While Proteus is usually thought to contain significant amounts of water ice, it has not been detected spectroscopically on the surface. These compounds may be responsible for the low albedo of the inner Neptunian moons. 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. The surface's color is neutral as the reflectivity does not change appreciably with the wavelength from violet to green. The surface of Proteus is dark-its geometrical albedo is about 10%. 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. It is more than 400 kilometres in diameter, larger than Nereid, the second to be discovered. Proteus is the second largest moon of Neptune. It rotates synchronously with the orbital motion, which means that one face always points to the planet. Proteus is the largest of the regular prograde satellites of Neptune. Its orbit has a small eccentricity and is inclined by about 0.5° to the planet's equator. Proteus orbits Neptune at the distance approximately equal to 4.75 equatorial radii of the planet. On 16 September 1991 S/1989 N 1 was named after Proteus, the shape-changing sea god of Greek mythology. 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. It received the temporary designation S/1989 N 1. Proteus was discovered from the images taken by Voyager 2 space probe two months before its Neptune flyby in August 1989. It probably accreted later from the debris created when the largest Neptunian satellite Triton was captured. Proteus is likely not an original body that formed with Neptune. There are also a number of scarps, grooves and valleys related to large craters. The largest crater is more than 150 km in diameter. Its surface is dark, neutral in color and heavily cratered. Proteus circles Neptune in a near equatorial orbit at the distance of about 4.75 equatorial radii of the planet.ĭespite being more than 400 km in diameter Proteus has a highly irregular shape with several slightly concave facets. Discovered by Voyager 2 spacecraft in 1989, it is named after Proteus, the shape-changing sea god of Greek mythology. Proteus (pronounced /ˈproʊtiəs/, or as in Greek Πρωτεύς), also known as Neptune VIII, is the second largest Neptunian moon, and Neptune's largest inner satellite.
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