• Neptune is the eighth and farthest planet from the Sun in the Solar System. Named for the Roman god of the sea, it is the fourth-largest planet by diameter and the third largest by mass. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus. On average, Neptune orbits the Sun at a distance of 30.1 AU, approximately 30 times the Earth–Sun distance. Its astronomical symbol is ♆, a stylized version of the god Neptunes trident.
Neptune’s Great Dark Spot (“Scooter”) moving east around planet, driven by strong, 700 miles/hour west winds
Discovery Neptune was the first planet found by mathematical prediction rather than by empirical observation. Unexpected changes in the orbit of Uranus led Alexis Bouvard to deduce that its orbit was subject to gravitational perturbation by an unknown planet. Neptune was subsequently observed on September 23, 1846 by Johann Galle within a degree of the position predicted by Urbain Le Verrier, and its largest moon, Triton, was discovered shortly thereafter, though none of the planets remaining 12 moons were located telescopically until the 20th century. Neptune has been visited by only one spacecraft, Voyager 2, which flew by the planet on August 25, 1989.
Lick Observatory image of Neptune and two of its moons
Neptune is similar in composition to Uranus, and both have compositions which differ from those of the larger gas giants, Jupiter and Saturn. Neptunes atmosphere, while similar to Jupiters and Saturns in that it is composed primarily of hydrogen and helium, along with traces of hydrocarbons and possibly nitrogen, contains a higher proportion of "ices" such as water, ammonia and methane. The interior of Neptune, like that of Uranus, is primarily composed of ices and rock.Composition
Dark, active nitrogen geysers on Triton;constant direction of streaks hints at a very thin atmosphere
“Cantelope terrain” on Triton hints at tectonic processes
Possible frozen lakes (of water, ammonia and salts) on Triton
Atmosphere• In contrast to the relatively featureless atmosphere of Uranus, Neptunes atmosphere is notable for its active and visible weather patterns. These weather patterns are driven by the strongest sustained winds of any planet in the Solar System, with recorded wind speeds as high as 2,100 km/h. Because of its great distance from the Sun, Neptunes outer atmosphere is one of the coldest places in the Solar System, with temperatures at its cloud tops approaching −218 °C . Temperatures at the planets centre are approximately 5,000 °C. Neptune has a faint and fragmented ring system, which may have been detected during the 1960s but was only indisputably confirmed in 1989 by Voyager 2.
Hubble and Keck views of Neptune, showing storms with Equatorial winds of 1,400 km/hour
Planetary Ring System Neptune has a planetary ring system, though one much less substantial than that of Saturn. The rings may consist of ice particles coated with silicates or carbon-based material, which most likely gives them a reddish hue. The three main rings are the narrow Adams Ring, 63000 km from the center of Neptune, the Le Verrier Ring, at 53000 km, and the broader, fainter Galle Ring, at 42000 km. A faint outward extension to the Le Verrier Ring has been named Lassell; it is bounded at its outer edge by the Arago Ring at 57000 km. Earth-based observations announced in 2005 appeared to show that Neptunes rings are much more unstable than previously thought. Images taken from the W. M. Keck Observatory in 2002 and 2003 show considerable decay in the rings when compared to images by Voyager 2. In particular, it seems that the Liberté arc might disappear in as little as one century.
The 2 most prominent, clumpy rings of Neptune (clumping of material) discovered by Voyager. Neptune is overexposed to show rings.
ClimateNeptunes weather is characterized by extremely dynamic storm systems, withwinds reaching speeds of almost 600 m/s—nearly attaining supersonic flow. At thecloud tops, the prevailing winds range in speed from 400 m/s along the equator to250 m/s at the poles. Most of the winds on Neptune move in a direction oppositethe planets rotation. The difference in flow direction is believed to be a "skin effect"and not due to any deeper atmospheric processes. The abundance of methane,ethane and ethyne at Neptunes equator is 10–100 times greater than at the poles.This is interpreted as evidence for upwelling at the equator and subsidence near thepoles. In 2007 it was discovered that the upper troposphere of Neptunes south polewas about 10 °C warmer than the rest of Neptune, which averages approximately−200 °C (70 K). The warmth differential is enough to let methane, which elsewherelies frozen in Neptunes upper atmosphere, leak out as gas through the south poleand into space. The relative "hot spot" is due to Neptunes axial tilt, which hasexposed the south pole to the Sun for the last quarter of Neptunes year, or roughly40 Earth years. As Neptune slowly moves towards the opposite side of the Sun, thesouth pole will be darkened and the north pole illuminated, causing the methanerelease to shift to the north pole. Because of seasonal changes, the cloud bands inthe southern hemisphere of Neptune have been observed to increase in size andalbedo. This trend was first seen in 1980 and is expected to last until about 2020.The long orbital period of Neptune results in seasons lasting forty years.
High-altitude cirrus-like clouds in Neptune’s atmosphere 50 – 150 km wide and maybe 50 km above main clouds
Natural satellites (Moons) Neptune has 13 known moons. The largest by far, comprising more than 99.5 percent of the mass in orbit around Neptune. Unlike all other large planetary moons in the Solar System, Triton has a retrograde orbit, indicating that it was captured rather than forming in place; it probably was once a dwarf planet in the Kuiper belt. From July to September 1989, Voyager 2 discovered six new Neptunian moons. Of these, the irregularly shaped Proteus is notable for being as large as a body of its density can be without being pulled into a spherical shape by its own gravity. Although the second-most-massive-Neptunian moon, it is only one-quarter of one percent the mass of Triton. Neptunes innermost four moons—Naiad, Thalassa, Despina and Galatea—orbit close enough to be within Neptunes rings. The next-farthest out, Larissa, was originally discovered in 1981 when it had occulted a star. Five new irregular moons discovered between 2002 and 2003 were announced in 2004. As Neptune was the Roman god of the sea, the planets moons have been named after smaller sea gods.
Neptune’s moon Triton Imaged by Voyager 2 in 1989 Triton moves around Neptune opposite to the planet’s rotation and movement around the Sun, but synchronous, i.e., the same side of Triton always faces Neptune The diameter of Triton is 2,705 km, 3/4 the size of our Moon It was probably captured, just like Pluto Its surface is made of solid methane (CH4) and N2, with T = - 400° F Its pinkish color is due to organic compounds that formed from CH4 and N2There are no large impact craters; hence, the surface is young (constantly being resurfaced) The albedo is very high, 0.80 and, hence, the moon is bright (presence of ices)S pole region has N2 fumaroles; ejecta are wind-blown, thus, there is a tenuous atmosphere The density = 2.1 g/cm2, hence, there must be ices present
Crescent of Neptunewith its moon Triton in the background
New moon of Neptune, 1989N1, discovered byVoyager. The large crater is 400 km in diameter.
Neptune’s moon Triton is 2,705 km in diameter. Bright ice cap in southern hemisphere (top) (notice black streaks), and dark terrain with “highways” below.
Neptune’s moon Nereid is 340 km in diameter.It has the most eccentric orbit of any moon in the Solar System. Separation from Neptune varies from 1.4 to 9.6 million km
Neptune’s moon Nereid Discovered in 1949 Not well imaged by Voyager 2, but the spacecraft discovered 6 smaller moons Most eccentric orbit of any moon in the Solar System,hence, it was probably also captured like Triton and Pluto Nereid is 340 km in diameter Its albedo is low, 0.15, so Nereid is dark like our Moon
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