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Proteus (pronounced /ˈproʊtiəs/,[note 5] or as in Greek Πρωτεύς), 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 near 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 highly irregular shape with several slightly concave facets. Its surface is dark, neutral in color and heavily cratered. The largest crater is more than 150 km in diameter. There are also a number of scarps, grooves and valleys related to large craters.

Proteus is likely not an original body that formed with Neptune. It probably accreted later from the debris created when the largest Neptunian satellite Triton was captured.

Discovered by Voyager 2
Stephen P. Synnott
Discovered on June 16, 1989
Orbital characteristics (Epoch J2000)
Semi-major axis 117,647 km (0.00079 AU)
Eccentricity 0.0005
Periastron 117,588 km
Apastron 117,706 km
Orbital period 1.122315 d
Orbital circumference 739,200 km (0.005 AU)
Orbital velocity max: 7.629 km/s
mean: 7.625 km/s
min: 7.621 km/s
Inclination 28.92° (to Ecliptic)
0.526° (to Neptune's equator)
0.026° (to the local Laplace plane)
Satellite of Neptune
Physical characteristics
Diameter 436 × 416 × 402 km
Surface area ~2,195,000 km2
Volume ~38,177,000 km3
Mass 5.0×1019 kg
Mean density 1.3 g/cm3
Surface gravity ~0.075 m/s2 (0.001 g)
Escape velocity ~0.18 km/s
Rotation period synchronous
Axial tilt zero
Albedo 0.10
Surface temp.
min mean max
K ~51 K K
Atmospheric pressure 0 kPa


Proteus was discovered from the images taken by Voyager 2 space probe two months before its Neptune flyby in August 1989. It received the temporary designation S/1989 N 1.[6] 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.[7]

On 16 September 1991 S/1989 N 1 was named after Proteus, the shape-changing sea god of Greek mythology.[8]

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.[1] 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.[2]

Physical characteristics

Proteus is the second largest moon of Neptune. It is more than 400 kilometres in diameter, larger than Nereid, 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.[6] 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.[6] 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 hydrocarbons or cyanides. 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.[9]

Proteus is irregularly shaped; scientists believe it is about as large as a body of its density can be without being pulled into a spherical shape by its own gravity.[10] Saturn's moon Mimas has a more spherical shape despite being slightly less massive than Proteus, perhaps due to its lower density. The shape of Proteus can be approximated by a triaxial ellipsoid, one axis of which is longer by about 30 km than other two axises. Still, this approximation is not adequate for this moon, because its shape is actually closer to an irregular polyhedron. Proteus has a few flat or slightly concave facets measuring from 150 to 200 km in diameter. They are probably degraded impact craters.[2]

Proteus is heavily cratered, showing no sign of any geological modification. The largest crater, Pharos, has a diameter of 150–230 km.[6] Its depth is about 10–15 km. The crater has a central dome on its floor a few kilometers high.[2] Pharos is the only named surface feature on this moon: the name is Greek and refers to the island where Proteus reigned.[11] In addition to Pharos there are several craters 50–100 km in diameter and many more with diameters less than 50 km.[2]

The second landform found on Proteus is linear features such as scarps, 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.[2]


Proteus like other inner satellites of Neptune is likely not an original body that formed with Neptune. It probably 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.[12] Only after Triton's orbit became circularised did some of the rubble disc re-accrete into the present-day satellites.[13]


1. ^ In the earlier papers slightly different dimensions were reported. Thomas and Veverka in 1991 reported 440×416×404 km.[3][2] Croft in 1992 reported 430×424×410 km.[2] The difference is caused by the use of different sets of images and by the fact that the shape of Proteus is not described well by a triaxial ellipsoid.[2]
2. ^ The mass was calculated by multiplying the volume from Stooke, 1994 by the assumed density of 1,300 kg/m3. If one uses slightly larger dimensions from the earlier papers the mass will increase to 5 × 1019 kg.[4]
3. ^ Surface gravity derived from the mass m, the gravitational constant G and the radius r: Gm/r2.
4. ^ Escape velocity derived from the mass m, the gravitational constant G and the radius r: √2Gm/r.
5. ^ In US dictionary transcription, us dict: prō′·tē·əs.

Moons of Neptune

see also: The Solar System

Astronomy Encyclopedia

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