Nix, formal designation (134340) Pluto II, is a natural satellite of Pluto, with a diameter of 49.8 km (30.9 mi) across its longest dimension. It was discovered along with Pluto's outermost moon Hydra on 15 May 2005 by astronomers using the Hubble Space Telescope, and was named after Nyx, the Greek goddess of the night. Nix is the third moon of Pluto by distance, orbiting between the moons Styx and Kerberos. Nix was imaged along with Pluto and its other moons by the New Horizons spacecraft as it flew by the Pluto system in July 2015. These images reveal a large reddish area on Nix that is likely an impact crater.
Discovery
Nix was independently discovered by Max Mutchler and Andrew Steffl, members of the Pluto Companion Search Team, using the Hubble Space Telescope. The New Horizons team had suspected that Pluto and its moon Charon might be accompanied by other moons, hence they used Hubble to search for faint moons around Pluto in 2005. Since Nix's brightness is about 5,000 times fainter than Pluto, long exposure images were taken in order to find it. The discovery images were taken on 15 May 2005 and 18 May 2005. The discoveries were announced on 31 October 2005, after confirmation by precovering archival Hubble images of Pluto from 2002. The two newly announced moons of Pluto were subsequently provisionally designated S/2005 P 1 for Hydra and S/2005 P 2 for Nix. The moons were informally referred to as "P1" and "P2", respectively by the discovery team.
Naming
The name Nix was approved by the International Astronomical Union (IAU) and was announced on 21 June 2006 along with the naming of Hydra in the IAU Circular 8723. Nix was named after Nyx, the Greek goddess of darkness and night and mother of Charon, the ferryman of Hades in Greek mythology. The two newly named moons were intentionally named that the order of their initials N and H honors the New Horizons mission to Pluto, similarly to how the first two letters of Pluto's name honors Percival Lowell. The original proposal for the naming of Nix was to use the classical spelling Nyx, but to avoid confusion with the asteroid 3908 Nyx, the spelling was changed to Nix, the Coptic spelling of the name. The adjectival form of the name is Nictian (cf. Russian Никта Nikta). The names of features on the bodies in the Pluto system are related to mythology and the literature and history of exploration. In particular, the names of features on Nix must be related to deities of the night from literature, mythology, and history.
Origin Pluto's smaller moons, including Nix, were thought to have formed from debris ejected from a massive collision between Pluto and another Kuiper belt object, similarly to how the Moon is believed to have formed from debris ejected by a large collision of Earth. The ejecta from the collision would then have coalesced into the moons of Pluto. However, the collisional hypothesis cannot explain how Nix maintained its highly reflective surface.
Physical characteristics Nix has an elongated shape, with its longest axis measured at 49.8 km (30.9 mi) across and its shortest axis 31.1 km (19.3 mi) across. This gives Nix the measured dimensions of 49.8 km × 33.2 km × 31.1 km (30.9 mi × 20.6 mi × 19.3 mi). It is the third-largest moon of Pluto, being slightly smaller than Hydra. Early research appeared to show that the surface of Nix is reddish in color. Contrary to this, other studies show that Nix is spectrally neutral, similar to the small moons of Pluto. The neutral spectrum of Nix signifies that water ice is present on its surface. Nix also appeared to vary in brightness and albedo, or reflectivity. The brightness fluctuations were thought to be caused by areas with different albedos on the surface of Nix. Images of Nix from the New Horizons spacecraft show a large reddish area approximately 18 km (11 mi) across, which could explain the two conflicting measurements of Nix's surface color. The reddish area is thought to be a large impact crater where the reddish material was ejected from underneath Nix's water ice layer and deposited on its surface. In this case, Nix would likely have regoliths originating from the impact. Another explanation suggests that the reddish material may have originated from a collision with Nix and another object with a different composition. However, there were no significant color variations on other impact craters on Nix.
The water ice present on the surface of Nix is responsible for its high reflectivity. Trace amounts of frozen methane may be also present on the surface of Nix, and could be responsible for the presence of reddish material, likely tholins, on its surface. In this case, tholins on the surface of Nix may have originated from the reaction of methane with ultraviolet radiation from the Sun. Derived from crater counting data from New Horizons, the age of Nix's surface is estimated to be at least four billion years old.
Rotation
Nix is not tidally locked and tumbles chaotically similarly to all smaller moons of Pluto; the moon's axial tilt and rotation period vary greatly over short timescales. Due to the chaotic rotation of Nix, it can occasionally flip its entire rotational axis. The varying gravitational influences of Pluto and Charon as they orbit their barycenter causes the chaotic tumbling of Pluto's small moons, including Nix. The chaotic tumbling of Nix is also strengthened by its elongated shape, which creates torques that act on the object. At the time of the New Horizons flyby, Nix was rotating with a period of 43.9 hours retrograde to Pluto's equator with an axial tilt of 132 degrees — it was rotating backwards in relation to its orbit around Pluto. The rotation rate of Nix had increased by 10 percent since Nix was discovered.
Features
Nix has two craters that are officially named after moon deities. The largest named crater of Nix, Gleti, is 13 km (8.1 mi) in diameter and is surrounded by a dark red ejecta blanket. The other named crater of Nix, Metztli, is roughly 10 km (6.2 mi) in diameter.
Orbit
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