Uranus: A Haywire Magnetosphere During Voyager 2 Flyby

The Uranian magnetic field is more expansive than previously thought, according to newly analyzed data from Voyager 2, making it easier to search for moons with oceans.

During its flyby of Uranus 38 years ago, NASA's space probe Voyager 2 witnessed the ice giant’s magnetosphere in a truly exceptional state: an unusually strong solar wind likely dramatically compressed the planet's magnetic shield at the time. This is the result of a new study, presented in the journal Nature Astronomy by an American-German research team involving the Max Planck Institute for Solar System Research (MPS) in Germany. The team revisited and analyzed the space probe’s data from the days before its cosmic encounter with the ice giant. For decades, the Uranian magnetosphere had been considered an oddity. The study shows, that this notion is apparently based at least in part on the highly atypical conditions during Voyager 2’s flyby. The new results give hope for future space missions searching for subsurface oceans on the Uranian moons Titania and Oberon. Their orbits are likely to pass within the Uranian magnetosphere after all; their oceans should therefore reveal themselves through induced currents and magnetic fields.

Uranus is one of the two great unknowns in our Solar System. While all other planets have already been visited several times or at least extensively by space probes, Uranus and Neptune were only encountered up close by Voyager 2 – 38 and 35 years ago, respectively. Since then, there has not been another mission to Uranus or Neptune. Most of what we know about Uranus today is therefore based on the snapshot that Voyager 2 captured with its instruments on January 24, 1986. In the current publication, the research team led by NASA’s Jet Propulsion Laboratory shows, that this short encounter likely led to a misleading impression of the size and composition of Uranus’ magnetosphere.

It is undisputed, that Voyager 2’s flyby of Uranus was a great success. The probe, which was launched in 1977 and is still sending signals to Earth from beyond the rim of the Solar System, obtained data that led, among other things, to the discovery of eleven previously unknown moons and two unknown rings. In addition, several instruments provided information about the magnetic field and the distribution of charged particles in the vicinity of the planet – with unexpected results: apparently, the line connecting Uranus’s magnetic north and south poles is inclined by almost 60 degrees to the planet's rotation axis; the center of the magnetic field and the center of the planet do not coincide. Another peculiarity: unlike Earth and Saturn, Uranus’s magnetic field primarily traps electrons and protons. There are hardly any heavier charged particles. Until now, researchers have interpreted this as an indication that Uranian moons do not harbor any active cryovolcanoes. Volcanic activity would hurl large amounts of heavy ions into space.

The size of the magnetosphere was also surprising. On the day side, it reached less than half a million kilometers into space. This corresponds to about seventeen times the planet's radius. For comparison: in the case of Saturn, this distance is around 20 to 35 times the planet's radius.

Data from the weeks before the flyby

In the current study, the researchers revisited the data recorded by Voyager 2 in the days and weeks before the flyby. During that time, the particle spectrometer PLS determined the energy of the solar wind surrounding the space probe. This constant stream of particles from the Sun travels at varying speeds of between 300 and 800 kilometers per second even to the edge of our Solar System, sometimes stronger, sometimes weaker. In the days before Voyager 2 plunged into Uranus’s magnetosphere, solar wind speed had apparently been increasing steadily; in the weeks before, it had blown much more gently. “We know from Earth and other planets that the speed and density of the solar wind strongly influences the size of the magnetosphere,” explained MPS scientist and co-author Dr. Norbert Krupp.

In times of strong solar wind, the particle bombardment from the Sun noticeably compresses a planet’s magnetic shield. The research team's analyses now show that the Uranian magnetosphere was in an exceptional state on January 24, 1986. Compared to the weeks before, its volume on the side facing the Sun was probably reduced by up to 78 percent. The researchers also argue that the entire dynamics of the charged particles trapped in the magnetosphere change under such compressed conditions. It is conceivable that in this extreme situation, heavy charged ions could escape. Such processes are known to occur on Earth and Saturn. The Uranian magnetosphere may therefore not usually be as devoid of heavy ions as Voyager 2 witnessed. “The state in which Voyager 2 found Uranus’ magnetosphere is likely to have been extremely atypical,” said MPS scientist and co-author Dr. Norbert Krupp, summarizing the results.

Search for subsurface oceans

The new results are good news for future missions to Uranus. While no missions with this destination are currently scheduled, the USA, Europa, and China are currently exploring the possibilities. In this endeavor, the Uranian moons Titania and Oberon are of particular interest. Of the five large moons of Uranus, they orbit the planet at the greatest distance. Researchers suspect that there are underground oceans beneath their icy crusts, similar to those found on some moons of Jupiter and Saturn.

Until now, the Voyager 2 flyby seemed to suggest that both moons orbit their planet outside its magnetosphere. This idea is now no longer tenable. “If the orbits of Titania and Oberon actually pass through the Uranus magnetosphere, underground oceans can be detected much more easily and, above all, from a distance,” explained MPS scientist Dr. Elias Roussos, one of the authors of the new study. On their journey around the planet, the changing Uranian magnetic field should induce electrical currents in the oceans. The moons would thus generate their own induced magnetic fields, which scientific instruments could detect during flyby.