Neptune is about 4.5 billion km from the Sun, making it about 1.5 times more distant than Uranus. It takes over 165 Earth years to orbit the Sun once, and it rotates on its axis once every 16.1 h. Because of its great distance from the Sun, Neptune receives only 0.1 % of the sunlight that Earth receives- so the planet’s surface is very cold.
The planet cannot be seen with the unaided eye from Earth, but it can be seen through a good telescope. It then appears as a tiny featureless disc that is barely distinguishable from a star. Most of our information about Neptune came from the Voyager 2 space probe that passed by Neptune in 1989. Voyager found Neptune to have a deep blue colour with an outer layer covered with whitish clouds (Fig. 12.1).
Fig. 12.1Neptune as seen by Voyager 2. At the north (top) is the Great Dark Spot, accompanied by bright, white clouds that undergo rapid changes in appearance. To the south is the bright feature that Voyager scientists have nicknamed “Scooter.” Still farther south is the feature called “Dark Spot 2,” which has a bright core. Each feature moves eastward at a different velocity, so it is only occasionally that they appear close to each other, such as at the time this picture was taken (Credit: NASA).
Early Views About Neptune
In Roman mythology Neptune was the god of the sea. Such a god was important to the Romans because sea travel formed a key part of their life. Neptune was the son of Saturn and Orps (called Cronus and Rhea by the Greeks). Neptune also resembled the Greek god Poseidon, which was god of the sea, earthquakes and horses.
Early astronomers did not know about Neptune as a planet because it could not be seen from Earth by the unaided eye. The discovery of Neptune was a great triumph of mathematics. After the discovery of Uranus in 1781, astronomers noticed that its orbit was not following the path predicted by Newton’s and Kepler’s laws of motion. Either these laws were wrong or there had to be another undiscovered planetary body in the solar system influencing Uranus’ orbit. In 1845 and 1846, John Couch Adams in England and Urbain Le Verrier in France, independently calculated the mass and location of such a body. The body, now called Neptune, was discovered in 1846 by German astronomer Johann Gottfried Galle, close to the predicted position. Within a few weeks the British astronomer William Lassell discovered a moon around Neptune—it was called Triton after the half-man, half-fish son of the sea god (Table 12.1).Table 12.1Details of Neptune
Distance from Sun
4,504,000,000 km (30.1 AU)
Diameter
49,532 km
Mass
1.02 × 1026 kg (17.1 times Earth’s mass)
Density
1.64 g/cm3 or 1640 kg/m3
Orbital eccentricity
0.010
Period of revolution
60,225 Earth days or 165 Earth years
Rotation period
16 h 7 min
Orbital velocity
19,548 km/h
Tilt of axis
29.6°
Average temperature
−225 °C
Number of Moons
14
Atmosphere
Hydrogen, helium, some methane
Strength of gravity
11.2 N/kg at surface
Probing Neptune
Astronomers know little about Neptune because it is so far from Earth. Neptune has been visited by only one spacecraft, Voyager 2 on 25 August 1989. Almost everything we know about Neptune has come from this encounter. Voyager had been travelling for about 12 years and had covered nearly 5 billion km to reach Neptune. The space probe came to within 5000 km of the planet and it collected a wealth of information about this most distant gas giant and its moons.
Voyager found Neptune to be a large blue planet, with many markings and cloud bands. Five thin rings were also found around the planet and six new moons were discovered to add to the two already known. The rings were found to be complete rings with bright clumps in them. One of the rings appeared to have a curious twisted structure. The rings are very dark and their composition is unknown (Table 12.2).Table 12.2Significant space probes to Neptune
Probe
Country of origin
Launched
Comments
Voyager 2
USA
1977
Passed by Neptune in August 1989
Position and Orbit
Neptune is the eighth planet from the Sun and the fourth largest planetary member of the solar system. Its orbit is slightly elliptical and lies beyond that of Uranus. Neptune has a mean distance from the Sun of about 4500 million km, placing it about 30.1 times further from the Sun than Earth. Neptune is so far away that when Voyager 2 was passing the planet the radio signals from the probe took over 4 h to reach Earth.
Neptune travels around the Sun once every 165 years and rotates with a period of 16 h 7 min. In its equatorial zone, winds blow westward at close to 1500 km/h, creating huge storms.
Neptune spins on its axis at an angle of 29° from the vertical. This amount of axial tilt is similar to that of Earth. Because of this angle, it was expected that Neptune’s poles would be colder than its equator. However, astronomers using Europe’s Very Large Telescope in Chile, found that the planet’s south pole is about 10 °C warmer than elsewhere. This imbalance in temperature probably explains why Neptune has such strong winds.
Neptune’s orbit has a profound impact on the region directly beyond it, known as the Kuiper belt. The Kuiper belt is a ring of small icy worlds, similar to the asteroid belt but far larger, extending from Neptune’s orbit at 30 AU out to about 55 AU from the Sun. Much in the same way that Jupiter’s gravity dominates the asteroid belt, shaping its structure, so Neptune’s gravity dominates the Kuiper belt. Over the age of the solar system, certain regions of the Kuiper belt became destabilised by Neptune’s gravity, creating gaps in the Kuiper belt’s structure.
Density and Composition
Neptune is a gaseous planet with a mass about 17 times that of Earth, but it is not as dense as Earth. The average density of Neptune is about 1.64 g/cm3, compared to Earth’s density of 5.52 g/cm3. This is mainly due to the different composition of each planet—Earth is a rocky planet, while Neptune is a gaseous one.
Neptune’s composition and interior is similar to that of Uranus. Both planets have a rocky core surrounded by frozen ammonia, methane and water. Hydrogen contributes only about 15 % of the planet’s total mass. Compared to Jupiter and Saturn, Neptune has more ammonia, methane and water.
Neptune’s interior consists of a small but dense core of melted rock, mostly iron, nickel and silicates. In the core, the temperature and pressure is very high. Although Neptune receives 40 % less sunlight than Uranus, their surface heat is almost same. More interestingly, Neptune gives off 2.6 times more energy than the energy it takes from the Sun! The rocky cores temperature is so hot that it can melt rocks. The high heat of Neptune and the cold temperature of the space around it create a huge temperature difference. This difference creates a huge wind blasting like a hurricane.
Because of Neptune’s greater density, its core is probably slightly larger than the core of Uranus. Surrounding the core is a mantle of water, ammonia and methane, and an outer layer or ocean of hydrogen, helium and methane. However, Neptune’s interior layers may not be distinct and uniform.
The strength of gravity on Neptune is greater than Earth’s gravity (11.2 compared to Earth’s 9.8 N/kg). This means that a 75 kg person weighing 735 N on Earth would weigh 840 N on Neptune.
The Surface
The surface of Neptune looks solid, but it is actually not solid. It is made out of gas. So what we actually see in pictures is not the surface of Neptune, but it is the top of the clouds of Neptune.
Most of the planet consists of compressed, frozen gases. The outer layer of the planet is best described as an ocean containing water mixed with methane and ammonia. Thick clouds cover the ocean so visibility would be difficult.
The Atmosphere
The atmosphere of Neptune contains a
mixture of hydrogen, helium and some methane. Like the other gas giants, layers of ammonia, ammonium hydrosulfide, and water ice are also thought to exist. Methane in the top of the atmosphere gives the planet its blue colour. This colour is the result of absorption of red light by the methane.
Neptune’s atmosphere is much more active than that of Uranus. Rapid changes in weather occur regularly, and westward moving winds reach speeds up to 2000 km/h (the fastest of all the planets). The winds are driven by the heat energy radiated out from Neptune’s interior.
As Voyager 2 passed within 5000 km of the Neptunian cloud tops, its cameras revealed a wide variety of features. Bright polar collars and broad bands in different shades of blue were prominent in the southern hemisphere. Also visible were bright streaks of cirrus cloud stretched out parallel to the equator. The Voyager pictures also detected shadows of these clouds thrown onto the main cloud layer some 50 km below (see Fig. 12.2).
Fig. 12.2Fluffy white clouds floating high in Neptune’s atmosphere (Voyager 2) (Credit: NASA).
Voyager 2 also photographed large storms in the atmosphere. One in particular was about half the size of Jupiter’s Great Red Spot, and oval in shape. Named the Great Dark Spot, it was observed to rotate anticlockwise over a period of about 10 days. Observation revealed this feature was a hole in the Neptunian clouds through which the lower atmosphere could be seen. Winds blew the spot westward at about 300 m/s. Cirrus-type clouds of frozen methane were seen forming and changing shape above and around the Great Dark Spot (see Fig. 12.4).
Fig. 12.3Interior structure of Neptune.
Fig. 12.4Neptune’s Great Dark Spot has a diameter about the size of Earth (Credit: NASA).
Voyager 2 also identified a smaller dark spot in the southern hemisphere and a small irregular white cloud that moves around Neptune every 16 h or so, known as the ‘Scooter’. This latter feature may be a gas plume rising from lower in the atmosphere.
Various white streaks and spots have been detected on Neptune but most have disappeared or changed greatly, while other features have emerged. In 1994 images from the Hubble Space Telescope showed the Great Dark Spot had disappeared (in contrast to Jupiter’s Great Red Spot, which has lasted hundreds of years). This indicates that Neptune’s atmosphere changes rapidly.
Neptune is also covered with a number of belts and zones that are fainter than those on Jupiter. A broad, darkish band is prominent at high southern latitudes. Embedded in this band is a smaller dark spot, about the size of Earth. White and wispy clouds seem to hover over the smaller dark spot. Scientists believe the darker clouds on Neptune contain hydrogen sulfide.
Neptune’s spectra suggest that its lower atmosphere is hazy due to the condensation of products of ultraviolet photolysis of methane, such as ethane and acetylene. There are also trace amounts of carbon monoxide and hydrogen cyanide. Elevated concentrations of hydrocarbons make this layer warmer than expected.
For reasons that remain unknown, the planet’s outer atmosphere layer has a high temperature of about 750 °C. The planet is too far from the Sun for this heat to be generated by ultraviolet radiation. One candidate for a 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.
The Rings
Astronomers on Earth thought Neptune might have some incomplete rings or arcs when they observed an occultation of a star by Neptune in 1984. However, pictures taken by Voyager 2 revealed that these arcs were part of a narrow ring system that contains three areas. The three main rings are the narrow Adams Ring (63,000 km from the centre of Neptune), the Le Verrier Ring (at 53,000 km), and the broader but fainter Galle Ring (at 42,000 km).
The rings may consist of ice particles coated with silicates or carbon-based material, which most likely gives them a reddish hue. They are hard to see because they consist of small particles that reflect little light. Their exact composition is unknown but, because the temperature is so low, they probably contain frozen methane. Some of this methane has been changed by radiation into other carbon compounds, thus making the rings appear dark (see Fig. 12.5).
Fig. 12.5Neptune’s rings and moons as seen by the Hubble Space Telescope. The image of the planet was blocked out to capture detail in the rings, and then reinserted (Credit: NASA/ESA).
Temperature and Seasons
The large distance between Neptune and the Sun, means that the average temperature on Neptune is a very cold −225 °C. This temperature is low enough to freeze methane. Bright clouds of methane ice form in the upper atmosphere, and cast shadows on the lower cloud layers.
Since Neptune takes 165 years to orbit the Sun, the time between any seasons is very long (40 years) and temperatures do not vary much season to season. The planet does have a north and south pole and it’s axis is tilted by nearly 30° from the vertical.
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.
Neptune’s interior is believed to be very similar in composition and structure to Uranus. Core temperature is therefore expected to be around 7000 °C and core pressure about 20,000 atm (similar to Uranus).
Magnetic Field
Neptune has a strong magnetic field—about 25 times greater than Earth’s. However, the magnetic axis is tilted 47° from Neptune’s rotational axis and it is off-centre by more than half the radius of the planet. The magnetic field is probably generated in middle regions where the pressure is high enough for water to conduct electrical currents.
Neptune’s bow shock, where the magnetosphere begins to slow the solar wind, occurs at a distance of 35 times the radius of the planet. The magnetopause, where the pressure of the magnetosphere counterbalances the solar wind, lies at a distance of 23–27 times the radius of Neptune. The tail of the magnetosphere extends out to at least 72 times the radius of Neptune, and likely much farther.
Observation of Neptune in the radio-frequency band shows that the planet is a source of both continuous emission and irregular bursts. Both sources are believed to originate from the planet’s rotating magnetic field. In the infrared part of the spectrum, Neptune’s storms appear bright against the cooler background, allowing the size and shape of these features to be readily tracked.
Moons of Neptune
Prior to the Voyager 2 encounter, only two moons were known to exist around Neptune. Today we know Neptune has 14 moons, following the discovery of new moons by Voyager 2 and the Hubble Space Telescope. William Lassell discovered the largest moon, Triton, in 1846. Triton is spherical in shape and has a nearly circular orbit. The other moons are small, irregular shaped objects with highly elliptical orbits, suggesting Neptune has captured them. One of the interesting things about Triton is that it has a retrograde orbit; that is, it orbits in the opposite direction to Neptune’s rotation. Some astronomers believe Neptune may also have captured Triton some 3 to 4 billion years ago.
Triton has a diameter of 2700 km and orbits Neptune every 5.88 days at a distance of 354,800 km. Its surface contains many interesting features including fault lines, cracks and ice (water, methane and ammonia) flows. There are not many impact craters, an indication that ice flows from the interior may have caused extensive resurfacing. The equatorial region contains a wrinkled terrain that resembles the skin of a cantaloupe or rock melon. Long narrow valleys rimmed by ridges cross the area. Such a region may have formed from repeated episodes of melting and cooling of the icy crust. Triton also has a few frozen lakes that may be the calderas of extinct ice volcanoes. In other areas, dark features surrounded by bright aureoles or rings are visible—these may be may be some of the geyser-like plumes detected by Voyager 2. The pinkish South Polar Region is covered by a cap of frozen methane and nitrogen and temperatures are around −245 °C. The pinkish colour of the polar cap is probably due to frozen nitrogen.
The very
thin atmosphere of Triton contains nitrogen and methane.
Triton’s density was found to be a little over 2 g/cm3, roughly twice the density of water. This suggests that Triton is made up of a mixture of rock and icy material.
Triton has a tidal effect on Neptune that is tending to pull Triton towards Neptune. In about a quarter of a billion years, Triton may be pulled apart by Neptune’s gravitational pull.
Fig. 12.6Neptune's magnetic field.
Table 12.3Moons of Neptune
Name of moon
Distance (km)
Period (days)
Diameter (km)
Discovered (year)
Naiad
48,200
0.29
66
1989
Thalassa
50,100
0.31
82
1989
Despina
52,500
0.34
150
1989
Galatea
62,000
0.43
176
1989
Larissa
73,500
0.56
194
1989
S/2004N1
The Solar System in Close-Up Page 21
