Moons of Neptune
Triton is massive enough to have achieved hydrostatic equilibrium and to retain a thin atmosphere capable of forming clouds and hazes.
[4] The third moon, later named Larissa, was first observed by Harold J. Reitsema, William B. Hubbard, Larry A. Lebofsky and David J. Tholen on 24 May 1981.
The astronomers were observing a star's close approach to Neptune, looking for rings similar to those discovered around Uranus four years earlier.
[6] In 2001, two surveys using large ground-based telescopes found five additional outer irregular moons, bringing the total to thirteen.
[7] Follow-up surveys by two teams in 2002 and 2003 respectively re-observed all five of these moons, which are Halimede, Sao, Psamathe, Laomedeia, and Neso.
[7] In 2013 Mark R. Showalter discovered Hippocamp while examining Hubble Space Telescope images of Neptune's ring arcs from 2009.
He used a technique similar to panning to compensate for orbital motion and allow stacking of multiple images to bring out faint details.
[9][10] After deciding on a whim to expand the search area to radii well beyond the rings, he found an unambiguous dot that represented the new moon.
[9] In 2021, Scott S. Sheppard and colleagues used the Subaru Telescope at Mauna Kea, Hawaii and discovered two more irregular moons of Neptune, which were announced in 2024.
The first group includes the seven inner moons, which follow circular prograde orbits lying in the equatorial plane of Neptune.
[19] In order of distance from Neptune, the regular moons are Naiad, Thalassa, Despina, Galatea, Larissa, Hippocamp, and Proteus.
Proteus is not significantly elongated, but not fully spherical either:[6] it resembles an irregular polyhedron, with several flat or slightly concave facets 150 to 250 km in diameter.
[25] Their spectra indicate that they are made from water ice contaminated by some very dark material, probably complex organic compounds.
[6] In order of their distance from the planet, the irregular moons are Triton, Nereid, Halimede, Sao, S/2002 N 5, Laomedeia, Psamathe, Neso, and S/2021 N 1, a group that includes both prograde and retrograde objects.
It was the second moon in the Solar System that was discovered to have a substantial atmosphere, which is primarily nitrogen with small amounts of methane and carbon monoxide.
[26] Its surface is covered by nitrogen, methane, carbon dioxide and water ices[27] and has a high geometric albedo of more than 70%.
[6] Voyager 2 observations revealed a number of active geysers within the polar cap heated by the Sun, which eject plumes to the height of up to 8 km.
[28] Because of its retrograde orbit and relative proximity to Neptune (closer than the Moon is to Earth), tidal deceleration is causing Triton to spiral inward, which will lead to its destruction in about 3.6 billion years.
Early measurements of Nereid showed large, irregular variations in its visible magnitude, which were speculated to be caused by forced precession or chaotic rotation combined with an elongated shape and bright or dark spots on the surface.
Thermal modeling based on infrared observations from the Spitzer and Herschel space telescopes suggest that Nereid is only moderately elongated which disfavours forced precession of the rotation.
[32] The thermal model also indicates that the surface roughness of Nereid is very high, likely similar to the Saturnian moon Hyperion.
[34] Neptune has the largest Hill sphere in the Solar System, owing primarily to its large distance from the Sun; this allows it to retain control of such distant moons.
In this scenario, Triton is the surviving member of a binary Kuiper belt object[note 3] disrupted by its encounter with Neptune.
