Larissa (moon)

Larissa, also known as Neptune VII, is the fifth-closest inner satellite of Neptune. It is named after Larissa, a lover of Poseidon (the Greek equivalent of the Roman god Neptune).

Larissa

Larissa from Voyager 2
Discovery
Discovered by	Harold J. Reitsema, William B. Hubbard, Larry A. Lebofsky, and David J. Tholen[1]
Discovery date	May 24, 1981
Designations
Designation	Neptune VII
Pronunciation	/ləˈrɪsə/[2]
Named after	Λάρισσα Lārissa
Alternative names	S/1989 N 2 S/1981 N 1
Adjectives	Larissean,[3] Larissan,[4] Larissian[5] /ləˈrɪs(i)ən/
Orbital characteristics[6][7]
Epoch 18 August 1989
Semi-major axis	73 548.26 km
Eccentricity	0.001393 ± 0.00008
Orbital period (sidereal)	0.55465332 ± 0.00000001 d
Inclination	0.251 ± 0.009° (to Neptune equator) 0.205° (to local Laplace plane)
Satellite of	Neptune
Physical characteristics
Dimensions	216 km × 204 km × 168 km (± 6 km × 16 km × 4 km) km[8]
Mean radius	97±3 km[8]
Volume	~3.64×106 km3[a]
Mass	~(0.19–5.7)×1018 kg[b]
Mean density	~0.052–1.57 g/cm3[c]
Surface gravity	~0.001–0.054 m/s2[d]
Escape velocity	~0.015–0.095 km/s[e]
Synodic rotation period	synchronous
Axial tilt	zero
Albedo	0.09[8][12]
Temperature	~51 K mean (estimate)
Apparent magnitude	21.5[12]

Larissa is also the eponymous nymph of the city in Thessaly, Greece.

Discovery

Voyager 2 was the space probe that confirmed the existence of Larissa

Larissa was first discovered by Harold J. Reitsema, William B. Hubbard, Larry A. Lebofsky and David J. Tholen, based on fortuitous ground-based stellar occultation observations^[13] on May 24, 1981. It was given the temporary provisional designation S/1981 N 1 and its supposed existence was announced on May 29, 1981.^[14] The moon was later recovered and confirmed to be the only object in its orbit during the Voyager 2 flyby in 1989^[15] after which it received the additional designation S/1989 N 2 on August 2, 1989.^[16] The announcement by Stephen P. Synnott spoke of "10 frames taken over 5 days", which gives a recovery date sometime before July 28. The name was given and then confirmed by the International Astronomical Union on September 16, 1991.^[17]

Characteristics

Size comparison between Larissa (lower left), the Moon (upper left) and Earth

Morphographic map of Larissa, showing whatever unclear surface features that Voyager 2 was able to image during its flyby.^[9]

Larissa is the fourth-largest satellite of Neptune. It is irregular (non-spherical) in shape and appears to be heavily cratered, with no sign of any geological modification. It is likely that Larissa, like the other satellites inward of Triton, is a rubble pile re-accreted from fragments of Neptune's original satellites, which were disrupted by perturbations from Triton soon after that moon's capture into a very eccentric initial orbit.^[18]

Larissa's orbit is nearly circular and lies below Neptune's synchronous orbit radius, which means it is slowly spiralling inward due to tidal deceleration and may eventually impact Neptune's atmosphere, or break up into a planetary ring upon passing its Roche limit due to tidal stretching, similarly to how Triton will eventually collide with Neptune or break into a planetary ring.

Compositionally, Larissa appears to be similar to other small inner Neptunian satellites, with a deep 3.0 micron feature attributed to water ice or hydrated silicate minerals. It has a 0.08 albedo at 1.4 and 2.0 microns, dropping to 0.03 at 3.0 microns, and increasing to 0.09 at 4.6 microns.^[19]

Exploration

Side-by-side comparison of Larissa imaged by Voyager 2 and an enhanced version to the right

Larissa has only been visited once by Voyager 2 in 1989. The probe was able to get some photographs with details of Larissa, showing its cratered surface; unlike the other inner moons of Neptune that only appeared as dots or smudges.

Notes

1. A volume of (3.5±1.0)×10⁶ km³ was obtained from a detailed shape model, assuming dimensions of 208 km × 192 km × 178 km.^[9] The long and short dimensions were estimated based on a single image, with the medium dimension simply assumed as halfway between those values.^[10] The volume has been scaled to reflect more recently obtained dimensions of 216 km × 204 km × 168 km based on two images.^[8]
2. A density of 0.05–1.5 g/cm³ was calculated when assuming the volume as a sphere with a radius of 97±3 km, and assuming Larissa and Proteus share similar densities.^[11] The mass was calculated with the provided density and volume.
3. Density obtained from the calculated mass and the scaled volume.
4. Surface gravity derived from the mass m, the gravitational constant G and the radius r: ${\displaystyle {\frac {Gm}{r^{2}}}}$
5. Escape velocity derived from the mass m, the gravitational constant G and the radius r: ${\displaystyle {\sqrt {\frac {2Gm}{r}}}}$