Proteus (moon)

Template:Short description {{#invoke:infobox|infoboxTemplate | class = vcard | titleclass = fn org | title = Proteus | image = {{#invoke:InfoboxImage|InfoboxImage|image=Proteus (Voyager 2).jpg|upright={{#if:||1.1}}|alt=}} | caption = Processed grayscale image of Proteus from Voyager 2, August 1989 (image processing date). The massive crater Pharos occupies much of the upper right, straddling Proteus's terminator | headerstyle = {{#if:|background-color:|background-color:#E0CCFF}} | labelstyle = max-width:{{#if:||11em}}; | autoheaders = y

| header1 = Discovery

| header10 = {{#if:|Designations|Designations}}

| header20 = Orbital characteristics{{#ifeq:|yes| (barycentric)}}<ref name="Jacobson2004" />

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| header60 = Proper orbital elements

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| header70 = Template:Anchor{{#if:| Physical characteristics|Physical characteristics}}

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| header110 = Atmosphere

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Proteus (Template:IPAc-en Template:Respell), 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.<ref name="IAUC 5347" /> Proteus orbits Neptune in a nearly equatorial orbit at a distance of about 4.75 times the radius of Neptune's equator.<ref name="Jacobson2004" />

Despite being a predominantly icy body more than Template:Convert in diameter, Proteus's shape deviates significantly from an ellipsoid.<ref name="Croft1992" /> It is shaped more like an irregular polyhedron with several slightly concave facets and relief as high as Template:Convert. Its surface is dark, neutral in color, and heavily cratered.<ref name="DumasSmithTerrile2003" /> Proteus's largest crater is Pharos, which is more than Template:Convert in diameter. There are also a number of scarps, grooves, and valleys related to large craters.

Proteus is probably not an original body that formed with Neptune. It could have accreted later from the debris formed when the largest Neptunian satellite, Triton, was captured.<ref name="Goldreich1989" />

Discovery and namingEdit

File:Voyager.jpg

Voyager 2, the space probe that discovered Proteus

Proteus was discovered from the images taken by the Voyager 2 space probe two months before its Neptune flyby in August 1989. Proteus was discovered 40 years after the discovery of Neptune's moon Nereid in 1949.<ref name="indepth"/>

Upon discovery, Proteus received the temporary provisional designation S/1989 N 1.<ref name="SmithSoderblom1989" /> 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.<ref name="IAUC 4806" />

On 16 September 1991, S/1989 N 1 was named after Proteus, the shape-changing sea god of Greek mythology<ref name="IAUC 5347" /> following the convention that moons of Neptune should be named after deities and creatures related to the sea.

OrbitEdit

Template:Annotated image Proteus orbits Neptune at a distance of approximately Template:Convert, nearly equal to 4.75 times the equatorial radius of the planet. The orbit of Proteus is nearly circular, having a small orbital eccentricity, and is inclined by about 0.5 degrees to Neptune's equator.<ref name="Jacobson2004" /> Proteus is tidally locked to Neptune, and rotates synchronously with its orbital motion, which means that Proteus always presents the same face to Neptune.<ref name="Stooke1994" />

Proteus may have once been in a 1:2 orbital resonance with Larissa, where Proteus made one orbit for every two orbits made by Larissa. Due to the outward tidal migration of Proteus over time, this is no longer the case.<ref name="ZhangHamiltonI"/> Proteus may have ceased its integral orbital resonance with Larissa several hundred million years ago.<ref name="ZhangHamiltonI"/><ref name="ZhangHamiltonII"/>

Physical characteristicsEdit

File:ProblematicProteus.jpg

Proteus compared to 1 Ceres and the Moon<ref name="Stooke1994" />

Proteus is the second-largest moon of Neptune and is the largest of its regular prograde moons. It is about Template:Convert in diameter, larger than Nereid, Neptune's third-largest moon. It was not discovered by Earth-based telescopes because Proteus orbits so close to Neptune that it is lost in the glare of reflected sunlight.<ref name="SmithSoderblom1989" />

CompositionEdit

File:Proteus Color.png

False-color Voyager 2 image of Proteus, with its large crater Pharos located at the right

The surface of Proteus is dark as it has a geometrical albedo of about 10 percent–meaning that its surface reflects 10 percent of the light it receives from the Sun. The color of its surface is neutral as the reflectivity does not change appreciably with the wavelength from violet to green.<ref name="SmithSoderblom1989" /> In the near-infrared range of around the wavelength of 2 μm, Proteus's surface becomes less reflective, suggesting the 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, there is no definite spectroscopic confirmation of its presence on the surface.<ref name="DumasSmithTerrile2003" /> However, James Webb Space Telescope NIRCam spectrophotometry of the moon has revealed a 3 μm absorption feature, indicating possible water ice or hydrated minerals on the surface of Proteus.<ref name=":0">Template:Cite journal</ref> Proteus's infrared albedo measured at 1.4, 2.1, 3.0, and 4.6 microns appears to match other dark Solar System small bodies, such as Template:Mpl-.<ref name=":0" />

ShapeEdit

The shape of Proteus is close to a sphere with a radius of about Template:Convert, although deviations from the spherical shape are large—up to Template:Convert; 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.<ref name="Croft1992" /> Proteus is slightly elongated in the direction of Neptune, although its overall 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.<ref name="Stooke1994" />

Surface featuresEdit

File:Proteus - August 25 1989 (31541728335).jpg

Voyager 2 image of Proteus, digitally processed showing surface features

File:Proteus map (Stooke).jpg

Map of Proteus

Proteus is heavily cratered, showing no sign of any geological modification.<ref name="SmithSoderblom1989" /> The largest crater, Pharos, has a diameter from Template:Nowrap.<ref name="Croft1992" /> Its depth is about 10–15 km.<ref name="Stooke1994" /> The crater has a central dome on its floor a few kilometres high.<ref name="Stooke1994" /> Pharos is the only named surface feature on this moon. In Greek mythology, it refers to the island where Proteus reigned.<ref name="usgsPharos" /> In addition to Pharos, there are several craters 50–100 km in diameter and many more with diameters less than 50 km.<ref name="Stooke1994" />

Other landforms on Proteus include 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 or as a result of tidal stresses from Neptune.<ref name="Stooke1994" /><ref name="Croft1992" />

Named featuresEdit

Protean craters are named after water-related spirits, gods, goddesses (excluding Greek and Roman names). As of May 2024, there is only one named crater on this body.<ref name="usgsPharos" />

Crater	Pronunciation	Diameter	Approval Year	Eponym	Ref
Pharos	Template:IPAc-en Template:Respell	230–260 km<ref name="Croft1992" />	1994	Lighthouse of Alexandria (Pharos), island where Proteus reigned	Template:WGPSN

OriginEdit

Proteus, like the other inner moons of Neptune, is unlikely to be an original body that formed with it, and is more likely to have accreted from the rubble that was produced 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 moons, causing them to collide and reduce to a disc of rubble.<ref name="Goldreich1989" /> Only after Triton's orbit became circularised did some of the rubble disc re-accrete into the present-day moons of Neptune.<ref name="BanfieldMurray1992" />

Proteus initially had a smaller orbit around Neptune, and may have formed closer to the planet. At the time of the formation of Proteus, its orbit was estimated to be Template:Convert less than its current distance from Neptune.<ref name="Showalter2019"/> Over time, Proteus migrated outward from Neptune through tidal interactions. During the outward migration of Proteus, collisions and impact events may have formed large craters and had ejected fragments of Proteus into orbit around Neptune. A collision of Proteus and another object could possibly explain the origin of Neptune's smaller moon Hippocamp, which orbits close to Proteus.<ref name="Showalter2019"/>

NotesEdit

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ReferencesEdit

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External linksEdit

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Proteus In Depth at NASA's Solar System Exploration site
Proteus page at The Nine Planets
Proteus, A Moon Of Neptune on Views of the Solar System
Ted Stryk's Proteus Page
Neptune's Known Satellites (by Scott S. Sheppard)

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