Uranus's interior

Uranus, the seventh planet from the Sun, has much in common with the other Jovian gas giants. Still, its interior is different from the interiors of the larger planets Jupiter and Saturn and more like that of Neptune.

Overview

Uranus is primarily composed of hydrogen and helium, and it is considered a Jovian planet or a gas giant. The interior of Uranus differs significantly from that of Jupiter and Saturn, however. It shares much more in common with Neptune. Like the interior of Jupiter and Saturn, Uranus’s interior cannot be sampled directly. Models of the interiors of the Jovian planets are inferred by external observation. For example, a magnetic field tells much about the nature and physical characteristics of a given planet’s interior.

Accurate determinations of orbital motions of Uranus’s satellites led to a precise value of the planet’s mass. With its size slightly above Neptune, Uranus is the third largest planet in the solar system but the second least dense. Uranus’s mass is 14.5 times that of Earth. With a mean radius four times the Earth’s, its overall density is 1.27 grams per cubic centimeter. Of course, the different structures of Uranus, from the core to the upper atmosphere, have specific characteristics of their own. In gross terms, Saturn is less dense than Uranus as a planet. Uranus is composed, like Jupiter and Saturn, primarily of hydrogen and helium, although the percentages of both are different in Uranus from the percentages of Jupiter and Saturn. Uranus contains methane, water vapor, ammonia, carbon monoxide, and carbon dioxide. Hydrocarbons such as ethane, acetylene, methyl acetylene, and diacetylene are also present, presumably created by the photolysis of methane in the atmosphere under the illumination of solar ultraviolet light.

Like Jupiter and Saturn, hydrogen in Uranus is found in gaseous form in the atmosphere, but deep inside the planet at greater pressure, hydrogen may exist in more exotic forms. Whether helium is found in states other than as a simple gas is debatable. Because of their size, temperature, pressure, composition, and interior structural differences from Jupiter and Saturn, Uranus and Neptune are often called ice giants.

Several models exist for Uranus’s interior. They agree on major features and differ in only minor ways. A great deal of the interior is understood to be composed of water, ammonia, and methane, with the percentage of water included in each model varying. That water percentage ranges between 9.3 to 13.5 Earth masses, depending on which model a scientist supports, with only 0.5 to 1.5 Earth masses of hydrogen and helium in the atmosphere and interior, which leaves between 0.5 to 3.7 Earth masses for what is considered rocky material.

Two inner layers are under the atmosphere of hydrogen, helium, and methane. First, an icy mantle incorporates the majority of the planet’s mass. Under that is the rocky core. The core represents less than 20 percent of the planet’s radius, and the relatively thin upper atmosphere of gases represents another 20 percent. This leaves the mantle as 60 percent of the planet, with as much as 13.5 Earth masses. Obviously, the atmosphere, mantle, and core densities are different, just as pressures and temperatures in these three distinctly different regions vary.

The mantle is icy because it is a hot, dense, electrically conducting mixture of water, ammonia, and less abundant volatile substances. In the mantle, under tremendous pressure, molecules dissolved in the water become ionized, creating the high electrical conductivity displayed by the mantle. This layer is often called a water-ammonia ocean because of its combined fluid and highly conductive nature. The latter quality of the mantle generates the planet’s complex magnetic field.

The core is far denser, at approximately nine grams per cubic centimeter. The planet’s central pressure and temperature are believed to reach eight million bars and 5,000 kelvins, respectively. High internal heat flows out to the atmosphere. Materials that have high electrical conductivities usually also have high thermal conductivities. In the case of Uranus, however, it is evident, based on atmospheric quiescence, that heat flow from the planet’s core to the atmosphere is less than Neptune's. Whereas Neptune radiates more energy than it intercepts by solar irradiance, Uranus is barely 6 percent greater in the infrared than the solar energy absorbed by Uranus’s atmosphere. All models agree that Uranus’s heat flow from the interior is only 0.042 watts per square meter. By comparison, the heat flow from the much smaller and less massive Earth is 0.075 watts per square meter. Uranus’s low heat flow and hence, cold atmosphere (portions of the troposphere have been recorded at a mere 49 kelvins) could be tied to the planet’s bizarre rotational configuration. Most planetary scientists believe that Uranus suffered a catastrophic event early in its history, presumably a collision with a sizeable planet-sized body to be left rotating on its side. During such an impact, Uranus lost much of its primordial interior heat. Such heat is left over from the gravitational collapse that created the planet. However, not all scientists subscribe to that explanation. Some propose that the planet’s mantle could be layered by composition so that heat flow toward the atmosphere could be diminished by convective action.

Methods of Study

The Voyager 2spacecraft has been the only spacecraft to encounter the Uranus system. Originally intended only to fly by Jupiter and Saturn, Voyager 2 was specially targeted to pass through the Uranian system in early 1986. While Voyager 2 cruised to Uranus, astronomers such as Heidi Hammel trained some of the best earthbound telescopes suitable for planetary studies to get a feel for what Voyager would encounter. Those studies provided additional information about Uranus’s atmosphere but also hinted that Uranus had some internal heat sufficient to drive cloud dynamics. However, earthbound telescopic studies did little to enhance understanding of Uranus’s interior. During the Uranus encounter, a significant discovery that aided in describing Uranus’s interior was the detection of a planetary magnetic field. The nature of Uranus’s magnetic field helped planetary scientists devise models for the planet’s interior, models quite different from those for the interiors of both Jupiter and Saturn. Then, in 1989, Voyager 2 fulfilled a similar task at Neptune. Planetary scientists realized that the relative similarities of Uranus and Neptune extended to their interiors.

It took the Voyager 2 spacecraft nearly five years to cross the gulf from the Saturn system to the Uranus system. The encounter phase began on November 4, 1985. Many fundamental questions were soon to be answered. At first, only increasingly revealing images were produced by Voyager 2. Scientists eagerly awaited the detection of a magnetosphere, which would indicate Uranus possessed a magnetic field. That would also tell much about the nature of the planet’s interior.

When Voyager 2 picked up radio signals indicating the spacecraft crossed the magnetosphere, it clearly revealed that Uranus has a magnetic field. After a jam-packed investigation, Voyager 2 ended the Uranus encounter on January 25, 1986, snapping a farewell image of the crescent planet.

Context

The planets Mercury through Saturn were known to the ancients. No individual can be credited with their discovery. Uranus, however, is the first planet for which definite records exist, indicating when the planet was first observed and charted. British astronomerWilliam Herschel was one of many to pay special attention to Uranus; others had misidentified it as an unnamed star. However, in 1781, Herschel recognized Uranus as a newly discovered object—a comet, the thought at first—orbiting the Sun. From that time to the dawn of the space age, even as Earth-based telescopes grew in size and resolution, little additional information could be learned substantively about Uranus other than its mass, size, mean distance from the Sun, rotation rate, and that it has a very unusual axial tilt relative to the ecliptic plane. It was realized that its atmosphere was quite different from that of Jupiter and Saturn, suggesting that Uranus’s interior differs from its two larger gas giant cousins due to its smaller mass. The interior was suspected to be more akin to Neptune, the next planet in the outer solar system to be discovered after Uranus.

Indeed, models of the two ice giants, generated based on Voyager 2's results and observations, are quite similar. Both planets are believed to have an outer envelope of molecular hydrogen, helium, and methane. Underneath that, both planets have mantles containing water, methane, and ammonia under high pressures and temperatures. Beneath that is an icy and rocky core. However, the difference between Uranus’s and Neptune’s interiors is that Uranus’s is less active: the planet does not have as great a heat flow from the interior to drive atmospheric dynamics.

Bibliography

Burgess, Eric. Uranus and Neptune: The Distant Giants. Columbia UP, 1988.

Elkins-Tanton, Linda T. Uranus, Neptune, Pluto, and the Outer Solar System. Rev. ed. Chelsea House, 2011.

Encrenaz, Thérèse and James Lequeux. The Solar System. 1 Telluric and Giant Planets Interplanetary Medium and Exoplanets. Wiley-ISTE, 2022.

Hunt, Garry E., and Patrick Moore. Atlas of Uranus. Cambridge UP, 1988.

Irwin, Patrick G. J. Giant Planets of Our Solar System: An Introduction. 2nd ed. Springer, 2006.

Lawrence, Ellen. Uranus: The Sideways-Spinning Planet. Sequoia Kids Media, 2022.

Miner, Ellis. Uranus: The Planet, Rings, and Satellites. Horwood, 1990.

Schmude, Richard W. Uranus, Neptune, and Pluto and How to Observe Them. Springer, 2008.

Tocci, Salvadore. A Look at Uranus. Watts, 2003.

"Uranus." NASA, 6 Apr. 2023. solarsystem.nasa.gov/planets/uranus/overview. 15 Sept. 2023.