Nix, formal designation(134340) Pluto II, is anatural satellite of Pluto, with a diameter of 49.8 km (30.9 mi) across its longest dimension.[3] It was discovered along with Pluto's outermost moonHydra on 15 May 2005 by astronomers using theHubble Space Telescope,[1] and was named afterNyx, the Greek goddess of the night.[10] Nix is the third moon ofPluto by distance, orbiting between the moonsStyx andKerberos.[11]
Nix was imaged along withPluto and its other moons by theNew Horizons spacecraft as it flew by the Pluto system in July 2015.[12] These images reveal a large reddish area on Nix that is likely animpact crater.[13]
Nix was independently discovered by Max Mutchler and Andrew Steffl, members of the Pluto Companion Search Team, using theHubble Space Telescope.[10] TheNew Horizons team had suspected that Pluto and its moonCharon might be accompanied by other moons, hence they used Hubble to search for faint moons around Pluto in 2005.[14] Since Nix's brightness is about 5,000 times fainter than Pluto, long exposure images were taken in order to find it.[15]
The discovery images were taken on 15 May 2005 and 18 May 2005. The discoveries were announced on 31 October 2005, after confirmation byprecovering archival Hubble images of Pluto from 2002.[16] The two newly announced moons of Pluto were subsequentlyprovisionally designated S/2005 P 1 for Hydra andS/2005 P 2 for Nix. The moons were informally referred to as "P1" and "P2", respectively by the discovery team.[17]
The name Nix was approved by theInternational Astronomical Union (IAU) and was announced on 21 June 2006 along with the naming of Hydra in the IAU Circular 8723.[18] Nix was named afterNyx, the Greek goddess of darkness and night and mother ofCharon, the ferryman ofHades inGreek mythology. The two newly named moons were intentionally named that the order of their initialsN andH honors theNew Horizons mission to Pluto, similarly to how the first two letters of Pluto's name honorsPercival Lowell.[19][14] The original proposal for the naming of Nix was to use theclassical spelling Nyx, but to avoid confusion with theasteroid3908 Nyx, the spelling was changed to Nix, theCoptic spelling of the name.[19] The adjectival form of the name is Nictian (cf. Russian НиктаNikta).
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.[20]
Pluto's smaller moons, including Nix, were thought to have formed fromdebris ejected from a massive collision between Pluto and anotherKuiper belt object, similarly to how theMoon is believed to have formed from debris ejected by alarge collision of Earth.[21] The ejecta from the collision would then have coalesced into the moons of Pluto.[22] However, the collisional hypothesis cannot explain how Nix maintained its highly reflective surface.[23]
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).[3] It is the third-largest moon of Pluto, being slightly smaller thanHydra.
Early research appeared to show that the surface of Nix is reddish in color.[24] Contrary to this, other studies show that Nix is spectrally neutral, similar to the small moons of Pluto.[9][5] The neutral spectrum of Nix signifies that water ice is present on its surface.[23] Nix also appeared to vary in brightness andalbedo, or reflectivity.[9] The brightness fluctuations were thought to be caused by areas with different albedos on the surface of Nix.[9] Images of Nix from theNew 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.[12][25]
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.[13] In this case, Nix would likely haveregoliths originating from the impact.[5] 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.[5]
The water ice present on the surface of Nix is responsible for its high reflectivity.[12][23] Trace amounts of frozenmethane may be also present on the surface of Nix, and could be responsible for the presence of reddish material, likelytholins, on its surface.[12] In this case, tholins on the surface of Nix may have originated from the reaction of methane withultraviolet radiation from theSun.[12] Derived fromcrater counting data fromNew Horizons, the age of Nix's surface is estimated to be at least four billion years old.[26][5]
Six images of Nix taken by the New Horizons in July 2015 (contrast enhanced)
Nix is nottidally locked andtumbles chaotically similarly to all smaller moons of Pluto; the moon's axial tilt and rotation period vary greatly over short timescales.[22] Due to thechaotic rotation of Nix, it can occasionally flip its entirerotational axis.[27] The varying gravitational influences of Pluto and Charon as they orbit theirbarycenter causes the chaotic tumbling of Pluto's small moons, including Nix.[22] The chaotic tumbling of Nix is also strengthened by its elongated shape, which createstorques that act on the object.[22][2] At the time of theNew Horizons flyby, Nix was rotating with a period of 43.9 hours retrograde to Pluto's equator with anaxial tilt of 132 degrees — it was rotating backwards in relation to its orbit around Pluto.[23] The rotation rate of Nix had increased by 10 percent since Nix was discovered.[23]
Nix orbits the Pluto-Charonbarycenter at a distance of 48,694 km (30,257 mi), between the orbits ofStyx andKerberos.[11] All of Pluto's moons including Nix have very circular orbits that arecoplanar to Charon's orbit; the moons of Pluto have very loworbital inclinations to Pluto's equator.[28][29] The nearly circular and coplanar orbits of Pluto's moons suggest that they may have gone through tidal evolutions since their formation.[28] At the time of the formation of Pluto's smaller moons, Nix may have had a moreeccentric orbit around the Pluto-Charon barycenter.[30] The present circular orbit of Nix may have been caused by Charon's tidal damping of the eccentricity of Nix's orbit, through tidal interactions. The mutual tidal interactions of Charon on Nix's orbit would cause Nix to transfer its orbital eccentricity to Charon, thus causing the orbit of Nix to gradually become more circular over time.[30]
Nix has an orbital period of 24.8546 days and its orbit is resonant with other moons of Pluto.[2] Nix is in a 3:2orbital resonance with Hydra, and a 9:11 resonance with Styx (the ratios represent numbers of orbits completed per unit time; the period ratios are the inverses).[2][31] As a result of this "Laplace-like" 3-body resonance, it has conjunctions with Styx and Hydra in a 2:3 ratio.
Nix's orbital period is close to a 1:4 orbital resonance with Charon, with a timing discrepancy of 2.8%; there is no active resonance.[2][24] A hypothesis explaining such a near-resonance is that the resonances originated before the outward migration of Charon following the formation of all five known moons, and is maintained by the periodic local fluctuation of 9 percent in the Pluto–Charon gravitational field strength.[c]
Sequence of conjunctions of Hydra (blue), Nix (red) and Styx (black) over one third of their resonance cycle. Movements are counterclockwise and orbits completed are tallied at upper right of diagrams (click on image to see the whole cycle).
TheNew Horizons spacecraft visited the Pluto system and photographed Pluto and its moons during its flyby on 14 July 2015. Of Pluto's smaller moons, only Nix and Hydra were imaged at resolutions high enough for surface features to be visible.[23] Prior to the flyby of the Pluto system, measurements of Nix's size were performed by theLong Range Reconnaissance Imager on boardNew Horizons, initially estimating Nix to be about 35 km (22 mi) in diameter.[32] The first detailed images of Nix taken byNew Horizons from a distance of about 231,000 km (144,000 mi) were downlinked, or received from the spacecraft on 18 July 2015 and released to the public on 21 July 2015.[25] With an image resolution 3 km (1.9 mi) per pixel, Nix's shape was often referred to as a "jelly bean" shape.[25] Enhanced color images from theRalph MVIC instrument ofNew Horizons show a reddish region on its surface.[25] From those images, another accurate measurement of Nix's dimensions was made, giving the approximate dimensions of 42 km × 36 km (26 mi × 22 mi).[25]
^The full list of discoverers includesH. A. Weaver,S. A. Stern, M. J. Mutchler, A. J. Steffl,M. W. Buie, W. J. Merline, J. R. Spencer, E. F. Young, and L. A. Young.[1]
^Given Nix's rotational north poledeclination of +42°,[5]: 5 subtracting this angle from the northcelestial pole of +90° gives the inclination with respect to thecelestial equator:i = +90° – (+42°) = +48°.
^ The instantaneous force in the Pluto–Charon–Nix alignment case is 9.46% larger than in the quadrature case (where Nix is 90° from the Pluto–Charon axis); the Charon–Pluto–Nix case is almost exactly halfway between these values. In Buieet al., the quote is "The gravitational force exerted by Pluto on either P1 or P2 varies by roughly 15% (peak-to-peak)." Pluto's gravitational pull,by itself, varies by 18% for Nix and 13% for Hydra.
^abcd"Planet and Satellite Names and Discoverers".Gazetteer of Planetary Nomenclature. International Astronomical Union (IAU) Working Group for Planetary System Nomenclature (WGPSN). Retrieved17 May 2021.
^abStern, Alan; Grinspoon, David (1 May 2018). "Chapter 7: Bringing It All Together".Chasing New Horizons: Inside the Epic First Mission to Pluto. Picador.ISBN978-1-250-09896-2.
^"Nix In Depth".solarsystem.nasa.gov. 21 November 2017. Retrieved23 February 2019.
^abBuie, Marc W.; Grundy, William M.; Young, Eliot F.; Young, Leslie A.; Stern, S. Alan (2006). "Orbits and Photometry of Pluto's Satellites: Charon, S/2005 P1, and S/2005 P2".The Astronomical Journal.132 (1):290–298.arXiv:astro-ph/0512491.Bibcode:2006AJ....132..290B.doi:10.1086/504422.S2CID119386667..a, i, e perJPL (site updated 2008 Aug 25)
^Stern, S. A.; Mutchler, M. J.; Weaver, H. A.; Steffl, A. J. (2008). "On the Origin of Pluto's Minor Moons, Nix and Hydra".arXiv:0802.2951 [astro-ph].
^abStern, S. A.; Mutchler, M. J.; Weaver, H. A.; Steffl, A. J. (2008). "The Effect of Charon's Tidal Damping on the Orbits of Pluto's Three Moons".arXiv:0802.2939 [astro-ph].
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