Pharos (crater)

Last updated

Pharos
Proteus (Voyager 2).jpg
A full-disc image of Proteus from the Voyager 2 spacecraft, with Pharos occupying much of the upper right
Feature typeImpact crater
Possible peak-ring impact basin
Location Proteus
Coordinates 10°0′S10°0′W / 10.000°S 10.000°W / -10.000; -10.000 [1]
Diameter255±12 km (Croft 1992) [2] : 409 
230 km (Stooke 1994) [3] : 47 
Depth~10 km (6.2 mi) [2]
Discoverer Voyager 2
Eponym Island of Pharos

Pharos ( /ˈfɛərɒs/ FAIR-oss) is the largest known impact crater on Neptune's moon Proteus. It is named after the island of Pharos, making it the only named surface feature on Proteus as of 2024. It measures 10–15 km deep and has a diameter of around 250 km (160 mi), making it more than half the diameter of Proteus itself. Debris ejected from the impact that created Pharos may have formed Hippocamp, a small moon whose orbit is unusually close to Proteus's. [4]

Contents

Observation and naming

Pharos was first discovered in the Voyager 2 spacecraft's flyby of Neptune and its system of moons on 25 August 1989, being one of the first identifiable features on Proteus. [5] [6] : 1447  The discovery of Proteus (then provisionally designated S/1989 N 1 [7] ) itself was announced roughly two months prior on 7 July 1989 as Voyager 2 approached the Neptune system. [8] The impact basin was provisionally named in a 1992 paper by Steven K. Croft after the island of Pharos, whose only surviving remnant is the Ras el-Tin promontory in modern-day Alexandria, Egypt. [3] : 31  [9] [10] The name was officially approved by the International Astronomical Union (IAU) in 1994. [10]

Geology

A false-color image of Proteus, with Pharos being the "facet" in the bottom right Proteus Color.png
A false-color image of Proteus, with Pharos being the "facet" in the bottom right

With a diameter between 230 and 267 kilometres (143 and 166 mi), [lower-alpha 1] Pharos is by far the largest known impact structure on Proteus. [3] : 47  As Proteus's mean radius is ~210 ± 7 km (130.5 ± 4.3 mi), [11] : 403  this makes Pharos over half the size of the satellite itself. [2] : 410  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. [3] : 39–43, 47  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 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 500 and 550 km (310 and 340 mi)—significantly larger than Proteus's mean diameter. Imagery and shape models derived from Voyager 2 data are not adequate to confirm this interpretation. [3] : 47–48, 52  [1] : 693–694 

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 100 km (62 mi) 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 20 km (12 mi) wide (located at roughly 5° S, 0° E); its origins are unknown, though a volcanic 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. The valley is roughly 12 km (7.5 mi) 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. [3] : 47, 50, 53 

Given Pharos's large size relative to Proteus itself, the impact event that created Pharos was likely highly disruptive. [12] 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. [2] : 407, 412, 417  [3] : 48–49, 51 

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 around Neptune in Proteus's orbit, similar to the gossamer rings of Jupiter's inner moons. Such a ring could potentially survive to the present day. [12] Fragments over 500 metres (1,600 feet) in diameter were ejected at velocities greater than 1.7 km/s (3,800 mph), enough to reach Triton's orbit. The fragments would impact at velocities of roughly 7.5 km/s (17,000 mph) relative to Triton, enough to excavate craters 7 kilometres (4.3 miles) 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. [13] Some of the escaped ejecta may have also accreted into the neighboring moon Hippocamp, whose volume is only roughly 2% of the estimated total volume of the Pharos basin. However, this model is uncertain and has several complications; if Hippocamp was created from debris ejected from the Pharos impact, then Proteus would have subsequently strongly excited Hippocamp's orbital eccentricity and inclination. 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. [14]

See also

Notes

  1. ↑ Diameter calculated as 255±12 km by Steven K. Croft in 1992 [2] : 409  and interpreted as "approximately 230 km" by Philip J. Stooke in 1994. [3] : 47 

Related Research Articles

<span class="mw-page-title-main">Impact crater</span> Circular depression in a solid astronomical body formed by the impact of a smaller object

An impact crater is a depression in the surface of a solid astronomical body formed by the hypervelocity impact of a smaller object. In contrast to volcanic craters, which result from explosion or internal collapse, impact craters typically have raised rims and floors that are lower in elevation than the surrounding terrain. Impact craters are typically circular, though they can be elliptical in shape or even irregular due to events such as landslides. Impact craters range in size from microscopic craters seen on lunar rocks returned by the Apollo Program to simple bowl-shaped depressions and vast, complex, multi-ringed impact basins. Meteor Crater is a well-known example of a small impact crater on Earth.

<span class="mw-page-title-main">Miranda (moon)</span> Moon of Uranus

Miranda, also designated Uranus V, is the smallest and innermost of Uranus's five round satellites. It was discovered by Gerard Kuiper on 16 February 1948 at McDonald Observatory in Texas, and named after Miranda from William Shakespeare's play The Tempest. Like the other large moons of Uranus, Miranda orbits close to its planet's equatorial plane. Because Uranus orbits the Sun on its side, Miranda's orbit is nearly perpendicular to the ecliptic and shares Uranus's extreme seasonal cycle.

<span class="mw-page-title-main">Triton (moon)</span> Largest moon of Neptune

Triton is the largest natural satellite of the planet Neptune. It is the only moon of Neptune massive enough to be rounded under its own gravity and hosts a thin but well-structured atmosphere. Triton orbits Neptune in a retrograde orbit— revolving in the opposite direction to the parent planet's rotation — the only large moon in the Solar System to do so. Triton is thought to have once been a dwarf planet from the Kuiper belt, captured into Neptune's orbit by the latter's gravity.

<span class="mw-page-title-main">Rhea (moon)</span> Moon of Saturn

Rhea is the second-largest moon of Saturn and the ninth-largest moon in the Solar System, with a surface area that is comparable to the area of Australia. It is the smallest body in the Solar System for which precise measurements have confirmed a shape consistent with hydrostatic equilibrium. Rhea has a nearly circular orbit around Saturn, but it is also tidally locked, like Saturn's other major moons; that is, it rotates with the same period it revolves (orbits), so one hemisphere always faces towards the planet.

<span class="mw-page-title-main">Thebe (moon)</span> Moon of Jupiter

Thebe, also known as Jupiter XIV, is the fourth of Jupiter's moons by distance from the planet. It was discovered by Stephen P. Synnott in images from the Voyager 1 space probe taken on March 5, 1979, while making its flyby of Jupiter. In 1983, it was officially named after the mythological nymph Thebe.

<span class="mw-page-title-main">Proteus (moon)</span> Large moon of Neptune

Proteus, also known as Neptune VIII, is the second-largest Neptunian moon, and Neptune's largest inner satellite. Discovered by Voyager 2 in 1989, it is named after Proteus, the shape-changing sea god of Greek mythology. Proteus orbits Neptune in a nearly equatorial orbit at a distance of about 4.75 times the radius of Neptune's equator.

<span class="mw-page-title-main">Larissa (moon)</span> Moon of Neptune

Larissa, also known as Neptune VII, is the fifth-closest inner satellite of Neptune. It is named after Larissa, a lover of Poseidon.

<span class="mw-page-title-main">Thalassa (moon)</span> Moon of Neptune

Thalassa, also known as Neptune IV, is the second-innermost satellite of Neptune. Thalassa was named after sea goddess Thalassa, a daughter of Aether and Hemera from Greek mythology. "Thalassa" is also the Greek word for "sea".

<span class="mw-page-title-main">Despina (moon)</span> Moon of Neptune

Despina, also known as Neptune V, is the third-closest inner moon of Neptune. It is named after Greek mythological character Despoina, a nymph who was a daughter of Poseidon and Demeter.

<span class="mw-page-title-main">Moons of Uranus</span> Natural satellites of the planet Uranus

Uranus, the seventh planet of the Solar System, has 28 confirmed moons. Most of them are named after characters that appear in, or are mentioned in, the works of William Shakespeare and Alexander Pope. Uranus's moons are divided into three groups: thirteen inner moons, five major moons, and ten irregular moons. The inner and major moons all have prograde orbits and are cumulatively classified as regular moons. In contrast, the orbits of the irregular moons are distant, highly inclined, and mostly retrograde.

<span class="mw-page-title-main">Moons of Neptune</span> Natural satellites of the planet Neptune

The planet Neptune has 16 known moons, which are named for minor water deities and a water creature in Greek mythology. By far the largest of them is Triton, discovered by William Lassell on 10 October 1846, 17 days after the discovery of Neptune itself. Over a century passed before the discovery of the second natural satellite, Nereid, in 1949, and another 40 years passed before Proteus, Neptune's second-largest moon, was discovered in 1989.

<span class="mw-page-title-main">Herschel (Mimantean crater)</span> Crater on Mimas

Herschel is the largest impact crater on the Saturnian moon Mimas. It is located on Mimas's leading hemisphere, centered on the equator at 112° longitude. It is named after the 18th-century astronomer William Herschel, who discovered Mimas in 1789.

<span class="mw-page-title-main">Crater chain</span> Line of craters along the surface of an astronomical body

A crater chain is a line of craters along the surface of an astronomical body. The descriptor term for crater chains is catena, plural catenae, as specified by the International Astronomical Union's rules on planetary nomenclature.

<span class="mw-page-title-main">North Polar Basin (Mars)</span> Large basin in the northern hemisphere of Mars

The North Polar Basin, more commonly known as the Borealis Basin, is a large basin in the northern hemisphere of Mars that covers 40% of the planet. Some scientists have postulated that the basin formed during the impact of a single, large body roughly 2% of the mass of Mars, having a diameter of about 1,900 km early in the history of Mars, around 4.5 billion years ago. However, the basin is not currently recognized as an impact basin by the IAU. The basin is one of the flattest areas in the Solar System, and has an elliptical shape.

In astronomy, an inner moon or inner natural satellite is a natural satellite following a prograde, low-inclination orbit inwards of the large satellites of the parent planet. They are generally thought to have been formed in situ at the same time as the coalescence of the original planet. Neptune's moons are an exception, as they are likely reaggregates of the pieces of the original bodies, which were disrupted after the capture of the large moon Triton. Inner satellites are distinguished from other regular satellites by their proximity to the parent planet, their short orbital periods, their low mass, small size, and irregular shapes.

<span class="mw-page-title-main">Exploration of Neptune</span> Overview of the exploration of Neptune

Neptune has been directly explored by one space probe, Voyager 2, in 1989. As of 2024, there are no confirmed future missions to visit the Neptunian system, although a tentative Chinese mission has been planned for launch in 2024. NASA, ESA, and independent academic groups have proposed future scientific missions to visit Neptune. Some mission plans are still active, while others have been abandoned or put on hold.

<span class="mw-page-title-main">Hippocamp (moon)</span> Smallest moon of Neptune

Hippocamp, also designated Neptune XIV, is a small moon of Neptune discovered on 1 July 2013. It was found by astronomer Mark Showalter by analyzing archived Neptune photographs the Hubble Space Telescope captured between 2004 and 2009. The moon is so dim that it was not observed when the Voyager 2 space probe flew by Neptune and its moons in 1989. It is about 35 km (20 mi) in diameter, and orbits Neptune in about 23 hours, just under one Earth day. Due to its unusually close distance to Neptune's largest inner moon Proteus, 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 S/2004 N 1 until February 2019, when it was formally named Hippocamp, after the mythological sea-horse symbolizing Poseidon in Greek mythology.

<span class="mw-page-title-main">Satellite system (astronomy)</span> Set of gravitationally bound objects in orbit

A satellite system is a set of gravitationally bound objects in orbit around a planetary mass object or minor planet, or its barycenter. Generally speaking, it is a set of natural satellites (moons), although such systems may also consist of bodies such as circumplanetary disks, ring systems, moonlets, minor-planet moons and artificial satellites any of which may themselves have satellite systems of their own. Some bodies also possess quasi-satellites that have orbits gravitationally influenced by their primary, but are generally not considered to be part of a satellite system. Satellite systems can have complex interactions including magnetic, tidal, atmospheric and orbital interactions such as orbital resonances and libration. Individually major satellite objects are designated in Roman numerals. Satellite systems are referred to either by the possessive adjectives of their primary, or less commonly by the name of their primary. Where only one satellite is known, or it is a binary with a common centre of gravity, it may be referred to using the hyphenated names of the primary and major satellite.

The following outline is provided as an overview of and topical guide to Neptune:

<span class="mw-page-title-main">Geology of Triton</span> Geologic structure and composition of Triton

The geology of Triton encompasses the physical characteristics of the surface, internal structure, and geological history of Neptune's largest moon Triton. With a mean density of 2.061 g/cm3, Triton is roughly 15-35% water ice by mass; Triton is a differentiated body, with an icy solid crust atop a probable subsurface ocean and a rocky core. As a result, Triton's surface geology is largely driven by the dynamics of water ice and other volatiles such as nitrogen and methane. Triton's geology is vigorous, and has been and continues to be influenced by its unusual history of capture, high internal heat, and its thin but significant atmosphere.

References

  1. 1 2 Thomas, P. C.; Veverka, J.; Helfenstein, P. (1995). "NEPTUNE'S SMALL SATELLITES". Neptune and Triton: 685–699. Bibcode:1995netr.conf..685T. ISBN   978-0-8165-1525-7.
  2. 1 2 3 4 5 Croft, S. K. (16 March 1992). "Proteus: Geology, shape, and catastrophic destruction". Icarus. 99 (2): 402–408. Bibcode:1992Icar...99..402C. doi:10.1016/0019-1035(92)90156-2.
  3. 1 2 3 4 5 6 7 8 Stooke, Philip J. (26 April 1994). "The Surfaces of Larissa and Proteus". Earth, Moon, and Planets. 65 (1): 31–54. Bibcode:1994EM&P...65...31S. doi:10.1007/BF00572198.
  4. ↑ "Tiny Neptune Moon Spotted by Hubble May Have Broken from Larger Moon". HubbleSite. Space Telescope Science Institute. 20 February 2019. Retrieved 4 October 2021.
  5. ↑ "Fact Sheet". JPL. Archived from the original on November 29, 2016. Retrieved March 3, 2016.
  6. ↑ Smith, B. A.; Soderblom, L. A.; Banfield, D.; et al. (15 December 1989). "Voyager 2 at Neptune: Imaging Science Results". Science. 246 (4936): 1422–1449. Bibcode:1989Sci...246.1422S. doi: 10.1126/science.246.4936.1422 . PMID   17755997.
  7. ↑ Marsden, Brian G. (September 16, 1991). "Satellites of Saturn and Neptune". IAU Circular. 5347. Retrieved 2011-10-24.
  8. ↑ Green, Daniel W. E. (July 7, 1989). "1989 N 1". IAU Circular. 4806. Retrieved 2011-10-24.
  9. ↑ Haag, Michael (2008). Vintage Alexandria: Photographs of the City, 1860–1960. American University in Cairo Press. p. 113. ISBN   9789774161926.
  10. 1 2 "Planetary Names: Crater, craters: Pharos on Proteus". Gazetteer of Planetary Nomenclature. USGS Astrogeology. Retrieved 4 November 2021.
  11. ↑ Karkoschka, Erich (2003). "Sizes, shapes, and albedos of the inner satellites of Neptune". Icarus. 162 (2): 400–407. Bibcode:2003Icar..162..400K. doi:10.1016/S0019-1035(03)00002-2.
  12. 1 2 Verbiscer, Anne J. (20 February 2019). "A new moon for Neptune". Nature. 566 (7744): 328–329. Bibcode:2019Natur.566..328V. doi:10.1038/d41586-019-00576-1. PMID   30787456.
  13. ↑ McKinnon, William B.; Singer, K. N.; Robbins, S. J.; et al. (March 2024). New Crater Counts on Triton, and Reassessment of the Case for Planetocentric Impactor Origin (PDF). 55th Lunar and Planetary Science Conference. The Woodlands, Texas, United States and virtually.
  14. ↑ Showalter, M. R.; de Pater, I.; Lissauer, J. J.; French, R. S. (19 December 2018). "The seventh inner moon of Neptune". Nature. 566 (7744): 350–353. Bibcode:2019Natur.566..350S. doi:10.1038/s41586-019-0909-9. PMC   6424524 . PMID   30787452. S2CID   67772685.
This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.