Proteus (moon)
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Proteus is the second-largest moon of Neptune after Triton and the largest of the planet's inner satellites. Despite a volumetric mean diameter of approximately 416 km, it retains an irregular, angular shape. Its dark surface is densely cratered and includes the enormous Pharos impact basin.[1][7]
Proteus was discovered by Voyager 2 in 1989. It completes one orbit around Neptune in approximately 27 hours, moving in the same direction as the planet rotates.[5][3]
Size and physical characteristics
Proteus has a volumetric mean radius of 208 ± 8 km. Its dimensions along three principal axes are approximately 440 × 416 × 404 km. The longest axis exceeds the shortest by about 36 km, and large depressions and facets increase the difference from a smooth ellipsoid.[1][4][7]
Proteus is slightly wider than Saturn's moon Mimas, but its outline is much less rounded. Differences in surface elevation reach approximately 20 km, a substantial fraction of the moon's mean radius.[1][7]
JPL lists a mass of approximately 3.9 × 1019 kg, but the uncertainty is almost as large as the value itself. The corresponding density is roughly 1.0 ± 1.0 g/cm³. These measurements do not yet provide a reliable estimate of the proportions of ice, rock and internal voids.[1]
Orbit and rotation
Proteus orbits at a mean distance of approximately 117,600 km from Neptune's centre. One revolution takes 1.122315 Earth days, or about 26 hours 56 minutes. The orbit is nearly circular and lies close to the planet's equatorial plane.[3][5]
Its rotation is synchronous: Proteus rotates once on its axis during each orbit and keeps approximately the same hemisphere facing Neptune.[4]
Hippocamp orbits inside the path of Proteus with a semi-major axis of approximately 105,300 km. Their mean orbital distances differ by about 12,300 km, and their motions are close to a 13:11 orbital resonance.[3][8]
Surface and topography
Proteus has a dark, densely cratered surface. Voyager 2 images also reveal linear features resembling grooves. The moon's overall form is similar to an irregular polyhedron, with several regions appearing almost flat or slightly concave.[7][9]
Early estimates indicate that the surface reflects only about 6% of incident sunlight. Its low brightness and apparent proximity to the much brighter Neptune helped prevent its discovery from Earth.[5]
Available images show no clear evidence of large-scale geological resurfacing. Impact structures dominate the visible landscape.[5]
Pharos impact basin
Pharos is the largest known impact structure on Proteus. Its diameter is estimated at approximately 230–260 km, more than half the mean diameter of the moon.[7][10]
Geological analysis of the Voyager 2 images placed the basin's depth at approximately 10–15 km. The collision substantially altered the shape of Proteus and left a depression occupying a large fraction of its visible surface.[10]
Pharos has also been proposed as the source of material from which Hippocamp formed. This hypothesis connects a major impact on Proteus with the later evolution of Neptune's entire inner satellite system.[11]
Possible origin of Hippocamp
Hippocamp may have formed from debris ejected from Proteus. Such an origin would explain why the tiny moon travels so close to a much larger body.[11]
Material thrown from Proteus by a large collision could have remained in orbit around Neptune and later accreted into a new moon. Tidal evolution then moved Proteus gradually farther from the planet, increasing the distance between the pair. The survival of a large impact basin on Proteus supports this sequence of events.[12]
Surface composition from JWST observations
JWST spectra of Proteus, Larissa, Galatea and Neptune's rings contain deep OH absorption bands near 3 μm. At the same time, the observations show no characteristic evidence of exposed water ice.[2]
A diagnostic band of magnesium-rich phyllosilicates near 2.72 μm was detected on Larissa and Galatea and in the rings, but not on Proteus. The compositions of the three inner moons therefore differ in important respects.[2]
The present inner moons may have reaccreted from the debris of older bodies destroyed during Triton's capture or from a dwarf planet disrupted by Neptune's tides. In either case, their surfaces could preserve material that once lay deep inside substantially larger objects.[2]
Discovery and exploration
Proteus was discovered by the Voyager 2 science team in 1989 as the spacecraft approached Neptune. It was initially designated S/1989 N1 and later named Proteus after the shape-changing sea god of Greek mythology. The moon is also designated Neptune VIII.[5][6]
In a Voyager 2 image obtained on 25 August 1989 from a distance of approximately 146,000 km, Proteus is about half illuminated. Its irregular outline, large depressions and linear structures are visible. A short exposure was used to reduce motion blur, leaving the image noticeably grainy.[9]
Voyager 2 first revealed the moon's shape and large-scale topography. Later observations by space-based and ground-based telescopes added astrometry, measurements of its interaction with neighbouring moons and spectroscopy of the surface.[8][2]
See also
References
- ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 Planetary Satellite Physical Parameters // NASA JPL. Proteus: GM = 2.58342 ± 2.42070 km³/s² from the NEP097 solution. Mass calculated using G = 6.67430 × 10−11 m³/(kg·s²).
- ↑ 2.0 2.1 2.2 2.3 2.4 Davis M. R. et al. Neptune’s Inner Moons and Rings Are Exposed Icy Body Interiors // Science Advances. 2026. Vol. 12. Article eaeb1437.
- ↑ 3.0 3.1 3.2 3.3 3.4 3.5 3.6 3.7 Planetary Satellite Mean Elements // NASA JPL. Proteus: mean elements for 1 January 2000, 12:00 TDB.
- ↑ 4.0 4.1 4.2 Neptunian Satellite Fact Sheet // NASA NSSDCA.
- ↑ 5.0 5.1 5.2 5.3 5.4 5.5 Proteus // NASA Science.
- ↑ 6.0 6.1 Neptune Moons // NASA Science. Table of moons and discovery circumstances.
- ↑ 7.0 7.1 7.2 7.3 7.4 Proteus Information Page // NASA Planetary Data System, Small Bodies Node.
- ↑ 8.0 8.1 Brozović M. et al. Orbits and Resonances of the Regular Moons of Neptune // Icarus. 2020. Vol. 338. Article 113462.
- ↑ 9.0 9.1 Proteus (Voyager 2); PIA00062 image description // NASA/JPL. Wikimedia Commons.
- ↑ 10.0 10.1 Croft S. K. Proteus: Geology, Shape, and Catastrophic Destruction // Icarus. 1992. Vol. 99. P. 402–408.
- ↑ 11.0 11.1 Showalter M. R. et al. The Seventh Inner Moon of Neptune // Nature. 2019. Vol. 566. P. 350–353.
- ↑ Tiny Neptune Moon May Have Broken from Larger Moon // NASA/ESA/STScI. 20 February 2019.
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