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Proteus (moon)
Encyclopedia
Proteus also known as Neptune VIII, is the second largest Neptunian moon
, and Neptune's largest inner satellite
. Discovered by Voyager 2
spacecraft in 1989, it is named after Proteus
, the shape-changing sea god of Greek mythology
. Proteus circles Neptune in a nearly equatorial orbit at the distance of about 4.75 equatorial radii of the planet.
Despite being more than 400 km in diameter Proteus has a somewhat irregular shape with several slightly concave facets and relief as high as 20 km. Its surface is dark, neutral in color and heavily cratered. The largest crater is more than 200 km in diameter. There are also a number of scarp
s, grooves and valleys related to large craters.
Proteus is probably not an original body that formed with Neptune; it may have accreted later from the debris created when the largest Neptunian satellite Triton
was captured.
space probe two months before its Neptune flyby in August 1989. It received the temporary designation S/1989 N 1. Stephen P. Synnott
and Bradford A. Smith announced its discovery on July 7, 1989, speaking only of “17 frames taken over 21 days”, which gives a discovery date of sometime before June 16.
On 16 September 1991 S/1989 N 1 was named after Proteus
, the shape-changing sea god of Greek mythology
.
Proteus orbits Neptune at the distance approximately equal to 4.75 equatorial radii of the planet. Its orbit has a small eccentricity
and is inclined by about 0.5° to the planet's equator. Proteus is the largest of the regular prograde satellites of Neptune. It rotates synchronously
with the orbital motion, which means that one face always points to the planet.
, the second to be discovered. It was not discovered by Earth
-based telescopes because it is so close to the planet that it is lost in the glare of reflected sunlight. The surface of Proteus is dark—its geometrical albedo is about 10%. The surface's color is neutral as the reflectivity
does not change appreciably with the wavelength from violet to green. In the near-infrared part of the spectrum the surface becomes less reflective around 2 μm pointing to a possible presence of complex organic compounds such as hydrocarbon
s or cyanide
s. These compounds may be responsible for the low albedo of the inner Neptunian moons. While Proteus is usually thought to contain significant amounts of water ice, it has not been detected spectroscopically on the surface.
The shape of Proteus is close to a sphere with the radius of about 210 km, although deviations from the spherical shape are large—up to 20 km; scientists believe it is about as large as a body of its density can be without being pulled into a perfect spherical shape by its own gravity. Saturn's moon Mimas
has a more spherical shape despite being slightly less massive than Proteus, perhaps due to the higher temperature near Saturn or tidal heating. Proteus is slightly elongated in the direction of Neptune, although its overall the shape is closer to an irregular polyhedron
than to a triaxial ellipsoid. The surface of Proteus shows several flat or slightly concave facets measuring from 150 to 200 km in diameter. They are probably degraded impact craters.
Proteus is heavily cratered
, showing no sign of any geological modification. The largest crater, Pharos, has a diameter from . Its depth is about 10–15 km. The crater has a central dome on its floor a few kilometers high. Pharos is the only named surface feature on this moon: the name is Greek
and refers to the island
where Proteus reigned. In addition to Pharos there are several craters 50–100 km in diameter and many more with diameters less than 50 km.
The second landform
found on Proteus is linear features such as scarp
s, valleys and grooves. The most prominent one runs parallel to the equator to the west of Pharos. These features likely formed as a result of the giant impacts, which formed Pharos and other large craters or as a result of tidal stresses from Neptune.
Natural satellite
A natural satellite or moon is a celestial body that orbits a planet or smaller body, which is called its primary. The two terms are used synonymously for non-artificial satellites of planets, of dwarf planets, and of minor planets....
, and Neptune's largest inner satellite
Inner satellite
In astronomy, an inner moon 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...
. Discovered by Voyager 2
Voyager 2
The Voyager 2 spacecraft is a 722-kilogram space probe launched by NASA on August 20, 1977 to study the outer Solar System and eventually interstellar space...
spacecraft in 1989, it is named after Proteus
Proteus
In Greek mythology, Proteus is an early sea-god, one of several deities whom Homer calls the "Old Man of the Sea", whose name suggests the "first" , as protogonos is the "primordial" or the "firstborn". He became the son of Poseidon in the Olympian theogony In Greek mythology, Proteus (Πρωτεύς)...
, the shape-changing sea god of Greek mythology
Greek mythology
Greek mythology is the body of myths and legends belonging to the ancient Greeks, concerning their gods and heroes, the nature of the world, and the origins and significance of their own cult and ritual practices. They were a part of religion in ancient Greece...
. Proteus circles Neptune in a nearly equatorial orbit at the distance of about 4.75 equatorial radii of the planet.
Despite being more than 400 km in diameter Proteus has a somewhat irregular shape with several slightly concave facets and relief as high as 20 km. Its surface is dark, neutral in color and heavily cratered. The largest crater is more than 200 km in diameter. There are also a number of scarp
Scarp
-Landforms and geology:* Cliff, a significant vertical, or near vertical, rock exposure* Escarpment, a steep slope or long cliff that occurs from erosion or faulting and separates two relatively level areas of differing elevations...
s, grooves and valleys related to large craters.
Proteus is probably not an original body that formed with Neptune; it may have accreted later from the debris created when the largest Neptunian satellite Triton
Triton (moon)
Triton is the largest moon of the planet Neptune, discovered on October 10, 1846, by English astronomer William Lassell. It is the only large moon in the Solar System with a retrograde orbit, which is an orbit in the opposite direction to its planet's rotation. At 2,700 km in diameter, it is...
was captured.
Discovery and orbit
Proteus was discovered from the images taken by Voyager 2Voyager 2
The Voyager 2 spacecraft is a 722-kilogram space probe launched by NASA on August 20, 1977 to study the outer Solar System and eventually interstellar space...
space probe two months before its Neptune flyby in August 1989. It received the temporary designation S/1989 N 1. Stephen P. Synnott
Stephen P. Synnott
Stephen P. Synnott is an American astronomer Voyager scientist who discovered several moons of Jupiter, Saturn, Uranus, and Neptune.He discovered Metis, Puck, Larissa , Proteus, and Thebe.-References:...
and Bradford A. Smith announced its discovery on July 7, 1989, speaking only of “17 frames taken over 21 days”, which gives a discovery date of sometime before June 16.
On 16 September 1991 S/1989 N 1 was named after Proteus
Proteus
In Greek mythology, Proteus is an early sea-god, one of several deities whom Homer calls the "Old Man of the Sea", whose name suggests the "first" , as protogonos is the "primordial" or the "firstborn". He became the son of Poseidon in the Olympian theogony In Greek mythology, Proteus (Πρωτεύς)...
, the shape-changing sea god of Greek mythology
Greek mythology
Greek mythology is the body of myths and legends belonging to the ancient Greeks, concerning their gods and heroes, the nature of the world, and the origins and significance of their own cult and ritual practices. They were a part of religion in ancient Greece...
.
Proteus orbits Neptune at the distance approximately equal to 4.75 equatorial radii of the planet. Its orbit has a small eccentricity
Orbital eccentricity
The orbital eccentricity of an astronomical body is the amount by which its orbit deviates from a perfect circle, where 0 is perfectly circular, and 1.0 is a parabola, and no longer a closed orbit...
and is inclined by about 0.5° to the planet's equator. Proteus is the largest of the regular prograde satellites of Neptune. It rotates synchronously
Synchronous orbit
A synchronous orbit is an orbit in which an orbiting body has a period equal to the average rotational period of the body being orbited , and in the same direction of rotation as that body.-Properties:...
with the orbital motion, which means that one face always points to the planet.
Physical characteristics
Proteus is the second largest moon of Neptune. It is about 420 kilometres in diameter, larger than NereidNereid (moon)
Nereid , also known as Neptune II, is the third-largest moon of Neptune. It has a highly eccentric orbit. Nereid was discovered by Gerard Kuiper in 1949 and was the second moon of Neptune to be discovered.- Discovery and naming :...
, the second to be discovered. It was not discovered by Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
-based telescopes because it is so close to the planet that it is lost in the glare of reflected sunlight. The surface of Proteus is dark—its geometrical albedo is about 10%. The surface's color is neutral as the reflectivity
Reflectivity
In optics and photometry, reflectivity is the fraction of incident radiation reflected by a surface. In general it must be treated as a directional property that is a function of the reflected direction, the incident direction, and the incident wavelength...
does not change appreciably with the wavelength from violet to green. In the near-infrared part of the spectrum the surface becomes less reflective around 2 μm pointing to a possible presence of complex organic compounds such as hydrocarbon
Hydrocarbon
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons from which one hydrogen atom has been removed are functional groups, called hydrocarbyls....
s or cyanide
Cyanide
A cyanide is a chemical compound that contains the cyano group, -C≡N, which consists of a carbon atom triple-bonded to a nitrogen atom. Cyanides most commonly refer to salts of the anion CN−. Most cyanides are highly toxic....
s. These compounds may be responsible for the low albedo of the inner Neptunian moons. While Proteus is usually thought to contain significant amounts of water ice, it has not been detected spectroscopically on the surface.
The shape of Proteus is close to a sphere with the radius of about 210 km, although deviations from the spherical shape are large—up to 20 km; scientists believe it is about as large as a body of its density can be without being pulled into a perfect spherical shape by its own gravity. Saturn's moon Mimas
Mimas (moon)
Mimas is a moon of Saturn which was discovered in 1789 by William Herschel. It is named after Mimas, a son of Gaia in Greek mythology, and is also designated Saturn I....
has a more spherical shape despite being slightly less massive than Proteus, perhaps due to the higher temperature near Saturn or tidal heating. Proteus is slightly elongated in the direction of Neptune, although its overall the shape is closer to an irregular polyhedron
Polyhedron
In elementary geometry a polyhedron is a geometric solid in three dimensions with flat faces and straight edges...
than to a triaxial ellipsoid. The surface of Proteus shows several flat or slightly concave facets measuring from 150 to 200 km in diameter. They are probably degraded impact craters.
Proteus is heavily cratered
Impact crater
In the broadest sense, the term impact crater can be applied to any depression, natural or manmade, resulting from the high velocity impact of a projectile with a larger body...
, showing no sign of any geological modification. The largest crater, Pharos, has a diameter from . Its depth is about 10–15 km. The crater has a central dome on its floor a few kilometers high. Pharos is the only named surface feature on this moon: the name is Greek
Greek mythology
Greek mythology is the body of myths and legends belonging to the ancient Greeks, concerning their gods and heroes, the nature of the world, and the origins and significance of their own cult and ritual practices. They were a part of religion in ancient Greece...
and refers to the island
Pharos
Pharos may refer to:Lighthouses:* The Pharos of Alexandria, a tower built on the island of Pharos that became one of the Seven Wonders of the Ancient World* The Pharos, either of two Roman lighthouses built at Dubris...
where Proteus reigned. In addition to Pharos there are several craters 50–100 km in diameter and many more with diameters less than 50 km.
The second landform
Landform
A landform or physical feature in the earth sciences and geology sub-fields, comprises a geomorphological unit, and is largely defined by its surface form and location in the landscape, as part of the terrain, and as such, is typically an element of topography...
found on Proteus is linear features such as scarp
Scarp
-Landforms and geology:* Cliff, a significant vertical, or near vertical, rock exposure* Escarpment, a steep slope or long cliff that occurs from erosion or faulting and separates two relatively level areas of differing elevations...
s, valleys and grooves. The most prominent one runs parallel to the equator to the west of Pharos. These features likely formed as a result of the giant impacts, which formed Pharos and other large craters or as a result of tidal stresses from Neptune.
Origin
Proteus, like the other inner satellites of Neptune, is unlikely to be an original body that formed with it, more probably having accreted from the wreak rubble that remained after Triton's capture. Triton's orbit upon capture would have been highly eccentric, and would have caused chaotic perturbations in the orbits of the original inner Neptunian satellites, causing them to collide and reduce to a disc of rubble. Only after Triton's orbit became circularised did some of the rubble disc re-accrete into the present-day satellites.External links
- Proteus Profile at NASA's Solar System Exploration site
- Proteus page at The
Nine8 Planets - Proteus, A Moon Of Neptune on Views of the Solar System
- Ted Stryk's Proteus Page
- Neptune's Known Satellites (by Scott S. SheppardScott S. SheppardScott S. Sheppard is an astronomer in the Department of Terrestrial Magnetism at the Carnegie Institution for Science. Starting as a graduate student at the Institute for Astronomy at the University of Hawaii, he was credited with the discovery of many small moons of Jupiter, Saturn, Uranus, and...
)