Pharos ( ) is the largest known

impact crater An impact crater is a depression (geology), depression in the surface of a solid astronomical body formed by the hypervelocity impact event, impact of a smaller object. In contrast to volcanic craters, which result from explosion or internal c ...

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 ...

's 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 is named after the island of

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 ...

, making it the only named surface feature on Proteus . It measures 10–15 km deep and has a diameter of around , making it more than half the diameter of Proteus itself. Debris ejected from the impact that created Pharos may have formed

Hippocamp The hippocampus, or hippocamp or ''hippokampos'' (plural: hippocampi or hippocamps; , from , and

Observation and naming

Pharos was first discovered in 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's flyby of

and its system of moons on 25 August 1989, being one of the first identifiable features on

. The discovery of Proteus (then provisionally designated S/1989 N 1) itself was announced roughly two months prior on 7 July 1989 as ''Voyager 2'' approached the Neptune system. The impact basin was provisionally named in a 1992 paper by Steven K. Croft after the island of

, whose only surviving remnant is the Ras el-Tin promontory in modern-day

Alexandria Alexandria ( ; ) is the List of cities and towns in Egypt#Largest cities, second largest city in Egypt and the List of coastal settlements of the Mediterranean Sea, largest city on the Mediterranean coast. It lies at the western edge of the Nile ...

Egypt Egypt ( , ), officially the Arab Republic of Egypt, is a country spanning the Northeast Africa, northeast corner of Africa and Western Asia, southwest corner of Asia via the Sinai Peninsula. It is bordered by the Mediterranean Sea to northe ...

. The name was officially approved 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 ...

(IAU) in 1994.

Geology

With a diameter between , Pharos is by far the largest known

impact structure An impact structure is a generally circular or craterlike geologic structure of deformed bedrock or sediment produced by impact on a planetary surface, whatever the stage of erosion of the structure. In contrast, an impact crater is the surface e ...

on Proteus. As Proteus's mean radius is ~, this makes Pharos over half the size of the satellite itself. The impact basin is so large that it significantly affects Proteus's overall shape, appearing as if a facet has been carved from its limb. Pharos is a relatively degraded crater, bound by an outer incomplete scarp interrupted by subsequent impacts. The floor of Pharos appears to be domed, but nevertheless lacks a clear central peak that characterizes central-peak impact craters. However, there may be a

peak ring A peak ring crater is a type of complex crater, which is different from a multi-ringed basin or central-peak crater. A central peak is not seen; instead, a roughly circular ring or plateau, possibly discontinuous, surrounds the Impact crater, crat ...

structure with a diameter approximately half that of Pharos's outer rim. A series of depressions further surround Pharos, with a system of north-south oriented valleys bordering the southeast rim being the most prominent. Furthermore, a ridge appears to run parallel to the northeastern rim. The origins of these structures are unknown, though they may represent an even larger outer ring of Pharos, bringing Pharos's total diameter to between significantly larger than Proteus's mean diameter. Imagery and shape models derived from ''Voyager 2'' data are not adequate to confirm this interpretation. Despite the relatively poor image resolution of ''Voyager 2'', several structures have been identified within Pharos. The most prominent are numerous smaller impact craters, the largest of which is roughly in diameter and occupies the southwestern floor of Pharos (located at roughly 40° S, 20° W). The northern floor contains a dome or hill-like structure roughly wide (located at roughly 5° S, 0° E); its origins are unknown, though a

volcanic A volcano is commonly defined as a vent or fissure in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface. On Earth, volcanoes are most often fo ...

origin is unlikely due to Proteus's small size and geological history. Much of Pharos's western floor is occupied by a broad, well-defined winding valley that resembles a

graben In geology, a graben () is a depression (geology), depressed block of the Crust (geology), crust of a planet or moon, bordered by parallel normal faults. Etymology ''Graben'' is a loan word from German language, German, meaning 'ditch' or 't ...

. The valley is roughly across and at least 100 km long, cutting across the northern rim of the 100 km crater. As the valley appears to extend along the terminator in ''Voyager 2'' imagery, it may continue further north. Imagery also indicates that the surface of Pharos's interior may be darker than the surrounding terrain, though this remains uncertain. Given Pharos's large size relative to Proteus itself, 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 created Pharos was likely highly disruptive. Nevertheless, that Pharos's structure is identifiable indicates that Proteus was not brought "to the brink" of destruction. Instead, Pharos's large size points towards Proteus's surface nearly being brought to the point of significant resurfacing. A system of possible tectonic faults and fractures oriented concentrically around Pharos may be related to stresses from the Pharos impact event. Additional linear features oriented radially from Pharos may exist, although S. K. Croft cautions that the features could be an illusory artifact of illumination. Alternatively, the extensive fracturing on Proteus may be the result of partial relaxation of Pharos, or tidal stresses from the reorientation of Proteus due to the Pharos impact.

Effect on other Neptunian moons

The Pharos impact would have also excavated large amounts of debris and ejecta, much of which would have escaped into direct Neptune orbit. The debris may have formed a dusty

ring (The) Ring(s) may refer to: * Ring (jewellery), a round band, usually made of metal, worn as ornamental jewelry * To make a sound with a bell, and the sound made by a bell Arts, entertainment, and media Film and TV * ''The Ring'' (franchise), a ...

around Neptune in Proteus's orbit, similar to the

gossamer rings The rings of Jupiter are a system of faint planetary rings. The Jovian rings were the third ring system to be discovered in the Solar System, after those of Saturn and Uranus. The main ring was discovered in 1979 by the ''Voyager 1'' space probe a ...

Jupiter Jupiter is the fifth planet from the Sun and the List of Solar System objects by size, largest in the Solar System. It is a gas giant with a Jupiter mass, mass more than 2.5 times that of all the other planets in the Solar System combined a ...

's inner moons. Such a ring could potentially survive to the present day. Fragments over in diameter were ejected at velocities greater than , enough to reach

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' ...

's orbit. The fragments would impact at velocities of roughly relative to Triton, enough to excavate craters in diameter. However, due to Pharos's ancient age and Triton's very rapid rate of resurfacing, it is unlikely any of Triton's observed craters are due to the Pharos impact. Some of the escaped ejecta may have also accreted into the neighboring moon

Hippocamp The hippocampus, or hippocamp or ''hippokampos'' (plural: hippocampi or hippocamps; , from , and orbital eccentricity In astrodynamics, the orbital eccentricity of an astronomical object is a dimensionless parameter that determines the amount by which its orbit around another body deviates from a perfect circle. A value of 0 is a circular orbit, values be ...

and

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 ...

. Hippocamp's eccentricity and inclination are both small, so if Hippocamp originated from the Pharos impact a mechanism is required to circularize Hippocamp's orbit. Nevertheless, Hippocamp's origin remains uncertain, and it may have formed independently, unrelated to Proteus and the Pharos basin. Assuming an origin from impact debris ejected from Proteus, as large impacts were more common early in the Solar System's history, Hippocamp is likely several billion years old.

Notes

References

{{reflist , refs= {{cite web , url=https://voyager.jpl.nasa.gov/news/factsheet.html , title=Fact Sheet , publisher=JPL , access-date=March 3, 2016 , archive-date=November 29, 2016 , archive-url=https://web.archive.org/web/20161129230752/http://voyager.jpl.nasa.gov/news/factsheet.html , url-status=live {{cite journal , last = Green , first = Daniel W. E. , title = 1989 N 1 , date = July 7, 1989 , journal = IAU Circular , volume = 4806 , url = http://www.cbat.eps.harvard.edu/iauc/04800/04806.html , access-date = 2011-10-24 {{cite journal , last = Marsden , first = Brian G. , title = Satellites of Saturn and Neptune , date = September 16, 1991 , journal = IAU Circular , volume = 5347 , url = http://www.cbat.eps.harvard.edu/iauc/05300/05347.html , access-date = 2011-10-24 {{cite web , title = Planetary Names: Crater, craters: Pharos on Proteus , publisher = USGS Astrogeology , work = Gazetteer of Planetary Nomenclature , url = https://planetarynames.wr.usgs.gov/Feature/4684?__fsk=-1708907535 , access-date = 4 November 2021 {{Cite journal , date=16 March 1992 , bibcode = 1992Icar...99..402C , pages = 402–408 , volume = 99 , first1 = S. K. , doi = 10.1016/0019-1035(92)90156-2 , title = Proteus: Geology, shape, and catastrophic destruction , last1 = Croft , journal = Icarus , issue = 2 {{cite news , title = Tiny Neptune Moon Spotted by Hubble May Have Broken from Larger Moon , url = https://hubblesite.org/contents/news-releases/2019/news-2019-04.html , website = HubbleSite , publisher = Space Telescope Science Institute , date = 20 February 2019 , access-date = 4 October 2021 {{Cite journal , title=NEPTUNE'S SMALL SATELLITES , date=1995 , last1=Thomas , first1=P. C. , last2=Veverka , first2=J. , last3=Helfenstein , first3=P. , journal=Neptune and Triton , pages=685–699 , bibcode=1995netr.conf..685T , isbn=978-0-8165-1525-7 , url=https://books.google.com/books?id=O8pDDQAAQBAJ&pg=PA685 {{cite book, title=Vintage Alexandria: Photographs of the City, 1860–1960, url=https://books.google.com/books?id=ks2Q6szR2DwC, first=Michael, last=Haag, publisher=American University in Cairo Press, year=2008, isbn=9789774161926, page=113 {{cite journal , last1=Stooke , first1=Philip J. , title=The Surfaces of Larissa and Proteus , date=26 April 1994 , journal=Earth, Moon, and Planets , volume=65 , issue=1 , pages=31–54 , doi=10.1007/BF00572198 , bibcode=1994EM&P...65...31S {{cite journal , last1=Smith , first1=B. A. , last2=Soderblom , first2=L. A. , last3=Banfield , first3=D. , display-authors=et al. , title=Voyager 2 at Neptune: Imaging Science Results , date=15 December 1989 , journal=Science , volume=246 , issue=4936 , pages=1422–1449 , doi=10.1126/science.246.4936.1422 , doi-access=free , pmid=17755997 , bibcode=1989Sci...246.1422S {{cite journal , doi = 10.1016/S0019-1035(03)00002-2, last = Karkoschka, first = Erich, year = 2003, title = Sizes, shapes, and albedos of the inner satellites of Neptune, journal = Icarus, volume = 162, issue = 2, pages = 400–407, bibcode = 2003Icar..162..400K {{cite journal , last1=Showalter , first1=M. R. , last2=de Pater , first2=I. , last3=Lissauer , first3=J. J. , last4=French , first4=R. S. , title=The seventh inner moon of Neptune , date=19 December 2018 , journal=Nature , volume=566 , issue=7744 , pages=350–353 , doi=10.1038/s41586-019-0909-9 , pmid=30787452 , bibcode=2019Natur.566..350S , s2cid=67772685 , pmc=6424524 {{cite conference , last1=McKinnon , first1=William B. , last2=Singer , first2=K. N. , last3=Robbins , first3=S. J. , display-authors=et al. , title=New Crater Counts on Triton, and Reassessment of the Case for Planetocentric Impactor Origin , date=March 2024 , conference=55th Lunar and Planetary Science Conference , location= The Woodlands, Texas, United States and virtually , url=https://www.hou.usra.edu/meetings/lpsc2024/pdf/1665.pdf {{cite journal , last1=Verbiscer , first1=Anne J. , title=A new moon for Neptune , date=20 February 2019 , journal=Nature , volume=566 , issue=7744 , pages=328–329 , doi=10.1038/d41586-019-00576-1 , pmid=30787456 , bibcode=2019Natur.566..328V Surface features of Neptune's moons Proteus (moon)

Observation and naming

Geology

Effect on other Neptunian moons

See also

Notes

References