The magnetic field and magnetosphere
Like the other giant planets, Uranus has a magnetic field that is generated by convection currents in an electrically conducting interior. The dipole field, which resembles the field of a small but intense bar magnet, has a strength of 0.23 gauss in its equatorial plane at a distance of one Uranian equatorial radius from the centre. The polarity of the field is oriented in the same direction as Earth’s present field—i.e., an ordinary magnetic compass would point toward the counterclockwise rotation pole, which for Earth is the North Pole (see Earth: The geomagnetic field and magnetosphere). The dipole axis is tilted with respect to the planet’s rotation axis at an angle of 58.6°, which greatly exceeds that for Earth (11.5°), Jupiter (9.6°), and Saturn (less than 1°). The magnetic centre is displaced from the planet’s centre by 31 percent of Uranus’s radius (nearly 8,000 km [5,000 miles]). The displacement is mainly along the rotation axis toward the north pole.
The magnetic field is unusual not only because of its tilt and offset but also because of the relatively large size of its small-scale components. This “roughness” suggests that the field is generated at shallow depths within the planet, because small-scale components of a field die out rapidly above the electrically conducting region. Thus, the interior of Uranus must become electrically conducting closer to the surface than on Jupiter, Saturn, and Earth. This inference is consistent with what is known about Uranus’s internal composition, which must be mostly water, methane, and ammonia in order to match the average density of the planet. Water and ammonia dissociate into positive and negative ions—which are electrically conducting—at relatively low pressures and temperatures. As on Jupiter, Saturn, and Earth, the field is generated by fluid motions in the conducting layers, but on Uranus the layers are not as deep.
As is the case for the other planets that have magnetic fields, Uranus’s field repels the solar wind, the stream of charged particles flowing outward from the Sun. The planetary magnetosphere—a huge region of space containing charged particles that are bound to the magnetic field—surrounds the planet and extends downwind from it. On the upwind side, facing the Sun, the magnetopause—the boundary between the magnetosphere and the solar wind—is 18 Uranian radii (460,000 km [286,000 miles]) from the centre of the planet.
The particles trapped within the Uranian magnetosphere comprise protons and electrons, which indicate that the planet’s upper atmosphere is supplying most of the material. There is no evidence of helium, which might originate with the solar wind, or of heavier ions, which might come from the Uranian moons. Because the largest Uranian moons orbit within the magnetosphere, they absorb some of the trapped particles. The particles behave as if they were attached to the magnetic field lines, so that those lines intersecting a moon in its orbit have fewer trapped particles than neighbouring field lines.
As is the case for Jupiter and Saturn, charged particles from the Uranian magnetosphere impinge on the upper atmosphere and produce auroras. Auroral heating can just barely account for the high temperature of Uranus’s exosphere (see above The atmosphere). One effect of the high temperature is that the atmosphere expands outward into the region occupied by ring particles and, by increasing drag, severely limits their orbital lifetime. This puts constraints on the age of the present material in the rings (see below The ring system).
Although Uranus has a somewhat lower density than Jupiter, it has a higher proportion of elements heavier than hydrogen and helium. Jupiter’s greater mass (by a factor of 22) leads to a greater gravitational force and thus greater self-compression than for Uranus. This additional compression adds to Jupiter’s bulk density. If Uranus were made of the same proportions of material as Jupiter, it would be considerably less dense than it is.
Different models proposed for the Uranian interior assume different ratios of rock (silicates and metals), ices (water, methane, and ammonia), and gases (essentially hydrogen and helium). At the high temperatures and pressures within the giant planets, the “ices” will in fact be liquids. To be consistent with the bulk density data, the mass of rock plus ice must constitute roughly 80 percent of the total mass of Uranus, compared with 10 percent for Jupiter and 2 percent for a mixture of the Sun’s composition. In all models Uranus is a fluid planet, with the gaseous higher atmosphere gradually merging with the liquid interior. Pressure at the centre of the planet is about five megabars.
Scientists have obtained more information about the interior by comparing a given model’s response to centrifugal forces, which arise from the planet’s rotation, with the response of the actual planet measured by Voyager 2. This response is expressed in terms of the planet’s oblateness. By measuring the degree of flattening at the poles and relating it to the speed of rotation, scientists can infer the density distribution inside the planet. For two planets with the same mass and bulk density, the planet with more of its mass concentrated close to the centre would be less flattened by rotation. Before the Voyager mission, it was difficult to choose between models in which the three components—rock, ice, and gas—were separated into distinct layers and those in which the ice and gas were well mixed. From the combination of large oblateness and comparatively slow rotation for Uranus measured by Voyager, it appears that the ice and gas are well mixed and a rocky core is small or nonexistent.
The fact that the mixed model of Uranus fits observations better than the layered model may reveal information on the planet’s formation. Rather than indicating a process in which Uranus formed from a rock-ice core that subsequently captured gas from the solar nebula, the mixed model seems to favour one in which large, solid objects were continually captured into a giant planet that already contained major amounts of the gaseous component.
Unlike the other three giant planets, Uranus does not radiate a substantial amount of excess internal heat. The total heat output is determined from the planet’s measured infrared emissions, while the heat input is determined from the fraction of incident sunlight that is absorbed—i.e., not scattered back into space. For Uranus the ratio of the two is between 1.00 and 1.14, which means that its internal energy source supplies, at most, 14 percent more energy than the planet receives from the Sun. (The equivalent ratios for the other giant planets are greater than 1.7.) The small terrestrial planets—Mercury, Venus, Earth, and Mars—generate relatively little internal heat; the heat flow from Earth’s interior, for example, is only about a ten-thousandth of what it receives from the Sun.
It is not clear why Uranus has such a low internal heat output compared with the other Jovian planets. All the planets should have started warm, since gravitational energy was transformed into heat during planetary accretion. Over the age of the solar system, Earth and the other smaller objects have lost most of their heat of formation. Being massive objects with cold surfaces, however, the giant planets store heat well and radiate poorly. Therefore, they should have retained large fractions of their heat of formation, which should still be escaping today. Chance events (such as collisions with large bodies) experienced by some planets but not others at the time of their formation and the resulting differences in internal structure are one explanation proposed to explain differences among the giant planets such as the anomalous heat output of Uranus.
Uranus’s moons and rings
Uranus’s 27 known moons are accompanied by at least 10 narrow rings. Each of the countless particles that make up the rings can be considered a tiny moon in its own orbit. In general, the rings are located closest to the planet, some small moons orbit just outside the rings, the largest moons orbit beyond them, and other small moons orbit much farther out. The orbits of the outermost group of moons are eccentric (elongated) and highly inclined to Uranus’s equatorial plane. The other moons and the rings are essentially coplanar with the equator.