Overview[]
Larissa, sometimes referred to as Neptune VII, is Neptune's fifth-closest inner moon in terms of distance. It's the fourth-largest Neptunian natural satellite, and the moon was given two pairs of temporary provisional designations known as S/1981 N 1 and S/1989 N 2 because previously, the moon was discovered by ground-based stellar occultations observations, although it was lost later on; afterwards, it was rediscovered again by the Voyager 2 probe. The moon was named after Larissa, a nymph and lover of Poseidon.[1] Just like the other inner moons of Neptune, its orbit lies within Neptune's synchronous orbit radius (i.e., where moons orbit faster than their planet rotates), which causes an effect called tidal deceleration, leading Larissa to gradually spiral inwards into Neptune. Because of this, in the far future, Larissa will either get fragmented into rings due to traversing too close to the Roche limit or disintegrate while passing the Neptunian atmosphere. Larissa was formed akin to the inner Neptunian moons from the debris that reaccreted from fragments from the original Neptunian moons, caused by Triton perturbing the moons after being captured into an impermanent elliptical orbit.[2]
Discovery[]
Larissa was initially discovered on 24 May, 1981, by Harold Reitsema, William Hubbard, Larry Lebofsky, and David Tholen from a stellar occultation detected by ground-based observations.[3] The stellar occultation has been predicted since January 1981, and it was predicted that the uncatalogued star would be very adjacent to Neptune, although it is not getting occulated, and takes place on 24 May.[4][5] The main goal of the observation was to discover Neptune's ring system, and analyze its atmosphere.[3][4] Utilizing two observatories, the uncatalogued star dimmed over 3-4% for over 8.1 seconds, suggesting the presence of a natural satellite orbiting Neptune, instead of a ring system.[3] Its minimum diameter was measured to be about 180 km (111 mi), and its distance from Neptune was calculated to be around 3 Neptune radii from Neptune's center if the moon orbits in Neptune's equatorial plane, based on the stellar occultation data alone.[3] Neptune's atmosphere occulated the uncatalogued star for at least a single minute, while Neptune's ring system occulated it twice, probably from Neptune's moons.[3] The presumed discovery of the moon was announced on 24 May, 1981 and given a provisional designation S/1981 N 1.[5]
S/1981 N 1 was rediscovered in late July 1989, when Despina, Galatea, and Larissa were found by the Voyager Imaging Team during Voyager 2's flyby mission of Neptune.[6] Afterwards, it received another additional provisional designation, S/1989 N 2, on 2 August, 1989,[6] but the team didn't recognize that S/1981 N 1 was related to S/1989 N 2 but instead researchers earlier suggested S/1981 N 1 as a ring arc,[7] although it was later confirmed that S/1981 N 1 and S/1989 N 2 are the same moon.[5] Larissa was given a proper name by the IAU in 16 September, 1991.[8]
Physical characteristics[]
Larissa is just like the other inner moons of Neptune; it is irregularly shaped and heavily bombarded by impact craters. In spite of that, Larissa is one of the only moons of Neptune that are imaged close enough for the Voyager 2 probe to see details instead of smudges; the other ones are Proteus and Triton. Its diameter is approximately 171.44 km. Larissa was created from material reaccreted from remnants of the original Neptunian moons, which experienced mutual collisions and self-destruction by Triton's gravitational influence perturbing them after being recently caught into an initial elliptical orbit. After Triton circularized its orbit, the second-generation inner moons of Neptune formed afterwards, one of which is Larissa.[9]
Motion[]
The moon completes its orbit around Neptune every 13 hours and 18 minutes, maintaining an average distance to Neptune of about 73,548 km. Its close distance to Neptune means it is probably tidally locked, like most of the inner moons of Neptune. Larissa orbits beyond the rings of Neptune, indicating that it isn't a shepherd moon, unlike Galatea and Despina. It's probable that Larissa and the larger inner moon, Proteus, were previously in a 1:2 orbital resonance; however, over a few hundred million years, Proteus steadily drifted outward due to tidal migration, and as a result, Proteus and Larissa are no longer in an orbital resonance.[10]
Gallery[]
- ↑ https://science.nasa.gov/neptune/moons/larissa/
- ↑ https://www.sciencedirect.com/science/article/abs/pii/001910359290155Z?via%3Dihub
- ↑ 3.0 3.1 3.2 3.3 3.4 https://ui.adsabs.harvard.edu/abs/1982Sci...215..289R/abstract
- ↑ 4.0 4.1 https://ui.adsabs.harvard.edu/abs/1981AJ.....86..135M/abstract
- ↑ 5.0 5.1 5.2 http://www.cbat.eps.harvard.edu/iauc/03600/03608.html
- ↑ 6.0 6.1 http://www.cbat.eps.harvard.edu/iauc/04800/04824.html
- ↑ http://www.cbat.eps.harvard.edu/iauc/04000/04022.html
- ↑ http://www.cbat.eps.harvard.edu/iauc/05300/05347.html
- ↑ https://www.sciencedirect.com/science/article/abs/pii/001910359290155Z?via%3Dihub
- ↑ https://ui.adsabs.harvard.edu/abs/2007Icar..188..386Z