiper belt, or it smashed into Neptune. Tritan has an interesting terrain, that resembles a cantaloupe. Whe Voyager 2 flew by the moon in 1989, it imaged 40% of Triton's surface, and discovered that Triton possessed a thin atmosphere and geological activity. Triton is covered in cryovolcanos, especially in Leviathan Patera, which spew out ices instead of lava. While Olympus Mons on Mars is widely known to be the largest volcano in the solar system, Leviathan is the second largest. While Saturn's moon Titan is the only other body in the solar system to have liquid confirmed to be present on its surface, other than Earth of course, Triton my have small slushy ice lakes in intense cryovolcanic regions. Triton has proven so interesting, that there was a mission proposal known as Trident that was hoping to get selected for funding. The mission wasn't selected however, in favour of new missions to Venus.
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| Geomorphology of Triton. Credit: NASA/JPL/Antonio Ciccolella CC BY-SA 4.0 |
Proteus
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| Image of Proteus from Voyager 2. Credit: NASA/JPL |
- Date Discovered: June 16, 1989 by Stephen Synnott/Voyager 2
- Mass: 5.035525935^19 Kilograms
- Radius: 232.5 Kilometres
- Density: 1.3 g/cm^3
- Surface Gravity: 0.076 m/sec^2
- Rotational Period: 26.88 hours
- Orbital Period: 26.88 Hours
Proteus is unusual, because even though it's large enough to reach hydrostatic equilibrium (a rough sphere shape), it is somewhat box shaped. This is because of a low density. Saturn's moon Mimas is slightly smaller than Proteus, yet it has a spherical shape. Even though Proteus is Neptune's second largest moon, it wasn't discovered until just before Voyager 2's flyby of Neptune. This is because Proteus has a dark surface, which gets outshined by the light reflected off Neptune. The only named feature on Proteus is its large crater in the upper-right of the image, which was named Pharos.
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| Map of Proteus. Credit: Philip J. Stooke |
Larissa
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| Voyager 2 Image of Larissa. Credit: NASA/JPL |
Larissa is one of Neptune's small inner moons that was able to be imaged at low resolution by Voyager 2. Larrisa is Neptune's fourth largest satellite, and may have many craters on its surface, though images are too low resolution to know for certain. Like some near Earth asteroids, such as Bennu and Ryugu, Larissa may be a rubble pile. This means that the moon isn't one cohesive object, but many smaller rocks held together by mutual gravity.
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| Map of Larissa. Credit: Philip J. Stooke |
Naiad
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| Image of Naiad from Voyager 2. The image is smeared due to the spacecraft's movement. Credit: NASA/JPL |
Naiad is Neptune's innermost known satellite, and was named after a type of nymph from Greek mythology. Like Larissa, Naiad is believed to be a rubble pile, but not much else is known about it.
Nereid
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| Image of Nereid from Voyager 2. Credit: NASA/JPL |
Despite being much smaller than Proteus, Nereid was the second moon of Neptune to be discovered. Gerard Kuiper, who also discovered Miranda, named Nereid after the sea-nymphs from Greek Mythology. While Neried is the third largest of Neptune's satellites, its still quite small, and orbits far out from the Neptune system. Due to Neptune's large size, and distance from the Sun, it has the largest Hill Sphere in the solar system. A Hill Sphere is the region around a planet were a stable orbit is possible. Neried's orbit is almost as long as an Earth year.
Hippocamp
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| Atist's impression of Hippocamp. Credit: ESA/Hubble |
While most of Neptune's moons were discovered during Voyager 2's flyby, Hippocamp remained hidden. The tiny moon was only discovered in 2013 by a team using the Hubble Space Telescope. In 2019, the moon was named Hippocamp, after sea-horses (not seahorses, but literal sea horses) from Greek Mythology.
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| 2009 Hubble image with Hipocamp circled. Credit: NASA/ESA/M. Showalter (SETI Institute) |
Neptune hosts several other small moons, which are listed below. Be sure to check out a similar article I did on the moons of Uranus here.
- Galatea: Nereid from Greek Mythology
- Despina: Daughter of Poseidon and Demeter
- Thalassa: Greek word for "sea"
- Halimede: Nereid from Greek Mythology
- Neso: Nereid from Greek Mythology
- Sao: Nereid from Greek Mythology
- Laomedeia: Nereid from Greek Mythology
- Psamathe: Nereid from Greek Mythology
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