Last updated September 29, 2024

The Moons of Neptune: A Procession of Ice and Mystery

The Last Planet of the Solar System

Neptune, the eighth planet of the Solar System, is surrounded by a complex system of 14 confirmed moons. These icy bodies, the most famous of which is Triton, form a dynamic and enigmatic ensemble. Triton, discovered in 1846, dominates this procession with its unique characteristics: retrograde orbit, geological activity, and nitrogen-rich composition. The other moons, such as Nereid, Proteus, or Larissa, have irregular orbits suggesting capture by Neptune's gravity.

The Major Moons of Neptune

The Neptunian system is dominated by Triton, which represents 99.7% of the total orbital mass. With a diameter of 2,700 km, it is the only major moon in the system. Its particularities include:

A retrograde orbit (opposite direction to Neptune's rotation), likely a sign of gravitational capture.
A young surface with nitrogen geysers and cryovolcanoes.
A tenuous atmosphere composed mainly of nitrogen.

The other moons, such as Nereid (discovered in 1949), have eccentric and inclined orbits, while Proteus (1989) is the second largest satellite after Triton, with a dark and cratered surface.

Origin and Orbital Dynamics

The moons of Neptune are divided into two groups:

Regular moons (such as Proteus and Larissa): Prograde and circular orbits, probably formed in situ.
Irregular moons (such as Triton and Nereid): Captured by Neptune, their orbits are inclined and eccentric.

The capture of Triton would have disrupted the original system, explaining the absence of large prograde moons. Simulations suggest that Triton originated from the Kuiper Belt, and its arrival would have ejected Neptune's primordial moons.

Characteristics of the major and minor moons of Neptune
Moon	Diameter (km)	Mass (×10¹⁹ kg)	Orbit (days)	Particularities
Triton	2 706	214	5.88 (retrograde)	Only major moon, geologically active
Proteus	420	0.5	1.12	Largest irregular satellite after Triton
Nereid	340	0.3	360.1	Most eccentric orbit (e = 0.75)
Larissa	194	0.05	0.55	Asymmetric shape, rich in water ice
Galatea	176	0.04	0.43	Influence on the Adams ring
Despina	150	0.02	0.33	Orbit inside the Le Verrier ring
Thalassa	82	0.001	0.31	Nearly circular orbit at 50,000 km
Naiad	66	0.0002	0.29	Closest known moon (48,227 km)
Halimede	62	0.0001	1 879	Possible fragment of Nereid
Psamathe	40	0.00004	9 074	Discovered in 2003, very distant orbit

Sources: NASA JPL, Icarus (2023), Nature Astronomy (2022).

Future Perspectives

No dedicated mission to Neptune is currently underway, but proposals such as Trident (NASA) or Odysseus (ESA) aim to study Triton and its interaction with Neptune's magnetosphere. The technical challenges are significant due to the distance (4.5 billion km from the Sun), but the scientific stakes justify these efforts:

Understanding the origin of Triton and its impact on the Neptunian system.
Studying cryovolcanic processes and potential organic chemistry.
Mapping minor moons to reconstruct the history of the system.

The moons of Neptune, particularly Triton, offer a unique window into the processes of capture and dynamic evolution in the outer Solar System. Their study could shed light on the formation of giant planets and the mechanisms of planetary migration. With future space missions, these icy worlds could reveal major surprises, reinforcing their status as natural laboratories for exoplanetology.