Uranus's magnetic field

Uranus is the seventh planet from the Sun. Although a Jovian or gas giant planet, it has more in common with Neptune than Jupiter or Saturn. Uranus’s magnetic field is believed to be produced in a manner similar to that which generates Neptune’s magnetic field.

Overview

Uranus was the first planet to be discovered through telescopic observations. In March 1781, while searching for binary stars using a two-meter telescope, British astronomerWilliam Herschel noted an object he initially thought was either a comet or a nebula. Recognizing its observed orbital motion to be that of a planet, Herschel first proposed the object be named in honor of King George III of England. However, the planet was finally named after the Greek god of the heavens.

Observations of Uranus continued over the next two centuries, but by and large, Uranus remained an enticing mystery. Five satellites were discovered between 1787 and 1948. The rotational axis of the planet proved extremely surprising: Uranus is a world rotating virtually on its side. The axis is inclined 97.8 degrees relative to the orbital axis of the solar system. Since Uranus’s atmosphere—as seen from Earth-based telescopes between the time of Herschel and the dawn of the space age—did not reveal significant features to observe over time, it was not until the second half of the twentieth century that this planet’s rotational period was accurately determined.

Knowledge of the orientation of Uranus’s rotational axis, its interior structure, and its rotational rate is key to determining the nature of Uranus’s magnetic field. Indeed, until the Voyager 2spacecraft encountered the planet close up in January 1986, there was no definitive proof that Uranus even possessed a magnetic field, although one was strongly suspected based on a contemporary model of the planet that shared similarities with gas giants Jupiter and Saturn.

Voyager 2 carried a magnetometer and a radioastronomy experiment used in concert to sample the magnetic environment of each planet (Jupiter, Saturn, Uranus, Neptune) that it encountered on its historic “Grand Tour.” Basically, a sophisticated magnetometer operates on the very fundamental principle of an induced voltage being produced in a coil of wire when it intercepts a time-varying magnetic flux. That flux is directly proportional to the instantaneous strength of the magnetic field. Spacecraft such as Voyager 2 detect and investigate magnetic environments in space by picking up radiation in radio wavelengths produced by oscillating charged particles, such as those in the solar wind or ions trapped in planetary magnetic fields.

Uranus and Neptune were considered to be gas giants like Jupiter and Saturn until it was realized, based largely upon computer models and Voyager 2 data, that being smaller in mass and having significant atmospheric differences from Jupiter and Saturn, Uranus and Neptune were better described as ice giants. Finding out about Uranus’s interior would be key to determining the means by which a magnetic field could be produced by the planet. However, the converse is true as well. Directly measuring the magnetic field of the planet would help to develop a model of Uranus’s interior. What was known about Uranus’s composition in the atmosphere prior to the Voyager 2 encounter was that the planet is composed primarily of hydrogen (83 percent) and helium (15 percent). However, Uranus has a pale blue-green color due to the presence of methane (2 percent), which selectively absorbs red wavelengths of light. Uranus also contains ices such as water, ammonia, and an assortment of hydrocarbons.

Knowledge Gained

Voyager 2 determined that the magnetic field generated by Uranus has quite unusual characteristics. The planet’s dipole moment is approximately fifty times that of Earth. The value of Uranus’s dipole moment is 3.8 × 10¹⁷ tesla meters cubed. By comparison to giant Jupiter, this is a mere 0.26 percent of the dipole moment of the largest planet in the solar system. Uranus’s average magnetic field strength, the maximum value or amplitude of the field, at the plant’s “surface” was measured to be twenty-three micro-teslas. However, magnetic field strength varied with latitude. At the surface in the southern hemisphere, the field strength was seen to dip as low as ten micro-teslas. Then again, at the surface in the northern hemisphere, the field was found to be as strong at 110 micro-tesla. This situation contrasts greatly with Earth, where the magnetic field is nearly as intense at both poles. Earth’s field is centered close to the planet’s physical center. However, that is not the case with Uranus. The center of Uranus’s magnetic field is actually displaced from the physical center of the planet by about a third of its radius; the magnetic center is closer to the south rotational pole. Further complicating the field is the fact that the magnetic axis, the line from the south to the north pole through the planet, is tilted 59 degrees relative to the line running between the north and south rotational poles.

Uranus’s magnetosphere thus displays a highly unusual tilt. That tilt and the rotational tilt of the planet give the dynamic behavior of Uranus’s magnetosphere a twisting structure. To make the magnetosphere’s character even stranger, it appears that the ring system around Uranus actually streams ions in the magnetosphere down into Uranus’s atmosphere. Auroras are produced, but they differ somewhat from the familiar auroral displays seen in Earth’s polar regions.

By detecting variations in radio waves produced by the planet’s magnetic field, astronomers have detected the rotational rate of the planet more accurately than by following the differential rotation of those atmospheric features that could be found. That rotation value is 17.233 hours.

Despite the unusual orientation and subsequent twisting behavior of the field lines as the planet rotates, Uranus’s magnetosphere does share some things with other planetary magnetic fields in our solar system. Its magnetosphere is affected by the solar wind, forming a bow shock ahead of the planet, a magnetopause, and a magnetotail. The bow shock was crossed at a distance equivalent to twenty-three Uranus radii, whereas the magnetopause was determined by Voyager 2 to be located at eighteen Uranus radii. The magnetotail appears as a corkscrew structure due to the twisting of the planet’s magnetic field lines. This magnetic field structure also has given Uranus radiation belts of trapped charged particles. Those particles consist primarily of protons and electrons but have a minor component of molecular hydrogen ions. Uranian satellites create gaps in the radiation belts by “sweeping up” charged particles as they revolve about that planet. Ring particles and the surfaces of the planet’s satellites struck by this ionizing radiation are darkened by that exposure.

What generates such an unusual planetary magnetic field? Whereas Earth’s field is created deep in the planet by a dynamo effect involving electrical currents generated by its molten core, Uranus’s magnetic field is speculated to be produced by the ice giant’s mantle. Between the atmosphere and the planet’s core, Uranus has a mantle layer believed to be composed of highly pressurized water, ammonia, and other ices that become ionized under that tremendous pressure and in the presence of temperatures in excess of 1,000 kelvins. Therefore, currents flow through the mantle. Some scientists do not accept this explanation, but there is no way, short of direct investigation of the planet’s interior, to validate or invalidate the mantle “ocean” hypothesis.

Why does Uranus’s magnetic field have such an unusual orientation relative to the planet’s rotational axis? There are two hypotheses: one suggests that the planet is in the process of reversing its magnetic field. (Rocks on Earth present a record that Earth’s magnetic field has reversed itself many times over geologic time.) The second hypothesis is that the disruption of the magnetic field alignment resulted from a collision between Uranus and one or more large bodies. Further study will be needed to decide between these two theories or replace them with a better explanation.

The Hubble Space Telescope, in concert with the Keck telescope in Hawaii, has imaged storms in Uranus’s atmosphere. If Hubble's service life is extended sufficiently, Hubble and Keck would, in addition to searching for atmospheric dynamics, produce data that could assist in developing a better understanding of Uranus’s interior. Such data could help explain Uranus’s complex magnetic field without getting direct measurements of the field characteristics, as would be possible only by sending another spacecraft to sample the field close to the planet. Launched in 2021, the James T. Webb Space Telescope has imaged incredibly clear pictures of Uranus’s rings, storm clouds, and polar caps, providing increasing data about the workings of the planet.

Context

Every 176 years, planetary alignments are such that it is possible through ingenious use of gravity-assist (or “slingshot”) maneuvers to send a pair of spacecraft to visit all the outer planets from Jupiter to Pluto. Prior to the authorization for the Mariner Jupiter-Saturn mission, which later was named Voyager, the National Aeronautics and Space Administration (NASA) had originally proposed sending a pair of sophisticated spacecraft on a journey that had been called the Grand Tour to take advantage of this rare opportunity to reach four planets. However, that ambitious plan was not funded. The end result was that Voyager 2 would be the only spacecraft to visit Uranus and Neptune in the twentieth century, and most likely for many decades to come after that initial spacecraft encounter. As such, Voyager 2 has provided the greatest share of data about the Uranian system.

The interpretation of those data led to our understanding of this unique and, in many ways, bizarre planet. Despite the tremendous insights provided by the Voyager 2 data, many questions remain unanswered. Among these are important questions concerning the internal structure of the planet, the production of the planet’s magnetic field, and the nature of the relatively minor amount of internal heating in the planet.

Just as the Galileo probe orbited Jupiter for a prolonged period of time and the Cassini spacecraft did the same at Saturn, the next logical step for Uranus studies would be to dispatch a dedicated orbiter, a spacecraft outfitted with a wide-ranging suite of scientific instruments to allow focused investigations of the planet’s atmosphere, internal structure, magnetic field, ring system, and collection of satellites. In 2022, NASA announced plans to make its Uranus Orbiter Probe (UOP) a priority within the space program. This probe would not only orbit Uranus but also dive into its atmosphere, producing valuable information about the planet. The UOP is slated to launch in the mid-twenty-first century. In the meantime, Uranus will continue to be studied using the Hubble Space Telescope, the James T. Webb Space Telescope, and ground-based facilities (such as the Keck telescope) in attempts to gain further insight into the many unanswered questions remaining from the Voyager encounter.

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