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Hippocamp, also designated Neptune XIV, is a small
moon The Moon is Earth's only natural satellite. It Orbit of the Moon, orbits around Earth at Lunar distance, an average distance of (; about 30 times Earth diameter, Earth's diameter). The Moon rotation, rotates, with a rotation period (lunar ...
of
Neptune Neptune is the eighth and farthest known planet from the Sun. It is the List of Solar System objects by size, fourth-largest planet in the Solar System by diameter, the third-most-massive planet, and the densest giant planet. It is 17 t ...
discovered on 1 July 2013. It was found by astronomer Mark Showalter by analyzing archived Neptune photographs the
Hubble Space Telescope The Hubble Space Telescope (HST or Hubble) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It was not the Orbiting Solar Observatory, first space telescope, but it is one of the largest and most ...
captured between 2004 and 2009. The moon is so dim that it was not observed when the ''
Voyager 2 ''Voyager 2'' is a space probe launched by NASA on August 20, 1977, as a part of the Voyager program. It was launched on a trajectory towards the gas giants (Jupiter and Saturn) and enabled further encounters with the ice giants (Uranus and ...
''
space probe Uncrewed spacecraft or robotic spacecraft are spacecraft without people on board. Uncrewed spacecraft may have varying levels of autonomy from human input, such as remote control, or remote guidance. They may also be autonomous, in which th ...
flew by Neptune and its moons in 1989. It is about in diameter, and orbits Neptune in about 23 hours, just under one
Earth day Earth Day is an annual event on April 22 to demonstrate support for environmental protection. First held on April 22, 1970, it now includes a wide range of events coordinated globally through earthday.org (formerly Earth Day Network) includin ...
. Due to its unusually close distance to Neptune's largest inner moon
Proteus In Greek mythology, Proteus ( ; ) is an early prophetic sea god or god of rivers and oceanic bodies of water, one of several deities whom Homer calls the "Old Man of the Sea" (''hálios gérôn''). Some who ascribe a specific domain to Prote ...
, it has been hypothesized that Hippocamp may have accreted from material ejected by an impact on Proteus several billion years ago. The moon was formerly known by its
provisional designation Provisional designation in astronomy is the naming convention applied to astronomical objects immediately following their discovery. The provisional designation is usually superseded by a permanent designation once a reliable orbit has been calcu ...
S/2004 N 1 until February 2019, when it was formally named Hippocamp, after the mythological sea-horse symbolizing
Poseidon Poseidon (; ) is one of the twelve Olympians in ancient Greek religion and mythology, presiding over the sea, storms, earthquakes and horses.Burkert 1985pp. 136–139 He was the protector of seafarers and the guardian of many Hellenic cit ...
in
Greek mythology Greek mythology is the body of myths originally told by the Ancient Greece, ancient Greeks, and a genre of ancient Greek folklore, today absorbed alongside Roman mythology into the broader designation of classical mythology. These stories conc ...
.


History


Discovery

Hippocamp was discovered by a team of
astronomer An astronomer is a scientist in the field of astronomy who focuses on a specific question or field outside the scope of Earth. Astronomers observe astronomical objects, such as stars, planets, natural satellite, moons, comets and galaxy, galax ...
s led by Mark Showalter of the
SETI Institute The SETI Institute is a not-for-profit research organization incorporated in 1984 whose mission is to explore, understand, and explain the origin and nature of life in the universe, and to use this knowledge to inspire and guide present and futu ...
on 1 July 2013. Showalter was examining archival
Hubble Space Telescope The Hubble Space Telescope (HST or Hubble) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It was not the Orbiting Solar Observatory, first space telescope, but it is one of the largest and most ...
images of Neptune from 2009, as part of his study on the ring arcs of Neptune. Since the inner moons and ring arcs of Neptune
orbit In celestial mechanics, an orbit (also known as orbital revolution) is the curved trajectory of an object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an artificial satellite around an ...
quickly, Showalter developed and used a technique similar to
panning Pan or PAN may refer to: Food * Pan (cooking), a piece of cooking equipment * Harina P.A.N., a pre-cooked corn meal * Pan or Paan, a North Indian term for betel Prefix * ''Pan-'', a prefix meaning "all", "of everything", or "involving all ...
, where multiple short- exposure images are gathered and digitally warped to compensate for orbital motion and to allow
stacking Stacking may refer to: Arts and media * ''Stacking'' (video game), a 2011 game from Double Fine * '' Stacking'', a 1987 TV movie directed and produced by Martin Rosen * Stacking, a technique in broadcast programming Language * Consonant stac ...
of multiple images to bring out faint details. On a whim, Showalter decided to extend his analysis to regions beyond Neptune's ring system; he then found Hippocamp as a faint but unambiguous white dot. To confirm the moon, Showalter further analyzed over 150 archival Hubble images going back to 2004. Within a week, Showalter repeatedly found Hippocamp in these images and was able to identify the moon at ten different observation times from 2004 to 2009. Showalter had also checked images from the ''
Voyager 2 ''Voyager 2'' is a space probe launched by NASA on August 20, 1977, as a part of the Voyager program. It was launched on a trajectory towards the gas giants (Jupiter and Saturn) and enabled further encounters with the ice giants (Uranus and ...
'' spacecraft to find any detections of Hippocamp during its 1989 Neptune flyby, but was unable to identify the moon since it was too dim to be detected by ''Voyager 2''s cameras. Nonetheless, the number of archival Hubble images with Hippocamp was enough to determine the moon's orbit. The discovery of Hippocamp was formally announced in a notice issued by the
International Astronomical Union The International Astronomical Union (IAU; , UAI) is an international non-governmental organization (INGO) with the objective of advancing astronomy in all aspects, including promoting astronomical research, outreach, education, and developmen ...
's
Central Bureau for Astronomical Telegrams The Central Bureau for Astronomical Telegrams (CBAT) is an official international clearing house for information relating to transient astronomical events. The CBAT collects and distributes information on comets, natural satellites, novae, mete ...
, along with a press release by the
Space Telescope Science Institute The Space Telescope Science Institute (STScI) is the science operations center for the Hubble Space Telescope (HST), science operations and mission operations center for the James Webb Space Telescope (JWST), and science operations center for the ...
on 15 July 2013. Given that the relevant images examined by Showalter were available to the public, the discovery could have been made by anyone.


Naming

The moon is named after the
hippocampus The hippocampus (: hippocampi; via Latin from Ancient Greek, Greek , 'seahorse'), also hippocampus proper, is a major component of the brain of humans and many other vertebrates. In the human brain the hippocampus, the dentate gyrus, and the ...
, a mythological creature depicted as having the upper body of a
horse The horse (''Equus ferus caballus'') is a domesticated, one-toed, hoofed mammal. It belongs to the taxonomic family Equidae and is one of two extant subspecies of ''Equus ferus''. The horse has evolved over the past 45 to 55 mi ...
with the lower body of a
fish A fish (: fish or fishes) is an aquatic animal, aquatic, Anamniotes, anamniotic, gill-bearing vertebrate animal with swimming fish fin, fins and craniate, a hard skull, but lacking limb (anatomy), limbs with digit (anatomy), digits. Fish can ...
in
Greek mythology Greek mythology is the body of myths originally told by the Ancient Greece, ancient Greeks, and a genre of ancient Greek folklore, today absorbed alongside Roman mythology into the broader designation of classical mythology. These stories conc ...
. The hippocampus symbolizes the Greek sea god
Poseidon Poseidon (; ) is one of the twelve Olympians in ancient Greek religion and mythology, presiding over the sea, storms, earthquakes and horses.Burkert 1985pp. 136–139 He was the protector of seafarers and the guardian of many Hellenic cit ...
as well as the Roman sea god
Neptune Neptune is the eighth and farthest known planet from the Sun. It is the List of Solar System objects by size, fourth-largest planet in the Solar System by diameter, the third-most-massive planet, and the densest giant planet. It is 17 t ...
. In
Roman mythology Roman mythology is the body of myths of ancient Rome as represented in the literature and visual arts of the Romans, and is a form of Roman folklore. "Roman mythology" may also refer to the modern study of these representations, and to th ...
, Neptune would often drive a sea-
chariot A chariot is a type of vehicle similar to a cart, driven by a charioteer, usually using horses to provide rapid Propulsion, motive power. The oldest known chariots have been found in burials of the Sintashta culture in modern-day Chelyabinsk O ...
pulled by hippocampi. Upon the announcement of its discovery, the moon was given the
provisional designation Provisional designation in astronomy is the naming convention applied to astronomical objects immediately following their discovery. The provisional designation is usually superseded by a permanent designation once a reliable orbit has been calcu ...
S/2004 N 1. The provisional designation indicates that it was the first Neptunian satellite identified in images dating from 2004. Follow-up Hubble observations of Hippocamp were conducted by Showalter in 2016, and the moon was later given its permanent
Roman numeral designation The naming of moons has been the responsibility of the International Astronomical Union's committee for Planetary System Nomenclature since 1973. That committee is known today as the Planetary nomenclature, Working Group for Planetary System Nomenc ...
by the
Minor Planet Center The Minor Planet Center (MPC) is the official body for observing and reporting on minor planets under the auspices of the International Astronomical Union (IAU). Founded in 1947, it operates at the Smithsonian Astrophysical Observatory. Funct ...
after its recovery. Hippocamp was formally numbered as Neptune XIV (14) on 25 September 2018, though it remained without an official name until February 2019. By the
International Astronomical Union The International Astronomical Union (IAU; , UAI) is an international non-governmental organization (INGO) with the objective of advancing astronomy in all aspects, including promoting astronomical research, outreach, education, and developmen ...
's (IAU's) nomenclature guidelines, name proposals for Neptune's moons must be based on a figure from
Greco-Roman mythology Classical mythology, also known as Greco-Roman mythology or Greek and Roman mythology, is the collective body and study of myths from the ancient Greeks and ancient Romans. Mythology, along with philosophy and political thought, is one of the ...
with a relationship to Poseidon or Neptune. Showalter and his team had sought for names since the announcement of their discovery; among the names considered was
Polyphemus Polyphemus (; , ; ) is the one-eyed giant son of Poseidon and Thoosa in Greek mythology, one of the Cyclopes described in Homer's ''Odyssey''. His name means "abounding in songs and legends", "many-voiced" or "very famous". Polyphemus first ap ...
, the gigantic one-eyed son of Poseidon and
Thoosa In Greek mythology, Thoosa (, ), also spelled Thoösa, was, according to Homer, the sea nymph daughter of the primordial sea god Phorcys, and the mother, by Poseidon, of the Cyclops Polyphemus.Homer, ''Odyssey'1.70–73 Heubeck, Hainsworth an ...
. Showalter then settled on the name Hippocamp in acknowledgement of the seahorse genus, ''
Hippocampus The hippocampus (: hippocampi; via Latin from Ancient Greek, Greek , 'seahorse'), also hippocampus proper, is a major component of the brain of humans and many other vertebrates. In the human brain the hippocampus, the dentate gyrus, and the ...
'', mainly for his passion for
scuba diving Scuba diving is a Diving mode, mode of underwater diving whereby divers use Scuba set, breathing equipment that is completely independent of a surface breathing gas supply, and therefore has a limited but variable endurance. The word ''scub ...
and the animal itself. Showalter's name proposal was approved by the IAU's naming committee on 20 February 2019, and the name was announced in a press release by the Space Telescope Science Institute.


Origin

The mass distribution of the Neptunian moons is the most lopsided of the satellite systems of the giant planets in the Solar System. One moon,
Triton Triton commonly refers to: * Triton (mythology), a Greek god * Triton (moon), a satellite of Neptune Triton may also refer to: Biology * Triton cockatoo, a parrot * Triton (gastropod), a group of sea snails * ''Triton'', a synonym of ''Triturus' ...
, makes up nearly all of the mass of the system, with all other moons together comprising only one third of one percent. The reason for the lopsidedness of the present Neptunian system is that Triton was captured from the
Kuiper belt The Kuiper belt ( ) is a circumstellar disc in the outer Solar System, extending from the orbit of Neptune at 30 astronomical units (AU) to approximately 50 AU from the Sun. It is similar to the asteroid belt, but is far larger—20 times ...
well after the formation of Neptune's original satellite system, much of which was destroyed in the process of capture. Triton's orbit upon capture is presumed to have been highly
eccentric Eccentricity or eccentric may refer to: * Eccentricity (behavior), odd behavior on the part of a person, as opposed to being "normal" Mathematics, science and technology Mathematics * Off- center, in geometry * Eccentricity (graph theory) of a ...
, which would have caused chaotic perturbations in the orbits of the original inner Neptunian satellites, leading to the ejection of some moons and the collisional destruction of others. At least some of Neptune's present inner satellites are thought to have then accreted from the resulting rubble after Triton's orbit was circularized by
tidal deceleration Tidal acceleration is an effect of the tidal forces between an orbiting natural satellite (e.g. the Moon) and the primary planet that it orbits (e.g. Earth). The acceleration causes a gradual recession of a satellite in a prograde orbit (satel ...
. Among these re-accreted moons are
Proteus In Greek mythology, Proteus ( ; ) is an early prophetic sea god or god of rivers and oceanic bodies of water, one of several deities whom Homer calls the "Old Man of the Sea" (''hálios gérôn''). Some who ascribe a specific domain to Prote ...
, the largest and outermost of Neptune's present inner moons. Proteus bears a large impact crater named
Pharos The Lighthouse of Alexandria, sometimes called the Pharos of Alexandria, was a lighthouse built by the Ptolemaic Kingdom of Ancient Egypt, during the reign of Ptolemy II Philadelphus (280–247 BC). It has been estimated to have been at least ...
, which has a diameter around —more than half the diameter of Proteus itself. This unusually large size of Pharos relative to Proteus implies that the
impact event An impact event is a collision between astronomical objects causing measurable effects. Impact events have been found to regularly occur in planetary systems, though the most frequent involve asteroids, comets or meteoroids and have minimal effe ...
that formed the crater would have nearly disrupted Proteus and ejected a significant amount of debris. The present orbit of Proteus is situated relatively close to that of Hippocamp, which orbits just interior of Proteus. Their orbital semi-major axes differ by only ten percent, implying that both had likely originated from the same position in the past. This is further evidenced by accounting for the moons' respective outward orbital migration rates, which also suggests that Hippocamp and Proteus were much closer together in the past. Ordinarily, two adjoining objects of disparate sizes would have either resulted in the smaller object being ejected or colliding with the larger object—this does not appear to be the case for Hippocamp and Proteus. Based on this evidence, Showalter and colleagues proposed that Hippocamp may have originated from debris ejected from Proteus by the
comet A comet is an icy, small Solar System body that warms and begins to release gases when passing close to the Sun, a process called outgassing. This produces an extended, gravitationally unbound atmosphere or Coma (cometary), coma surrounding ...
ary impact that formed its largest crater, Pharos. In this scenario, Hippocamp would be considered as a third-generation satellite of Neptune, originating from impacts on Neptune's reformed
regular moon In astronomy, a regular moon or a regular satellite is a natural satellite following a relatively close, stable, and circular orbit which is generally aligned to its primary's equator. They form within discs of debris and gas that once surroun ...
s after the capture of Triton. The regular moons of Neptune are thought to have been disrupted by cometary impacts multiple times, with only Proteus surviving intact despite being nearly disrupted by the Pharos impact event. Some of the debris ejected by the impact settled into a stable orbit interior to Proteus, and coalesced into Hippocamp. However, Hippocamp only accounts for two percent of the missing volume of material generated by the Pharos impact event, and the reason for the absence of the rest of the debris remains unknown. As with the other small inner moons of Neptune, Hippocamp is thought to have been repeatedly disrupted by comet impacts after it had coalesced from debris ejected from Proteus. Based on the formation rate of large craters on Proteus, Hippocamp is estimated to have been disrupted about nine times in the past 4 billion years, re-accreting back after each disruption event. These disruption events substantially reduce the moon's orbital eccentricity and inclination, providing an explanation for Hippocamp's present circular orbit despite its proximity to Proteus. Hippocamp had also likely lost some of its mass during these disruption events, possibly accounting for some of the missing volume of material ejected from the Pharos impact event. Proteus has since receded over from Neptune owing to tidal interactions with the planet, while Hippocamp remained close to its initial position where it formed as it migrates more slowly due to its smaller size.


Physical characteristics

Hippocamp is the second-smallest known moon of Neptune, with a diameter estimated at . It is about 1,000 times less massive and 4,000 times less voluminous than its hypothesized progenitor, Proteus. Based on Hippocamp's estimated
apparent magnitude Apparent magnitude () is a measure of the Irradiance, brightness of a star, astronomical object or other celestial objects like artificial satellites. Its value depends on its intrinsic luminosity, its distance, and any extinction (astronomy), ...
of 26.5, its diameter was initially thought to be around , but more recent observations revise this value upward two-fold. Nevertheless, it remains by a wide margin the smallest of Neptune's inner, regular, satellites. The surface properties of Hippocamp are unknown as it was not extensively studied through different wavelengths of light, particularly in the near-infrared spectrum. Hippocamp is assumed to resemble Neptune's other inner satellites in having a dark surface. Their geometrical albedos range from 0.07 to 0.10, with the average being about 0.09. The Hubble Space Telescope's
NICMOS The Near Infrared Camera and Multi-Object Spectrometer (NICMOS) is a scientific instrument for infrared astronomy, installed on the Hubble Space Telescope (HST), operating from 1997 to 1999, and from 2002 to 2008. Images produced by NICMOS contain ...
instrument has examined Neptune's large inner moons in the near-infrared, and has found evidence that similar dark, reddish material, characteristic of small outer Solar System bodies, appears to be present on all their surfaces. The data is consistent with
organic compound Some chemical authorities define an organic compound as a chemical compound that contains a carbon–hydrogen or carbon–carbon bond; others consider an organic compound to be any chemical compound that contains carbon. For example, carbon-co ...
s containing C−H and/or C≡N bonds, but spectral resolution was inadequate to identify the molecules. Water ice, abundant in the outer Solar System, is believed to be present, but its spectral signature could not be observed (unlike the case of small Uranian moons).


Orbit

Hippocamp completes one revolution around Neptune every 22 hours and 48 minutes (0.95 days), corresponding to a
semi-major axis In geometry, the major axis of an ellipse is its longest diameter: a line segment that runs through the center and both foci, with ends at the two most widely separated points of the perimeter. The semi-major axis (major semiaxis) is the longe ...
, or orbital distance of . For comparison, this distance is approximately 4.3 Neptune radii, or just over a quarter of the
Earth Earth is the third planet from the Sun and the only astronomical object known to Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...
Moon The Moon is Earth's only natural satellite. It Orbit of the Moon, orbits around Earth at Lunar distance, an average distance of (; about 30 times Earth diameter, Earth's diameter). The Moon rotation, rotates, with a rotation period (lunar ...
distance. Both its
inclination Orbital inclination measures the tilt of an object's orbit around a celestial body. It is expressed as the angle between a reference plane and the orbital plane or axis of direction of the orbiting object. For a satellite orbiting the Eart ...
and
eccentricity Eccentricity or eccentric may refer to: * Eccentricity (behavior), odd behavior on the part of a person, as opposed to being "normal" Mathematics, science and technology Mathematics * Off-Centre (geometry), center, in geometry * Eccentricity (g ...
are close to zero. It orbits between
Larissa Larissa (; , , ) is the capital and largest city of the Thessaly region in Greece. It is the fifth-most populous city in Greece with a population of 148,562 in the city proper, according to the 2021 census. It is also the capital of the Larissa ...
and Proteus, making it the second outermost of Neptune's regular satellites. Its small size at this location runs counter to a trend among the other regular Neptunian satellites of increasing diameter with increasing distance from the primary. Being situated at a relatively close distance to the much larger Proteus, Hippocamp is subjected to its significant gravitational influence. Its orbit is particularly sensitive to the mass of Proteus; orbital solutions using a variety of assumed masses for Proteus show that Hippocamp displays a significant in-orbit difference of around . This can allow for an estimate of the mass of Proteus by observing its influence on Hippocamp's orbit for over a period of several decades. Proteus and Hippocamp are nearly in a 11:13
mean-motion resonance In celestial mechanics, orbital resonance occurs when orbiting bodies exert regular, periodic gravitational influence on each other, usually because their orbital periods are related by a ratio of small integers. Most commonly, this relation ...
, which may be the reason for Hippocamp's sensitivity to the mass of Proteus. Both moons are outside the Neptune-
synchronous Synchronization is the coordination of events to operate a system in unison. For example, the conductor of an orchestra keeps the orchestra synchronized or ''in time''. Systems that operate with all parts in synchrony are said to be synchrono ...
orbit (Neptune's rotational period is 0.67 days or 16.1 hours) and are thus being tidally accelerated by Neptune and are migrating outward. Compared to Hippocamp, Proteus migrates at a faster rate due to its higher mass and thus stronger tidal interaction with Neptune. Based on its orbital migration rate, Proteus is estimated to recede about from Neptune in 18 million years, in which it will enter a true 11:13 resonance with Hippocamp. Additionally, the present orbital periods of Larissa and Hippocamp are within about one percent of a 3:5
orbital resonance In celestial mechanics, orbital resonance occurs when orbiting bodies exert regular, periodic gravitational influence on each other, usually because their orbital periods are related by a ratio of small integers. Most commonly, this relation ...
.


See also

*
Pharos The Lighthouse of Alexandria, sometimes called the Pharos of Alexandria, was a lighthouse built by the Ptolemaic Kingdom of Ancient Egypt, during the reign of Ptolemy II Philadelphus (280–247 BC). It has been estimated to have been at least ...
– The impact crater on Proteus whence Hippocamp may originate from


Notes


References


External links


Hippocamp In Depth
, NASA Solar System Exploration, updated 19 December 2019
A new moon for Neptune
Anne J. Verbiscer, ''
Nature Nature is an inherent character or constitution, particularly of the Ecosphere (planetary), ecosphere or the universe as a whole. In this general sense nature refers to the Scientific law, laws, elements and phenomenon, phenomena of the physic ...
'', 20 February 2019
Tiny New Moon Discovered around Neptune
Miriam Kramer,
Scientific American ''Scientific American'', informally abbreviated ''SciAm'' or sometimes ''SA'', is an American popular science magazine. Many scientists, including Albert Einstein and Nikola Tesla, have contributed articles to it, with more than 150 Nobel Pri ...
, 17 July 2013
How to Photograph a Racehorse …and how this relates to a tiny moon of Neptune
Mark Showalter, ''Cosmic Diary'' (Mark Showalter's Blog), 15 July 2013 {{DEFAULTSORT:Hippocamp (moon) Moons of Neptune 20130701 Moons with a prograde orbit