IV.

Composition and Structure

A.

Interior of Neptune

Neptune contains mostly rock and water, with hydrogen and helium (and trace amounts of methane) in its dense atmosphere. Astronomers believe that Neptune formed from frozen water and rock supplied by icy comet-like material found in the outer regions of the solar system. As the planet grew in size, pressures and temperatures in the planet’s interior increased, heating the planet’s frozen water into a hot, dense liquid sometimes described as slushy “ice.”

Although Neptune is one of the giant planets, it is smaller and has a different chemical composition than those of Saturn and Jupiter. While Saturn and Jupiter are made of mostly hydrogen and helium, Neptune captured a much smaller amount of these elements as the solar system formed. Instead, Neptune captured mostly water. Because water is more dense than hydrogen or helium, Neptune is more compact than either Jupiter or Saturn. Jupiter, for example, has a radius of 71,355 km (44,338 mi), while Neptune has a radius of about 24,760 km (about 15,383 mi). Neptune is also more massive and compact than Uranus, which has a radius of 25,560 km (15,882 mi).

Neptune likely has a solid core no larger than Earth (Earth’s diameter is 12,756 km/7,926 mi); this core could be composed primarily of iron and magnesium silicates. Neptune’s core may be small because most of the rock composing the planet remains mixed with the vast ocean that extends upward from the core to the atmosphere.

Neptune’s vast body of liquid accounts for most of its volume. Scientists think this pressurized ocean or mantle of slushy ice is composed mostly of water as well as molecules of methane and ammonia. Neptune’s ocean is extremely hot (about 4700°C/about 8500°F). The ocean remains liquid at this temperature instead of evaporating because the pressure deep in Neptune is several million times higher than the atmospheric pressure on Earth. Higher pressure holds molecules in liquid closer together and prevents them from spreading apart to form vapor.

B.

Atmosphere

The gaseous atmosphere of Neptune contains hydrogen, helium, and about 3 percent methane. It extends about 5,000 km (about 3,000 mi) above the planet’s ocean. Light reflected from Neptune’s deep atmosphere is blue, because the atmospheric methane absorbs red and orange light but scatters blue light. In 1998 astronomers also identified molecules of methyl in Neptune’s atmosphere. Methyl molecules each contain one carbon atom and three hydrogen atoms. Methyl molecules are known as hydrocarbon radicals because they are short-lived and highly reactive. They combine with each other to form ethane (C₂H₆), a flammable, colorless gas. The discovery of methyl in Neptune’s atmosphere marked the first observation of a hydrocarbon radical in the atmosphere of the outer planets. Astronomers hypothesize that great storm systems on Neptune eject methane into the upper atmosphere. Once in the upper atmosphere, the Sun’s energy breaks the methane down into methyl molecules. Below Neptune's methane clouds, at levels where the pressure rises to more than four times the atmospheric pressure at sea level on Earth, there may be a dense cloud layer composed of hydrogen sulfide particles.

Neptune emits about 2.7 times the amount of heat it absorbs from the Sun. Astronomers believe the excess heat that Neptune radiates comes from comet-like material that crashed into Neptune as the planet formed. Due to the force of gravity in the planet’s interior, the material in Neptune’s core is continually being pulled inward. As the material compacts, the molecules strike each other more frequently and with more force, releasing energy in the form of heat. Neptune’s core, which reaches temperatures of 5149°C (9300°F), is hotter than the Sun’s surface.

Neptune has an active atmosphere, with winds and massive storms that may be caused by heat escaping the planet’s interior. Neptune’s winds, which blow in a latitude direction, are faster in the planet’s polar regions than they are at Neptune’s equator. Neptune has the fastest winds in the solar system, reaching speeds of 2,000 km/h (1,200 mph).

Using the Hubble Space Telescope, astronomers have observed storms thousands of kilometers across in Neptune’s atmosphere. These storms are often visible as dark spots that appear and disappear in Neptune’s atmosphere over many months. The largest storm, known as the Great Dark Spot, appeared in the planet’s southern hemisphere and was photographed extensively in 1989 by the Voyager 2 spacecraft. Scientists estimated that the Great Dark Spot was as large in diameter as Earth is. By 1994 images transmitted to Earth by the Hubble Space Telescope showed that the Great Dark Spot had disappeared. Scientists believe this dark spot was an immense storm that either dissipated or was covered by other atmospheric features. From 1994 through 1998, astronomers used the Hubble Space Telescope to observe the emergence of additional large dark spots in Neptune’s northern hemisphere, indicating that the planet’s atmosphere changes rapidly. The chemical makeup of the cloud particles that form Neptune's Great Dark Spots is not known. Some scientists believe that the bright clouds rimming the poleward edges of the dark spots are composed of condensed methane particles.

C.

Magnetic Field

Neptune, like Earth, is surrounded by a magnetic field, a region of space that exerts a small force on electrically charged or magnetic material. Scientists believe that the slow escape of heat from the planet’s core circulates currents of electrically charged particles in Neptune’s deep ocean, generating a magnetic field. Neptune’s magnetic axis, the line indicating the direction of the force the planet’s magnetic field exerts, is aligned at an angle of 47° to Neptune’s axis of rotation. The influence of Neptune’s magnetic field extends for several hundred thousand kilometers above the planet.