European Geosciences Union

• Publications
• Meetings
• Membership
• Contact
• Login

(What is this?)

Neptune

Neptune is the 8^th and ultimate planet of our solar system (since 2006 when the International Astronomical Union defined the term “planet” and declassified Pluto as a “dwarf planet”). Unseen from Earth with the naked eye (it has a small magnitude 8.02 to 7.78), Neptune hadn’t been discovered until September 23, 1846 when it was observed by Johann Galle (Berlin Observatory) within 1° of the position indicated by Urbain Le Verrier, making Neptune the first planet to be found after mathematical prediction rather than by observation. Although the planet was included in Galileo’s drawings of 1612 & 1613, it had just turned retrograde at that time, therefore seemingly stationary and as a result Galileo mistook it for a star. In 1821 Alexis Bouvard calculated the orbit of Uranus but observations revealed unexpected deviations, which led astronomers to seek for an unknown planet that would cause these gravitational perturbations.  The planet’s greenish blue hue (along with the other planets’ nomenclature) inspired astronomers to name Neptune after the Roman god, similar or identical to the Greek god Poseidon, master of the oceans and the seas, “earth-shaker” and horse tamer, son of Cronus and Rhea, brother of Zeus and Pluto.

The furthest from the Sun planet Neptune has orbit is very close to the ecliptic with an inclination 1.8°, a semi-major axis 30.1 AU, with aphelion 30.4 AU, perihelion 29.8 AU, eccentricity 0.011, orbital period 164.79 years and synodic period almost a year 367.49 days. Amongst the planets of our solar system, Neptune is the 3^rd largest by mass (17 M_earth) and the 4^th largest by radius (equatorial radius 3.88 R_earth polar one 3.829 R_earth) and mean density 1.638 g/cm3 which is the largest of all giant gassy planets, and a gravity almost equal to the one of Earth, 1.14 g. Sidereal rotation period 16 h 6 m and the axial tilt 28.32° close to the Earth’s tilt.

Surface temperature is extremely low, as Neptune is very far from the Sun. At the distance of 30 AUs receives 1/1000^th of the power the Earth receives from the Sun and radiates 160% more energy than it receives from our star, energy that comes from radiogenic sources or many more exotic ones. The measured temperature at 1 bar 72 K and at 0.1 bar 55 K. The albedo is 0.29 (bond) and 0.41 (geometric).

The chemical composition of the atmosphere is 80±3.2 molecular hydrogen, 19±3.2% helium, 1.5±0.5% methane and small amounts of hydrogen deuteride, ethane, ammonia, water, ammonium hydrosulfide, the last ones in form of ice.

The only spacecraft that has visited Neptune so far is Voyager 2, a robotic space probe launched by NASA on August 20, 1977 in order to study our solar system. Voyager 2 flew by Neptune’s moon Nereid before making its closest approach to the planet on August 25, 1989, entering within 4400km of Neptune’s atmosphere. Later on the same day the spacecraft made a close encounter with the planet’s largest moon Triton. The mission revealed 6 new moons, more than one rings and a very active weather system. Neptune’s magnetic field was verified (found similar to that of Uranus) and measurements of radio emissions were used to determine the planet’s rotation period.

The planet has 13 moons, Triton being the largest, the only one massive enough to be spherical and the only one, amongst all planetary moons in the solar system, that has a retrograde orbit (suggesting he was once an object of the Kuiper belt, then got captured by Neptune’s gravity). Other moons are Proteus, Nereid, Naiad, Thalassa, Despina, Galateia & Larissa.  

Neptune is in many ways similar to Uranus; as a result they are often referred to as the “ice giants”, having a smaller size and higher abundances of volatiles compared to Jupiter and Saturn. Atmosphere consists of hydrogen (80%), helium(19%) and traces of methane. The latter absorb red & infrared light, causing the planet’s blue appearance. In the lower atmospheric regions there are higher abundances of methane, ammonia & water, gradually condensing into a superheated liquid mantle of water, methane & ammonia “ices”, highly dense & electrically conductive, where temperature may reach 2000 – 5000K. The “rocky” core consists of iron, nickel & silicates.

Convective fluid motions of the mantle’s material generate, with dynamo mechanism Neptune’s complex magnetic field whose dipole component is tilted at 47° relative to the rotational axis and offset at least 0.55 radii from the planet’s centre, plus there are strong contributions from a quadrupole component. Neptune’s atmosphere is characterized by active weather patterns, driven by almost supersonic winds (as high as 2100km/h) most of which flow in an opposite direction regarding the planet’s rotation, while its upper clouds reach temperatures as low as -218K. Neptune radiates 2.61 times the energy he receives from the sun. Possible explanations include radiogenic heating from the core, conversion of methane into hydrogen or diamond under high pressure & gravity waves above the tropopause caused by convection in the lower atmosphere.

LINKS:

http://solarsystem.nasa.gov/planets/profile.cfm?Object=Neptune

http://voyager.jpl.nasa.gov/image/neptune.html

PICTURES:

Neptune Great Dark Spot, a large anticyclonic structure like Jupiter's Great Red Spot, NASA, LPL.

Credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA)

Extremely dynamic storm systems and vortex stuctures become visible, like the Great Dark Spot of the southern hemisphere, an anti-cyclonic storm system discovered by NASA’s Voyager 2 in 1989. When the Hubble Space Telescope took pictures of Neptune in 1994, the Great Dark Spot had disappeared while a similar storm was found in the northern hemisphere.

Planet Neptune along with some of his moons, picture taken by Hubble SpaceTelescope.

From http://pds.nasa.gov/planets/special/neptune.htm  

• Publications
• Meetings
• Membership
• Imprint
• Sitemap
• Contact

• Top