Neptune has a solid, rocky core. It is about 4,600 miles (7,400 km) in radius. That is 30 percent of the planet's total radius. Neptune's mantle is a scalding hot ocean of water, ammonia, and methane. It is about 6,200 miles (10,000 km) deep, or 40 percent of the planet's radius.
Neptune's atmosphere is about 4,600 miles (7,400 km) deep, or 30 percent of the planet's radius. It is made of hydrogen, helium, and methane gases.
Atmosphere
Neptune's atmosphere extends about 4,600 miles (7,400 km) above the mantle. It is 87 percent hydrogen, 12 percent helium, and 0.5 percent methane. In the middle levels of the atmosphere are dark clouds of hydrogen sulfide. Above these clouds is a thick layer of blue clouds. This layer is made of methane gas. White clouds made of methane crystals drift above the blue cloud deck. Above this is a haze of hydrocarbons.
The main cloud deck is blue because it is made of methane. Methane absorbs red and yellow light. It reflects blue light. The light reflected from this layer gives the planet its bluish color. Neptune is much farther from the Sun than Uranus is. So, it should be colder. However, the temperature at the top of Uranus's atmosphere is about the same as Uranus's. Neptune's clouds may be warmed by heat rising from the planet's core.
Heat rising from the core pushes the clouds. In some areas, the winds blow at speeds of around 1,300 miles per hour (2000 kph). The rate at which Neptune's clouds move is the fastest in the solar system. The clouds near the equator move from east to west. That is in the opposite direction to the rotation of the planet. Clouds nearer to Neptune's poles move from west to east.
In 1989, Voyager 2 flew by Neptune. It discovered a huge dark spot on the southern hemisphere. The Great Dark Spot was an anti-cyclone about the size of Earth. It circled the planet every 18.3 hours. Further south, the space probe found a formation of white clouds and a smaller dark spot. The smaller dark spot circled the planet in 16 hours. The formation of white clouds, called the Scooter, circled in 16.8 hours. The Scooter appears to be a storm that moves at a speed of about 1,800 miles per hour (2,900 kph).
A new theory suggests that the dark spots on Neptune may not be cyclones and anti-cyclones. Instead, they may be holes in the blue methane clouds. They may have been opened up by high-pressure systems, showing the darker ammonia and sulfur clouds below. In 1994, Hubble Space Telescope photos showed that the Great Dark Spot had disappeared. The photos showed that another spot had appeared in the northern hemisphere. Scientists have named the latest spot the Great Dark Spot 1994. Voyager's earlier discovery is called the Great Dark Spot 1989.
Orbit and Rotation
Orbit
Neptune is the eighth planet from the Sun. The farther a planet's orbit is from the Sun, the slower the planet travels. Neptune's orbital velocity is only 12,100 mph (19,440 kph). Neptune's orbit covers a distance of 17 billion miles. It takes almost 165 years to complete a single orbit. Since the planet was discovered in 1846, Neptune hasn't completed one orbit! Neptune's average distance from the Sun is 2.8 billion miles (4.5 billion km). Its orbit is elliptical. However, Neptune's orbit is very close to being a circle. The difference between its closest point to the Sun and its farthest is only 49 million miles (79 million km).
Neptune's orbital plane is tilted at an angle of 1.8¬° to the ecliptic. Pluto is the outermost known planet in the solar system. It orbits farther from the Sun than Neptune. However, Pluto isn't always the farthest planet. Pluto's orbit is very eccentric. Part of Pluto's orbit is inside Neptune's orbit. For 20 years of Pluto's 248-year orbit, Pluto is inside the path of Neptune's orbit. Pluto and Neptune will never collide because their orbits are not in the same plane. At the two points where their orbits are in the same plane, their orbital paths are thousands of miles apart. In 1979, the orbit of Pluto took it inside the orbit of Neptune. So Neptune is now the farthest planet from the Sun. Neptune will remain the outermost planet until 1999.
Rotation
Like Earth, Neptune spins around its axis from west to east. However, a day on Neptune is shorter than an Earth day. Neptune completes one rotation in 16 hours, 7 minutes. The equator rotates at a speed of about 6,000 miles per hour (9,600 kph). This is slower than any of the other gas giants. It is still fast enough to cause the planet to bulge slightly at the equator. However, Neptune has the smallest equatorial bulge of the four gas giants.
Neptune's axial tilt is 29.6¬°. This is slightly more than Earth's axial tilt. This may mean that Neptune has seasons. However, the planet is so far from the Sun that it receives very little heat. So, changes in seasonal temperature are probably very small, or non-existent.
Moons and Rings
Neptune has eight moons. Six of them weren't discovered until the Voyager 2 mission in 1989. They are Triton, Proteus, Nereid, Larissa, Despina, Galatea, Thalassa, and Naiad.
The smallest moon is also the closest to the planet. It is Naiad. Naiad is about 31 miles (50 km) across. It orbits 30,000 miles (48,230 km) from the planet. It is so close to Neptune that it orbits inside Neptune's thin ring system. Naiad is very dark. It is shaped more like a potato than a ball. It orbits the immense planet in only eight hours!
Nereid is the outermost moon. It is about 210 miles (340 km) in diameter. Its orbit is very elliptical. At its closest, it is 870,000 miles (1.4 million km) from Neptune. At its farthest, the moon is 6,000,000 miles (9.7 million km) away. This is the most eccentric orbit in the entire solar system. Nereid may have been an asteroid that was captured by Neptune's gravity. It revolves around Neptune every 360 days.
The largest of Neptune's moons is Triton. It is about 1,680 miles (2700 km) in diameter. That is just slightly smaller than Earth's Moon. It orbits Neptune at a distance of 221,000 miles (355,000 km). Triton takes 5 days and 21 hours to circle the planet. It is very bright. Its albedo is 0.8. Triton has very few craters. Old craters have been smoothed over by water and ammonia slush flowing through cracks in the surface. Neptune's surface is mostly frozen lakes and fractured plains. Triton's fractured plains look a little like a cantaloupe. This surface exists nowhere else. Because Triton has very few new craters, it has one of the youngest surfaces in the solar system.
Triton is very unusual in several ways. First, Triton circles the planet from east to west. This means it moves clockwise around Neptune when watched from above Neptune's north pole. All other large moons orbit their planet in a counterclockwise direction. Also, Triton's orbit is tilted a steep 23¬° to the plane of Neptune's equator. Most moons orbit over their planet's equator. Earth's moon is tilted only 5¬° to Earth's equator.
Another amazing thing about Triton is that it has an atmosphere. Triton's atmosphere isn't very thick. The air pressure is 70,000 times lower than Earth's. It is made of nitrogen and methane. Frozen methane and nitrogen cover Triton's surface. In the summer, the Sun's weak warmth combines with heat from the moon's core. Then, the ices beneath the surface change to a gas. The pressure of the newly thawed gas is released in geyser-like eruptions. These eruptions of gas and surface material spew out up to six miles into the atmosphere.
Triton is the coldest place in the solar system. Its surface temperature is -395 ¬°F (-237¬°C)! The gases that escape from beneath the surface quickly cool when exposed to the cold atmosphere. They fall back to the surface as ice.
Tidal forces from the gravity of Neptune constantly pull on Triton. This creates the moon's internal heat. The tidal forces also cause Triton to spiral closer to the planet. In 10 million to 1 billion years, Triton's orbit will be so close to the planet that the moon will be torn apart by Neptune's gravity. Its fragments might form another ring around the planet. This ring would be much larger and more visible than the rings Neptune has now.
Rings
Neptune has four rings. The rings are made up of millions of dark particles. The particles range in size from dust particles to small trucks. They are composed of a larger proportion of dust than are the rings of the other gas giants. The thin, innermost ring is called the Galle ring. It is 26,000 miles (42,000 km) from the planet's center. Next is the Leverrier ring. This ring is 33,000 miles (53,000 km) from Neptune's center. It is close to a broad ring called the Plateau. The Plateau stretches across the distance between the Leverrier and the final ring. The final ring is called the Adams ring. It is around 39,000 miles (63,000 km) from Neptune's center. The Galle, Leverrier, and Adams rings were named after the discoverers of Neptune.
Neptune's rings are very thin. They are so thin that they do not block starlight. This makes them very difficult to see from Earth, even with powerful telescopes. Before the Voyager 2 mission in 1989, it was believed that Neptune's outer ring was not complete. Only sections of a ring, called ring arcs were ever seen. Voyager 2 showed that the outer ring is complete. It is thicker in certain sections than in others. The thin sections cannot be seen from Earth. The thick sections appear as ring arcs. Shepherd moons may be causing particles in the outer ring to bunch together. These bunched particles form the thicker sections of the ring.
Magnetic Field
Neptune has a weak magnetic field. Its mantle is so hot and under so much pressure that it may conduct electricity. As Neptune rotates, the liquid mantle moves and creates an electrical current. The current produces the magnetic field. Neptune's magnetic field is 10 times weaker than Jupiter's. It is about equal to Earth's and Saturn's magnetic fields.
Neptune's magnetic field is tilted 47¬° from the planet's axis of rotation. A line drawn between the magnetic poles, wouldn't pass through the center of the planet. It misses it by more than 6,000 miles (9700 km). This is evidence that the magnetic field is produced in the mantle and not by the core.
Gravity
Neptune has a lot more mass than Earth has. However, it is also much larger than Earth. Neptune doesn't have a lot of mass for its size. So, despite its mass, Neptune's gravity is not very strong. Also, the farther you move from the center of a planet, the more its gravity decreases. Earth's surface is about 3,963 miles (6738 km) from its center. Neptune's gravity is measured from the top of its atmosphere. The top of its atmosphere is about 15,400 miles (24,800 km) from its center. So, Neptune's gravitational pull is just 12 percent greater than Earth's.
