Science – Adopt a Planet

NEPTUNE

• 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 Neptune's 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 planet's 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

Composition

Neptune is similar in composition to Uranus, and both have compositions which differ from those of the larger gas giants, Jupiter and Saturn. Neptune's atmosphere, while similar to Jupiter's and Saturn's 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.

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,

Neptune's 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, Neptune's outer atmosphere is one of the coldest places in the Solar System, with temperatures at its cloud tops approaching −218 °C . Temperatures at the planet's 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.

Magnetic field of Neptune.

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 Neptune's 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.

ClimateNeptune's weather is characterized by extremely dynamic storm systems, with winds reaching speeds of almost 600 m/s—nearly attaining supersonic flow. At the cloud tops, the prevailing winds range in speed from 400 m/s along the equator to 250 m/s at the poles. Most of the winds on Neptune move in a direction opposite the planet's 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 Neptune's equator is 10–100 times greater than at the poles. This is interpreted as evidence for upwelling at the equator and subsidence near the poles. In 2007 it was discovered that the upper troposphere of Neptune's south pole was 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 elsewhere lies frozen in Neptune's upper atmosphere, leak out as gas through the south pole and into space. The relative "hot spot" is due to Neptune's axial tilt, which has exposed the south pole to the Sun for the last quarter of Neptune's year, or roughly 40 Earth years. As Neptune slowly moves towards the opposite side of the Sun, the south pole will be darkened and the north pole illuminated, causing the methane release to shift to the north pole. Because of seasonal changes, the cloud bands in the southern hemisphere of Neptune have been observed to increase in size and albedo. 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. Neptune's innermost four moons—Naiad, Thalassa, Despina and Galatea—orbit close enough to be within Neptune's 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 planet's 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 N2

There 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 Neptune with its moon Triton in

the background

New moon of Neptune, 1989N1, discovered by

Voyager. 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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