Things about Neptune

 What we know about the Blue Planet

Introduction:

Neptune is the eighth and the farthest known planet from the Sun. It is the fourth-largest planet by diameter in the solar system after Jupiter, Saturn, and Uranus. Neptune is also the densest of all the gas giants. It is 17 times the mass of the Earth and slightly more massive than its near-twin Uranus. Neptune is denser and physically smaller than Uranus because its greater mass causes more gravitational compression of its atmosphere. The planet orbits the Sun once every 164.8 years at an average distance of 4.5 billion kilometers from its star. Neptune is not visible to the unaided eye and is the only planet in the solar system found by mathematical predictions rather than by empirical observation. The planet's distance from the Earth gives it a very small apparent size, making it challenging to study with Earth-based telescopes.

While studying about Uranus, Alexis Bouvard noticed some unexpected changes in the orbit of Uranus. This led her to deduce that it was because of the gravitational attraction of some unknown planet. Neptune was subsequently observed with a telescope on 23rd September 1846 by Johann Galle. Neptune was visited by Voyager 2 when it flew by the planet on 25th August 1989; Voyager 2 remains the only spacecraft to visit Neptune. The advent of the Hubble Space Telescope and large ground-based telescopes with adaptive optics has recently allowed for additional detailed observations from afar.

 

Atmosphere and the interior:

Neptune is very similar to all the other gas giants, comprising of 80% hydrogen and 19% helium, along with traces of hydrocarbons and possibly nitrogen; though it contains a higher proportion of 'ices' such as water, ammonia, and methane. Uranus and Neptune are normally considered 'ice giants' to emphasize this distinction. Traces of methane in the outermost regions in the part account for the planet's blue appearance, though an unknown component is believed to color Neptune a deeper blue compared to Uranus. 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 IRS cloud tops approaching 55 K (-218℃). Temperatures at the planet's center are approximately 5,400 K (5,100℃). Neptune's atmosphere is subdivided into two main regions: the lower troposphere, where temperature decreases with altitude, and the stratosphere, where temperature increases with altitude. Seeing Neptune's spectra, it is believed that its lower stratosphere is hazy due to the condensation of products of ultraviolet photolysis of methane, such as ethane and ethyne. The stratosphere of Neptune is warmer than that of Uranus due to the elevated concentration of hydrocarbons.

Neptune's atmosphere has active and visible weather patterns. For example, at the time of the Voyager 2 flyby in 1989, the planet's southern hemisphere had a Great Dark Spot comparable to the Great Red spot on Jupiter. More recently in 2018, a newer main dark spot and a smaller dark spot were identified and studied. Most of the winds on Neptune move in a direction opposite in the planet's rotation. Neptune's weather is characterized by extremely dynamic storm systems, with winds reaching speeds of almost 2,200 km/hr (nearly reaching supersonic flow). The planet is also too far from the Sun for heat to be generated by ultraviolet radiation. One candidate for the heating mechanism is atmospheric interaction with ions in the planet's magnetic field. Other candidates are gravity waves from the interior that dissipate in the atmosphere. 

Its interior is primarily composed of ice and rock. Neptune's internal structure resembles that of Uranus; its atmosphere forms about 5 to 10% of its mass and extends perhaps 10 to 20% of the way towards the core. Increasing concentrations of methane, ammonia, and water are found in the lower regions of the atmosphere. The mantle is equivalent to 10 to 15 Earth masses and is rich in, again, water, ammonia, and methane. As is customary in planetary science, this mixture is referred to as icy even though it is a hot, dense fluid. This fluid, which has high electrical conductivity, is sometimes called a water-ammonia ocean. The mantle may consist of a layer of ionic water in which the water molecules break down into a soup of hydrogen and oxygen ions, and deeper down superionic water in which the oxygen crystallizes but the hydrogen ions float around freely within the oxygen lattice. 

At a depth of 7,000 km, the conditions may be such that methane decomposes into diamond crystals that rain downwards like hailstones. very high-pressure experiments at the Lawrence Livermore National Laboratory suggest that the top of the mantle may be an ocean of liquid carbon with floating solid 'diamonds'. The core of Neptune is likely composed of iron, nickel, and silicates, with an interior model giving a mass of about 1.2 times that of the Earth.

Magnetosphere:

Before Voyager 2's arrival at Neptune, it was hypothesized that Uranus' tilted magnetosphere was the result of its sideways rotation. On comparing the magnetic field of the two planets, scientists now think the extreme orientation may be characteristic of flows in the planets' interiors. This field may be generated by convective fluid motions in a thin spherical shell of electrically conducting liquids (probably a combination of ammonia, methane, and water).

Moons and rings:

Neptune has 14 known moons. Triton is the largest Neptunian moon, comprising more than 99.5% of the mass in the 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 was discovered shortly after the discovery of Neptune itself, though none of the planet's remaining 13 known moons were located telescopically until the 20th century. Neptune's second-known natural satellite, the irregular moon Nereid, has one of the most eccentric orbits of any satellite in the Solar System. 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. 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. A new moon and the smallest yet, Hippocamp, was found in 2013 by combining multiple Hubble images.

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, 63,000 km from the center of Neptune, the Le Verrier Ring, at 53,000 km, and the broader, fainter Galle Ring, at 42,000 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 57,000 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 Liberte arc might disappear in as little as one century.