V.

Composition and Structure

A.

Interior of Uranus

Uranus contains mostly rock and water, with hydrogen and helium (and trace amounts of methane) in its dense atmosphere. Astronomers believe that Uranus, like Neptune, formed from the same material—principally frozen water and rock—that composes most of the planet’s moons. As the planet grew, pressures and temperatures in the planet’s interior increased, heating the planet’s frozen water into a hot liquid.

Uranus probably has a relatively small rocky core (smaller in size than Earth’s core), with a radius no larger than 2,000 km (1,240 mi) and a temperature of about 6650°C (12,000°F). Uranus’s core may be small because most of the rock composing the planet remains mixed with the body of water that surrounds the core and extends upward to the planet’s atmosphere.

The vast body of liquid on Uranus accounts for most of the planet’s volume. This compressed, slushy liquid is sometimes described as an ocean or as ice. Scientists think this ocean consists mostly of water molecules, which are mixed with silicate, magnesium, nitrogen-bearing molecules such as ammonia, and hydrocarbons (molecules composed of carbon and hydrogen) such as methane. Uranus’s ocean is extremely hot (about 6650°C/about 12,000°F). Water at the surface of Earth evaporates, or boils, at 100°C (212°F). The ocean on Uranus remains liquid at such a high temperature, however, because the pressure deep in Uranus is about five million times stronger than the atmospheric pressure on Earth at sea level. Higher pressure holds molecules in liquids close together and prevents them from spreading out to form vapor.

B.

Atmosphere

The atmosphere of Uranus, which contains hydrogen, helium, and trace amounts of methane, extends about 5,000 km (about 3,100 mi) above the planet’s ocean. At the time of the Voyager 2 flyby in 1986, the atmosphere was relatively calm and inactive, with few storms or clouds, but Hubble Space Telescope images showed more activity in 2001. Winds blow parallel to the equator of Uranus, moving in the same direction as the planet’s rotation at high latitudes, and opposite to the rotation at low latitudes. These winds layer Uranus’s clouds into bands. Light reflected from Uranus’s deep atmosphere is blue-green, because the atmospheric methane absorbs red and orange light. Unlike the other giant planets, Uranus radiates little heat into space from its deep interior.

Although Uranus is one of the giant planets, it is smaller and has a different chemical composition than Saturn and Jupiter. While Saturn and Jupiter are made of mostly hydrogen and helium, Uranus captured a much smaller amount of these elements as the solar system formed. Instead, Uranus captured mostly water. Because water is more dense than hydrogen and helium, Uranus is more compact than Jupiter or Saturn. Jupiter, for example, has a radius of 71,355 km (44,338 mi) while Uranus has a radius of 25,548 km (15,875 mi). If Uranus had the same mass it has now but consisted of the lighter elements hydrogen and helium, the planet would be larger but much less dense than Jupiter. Uranus is also slightly less massive, and thus less dense and less compact, than Neptune, which is otherwise very similar in composition. As a result, the radius of Uranus is slightly larger than the radius of denser Neptune.

C.

Magnetic Field

Uranus, like Earth, is surrounded by a magnetic field, a region of space that exerts a small force on electrically charged or magnetic material. Uranus’s deep oceans contain electrically charged particles called ions. Ocean currents on Uranus circulate these charged particles, which in turn creates a magnetic field. Scientists believe that ocean currents in the other Jovian planets—Neptune, Saturn, and Jupiter—are created by heat released from these planets’ cores. The core of Uranus releases less heat than the other three Jovian planets, however, and astronomers are unsure about what causes ocean currents in the planet’s fluid interior. Uranus’s magnetic field is similar in strength to Earth’s magnetic field. Uranus’s magnetic axis (the line joining the north and south poles of its magnetic field) is aligned with the planet’s strongly tilted rotational axis, although the magnetic field is offset from the center of the planet. The influence of Uranus’s magnetic field extends for several hundred thousand kilometers above the planet.