This illustration shows the internal structure of Uranus with four layers: hydrogen (light blue), water (dark blue), hydrocarbons (red), and a rocky core (yellow). Uranus has a disordered magnetic field that originates from its water layer. Credit: B. Militzer and NASA
In 1986 and 1989, Voyager 2 made the final two stops on its grand tour of the outer solar system when it passed by Uranus and Neptune, respectively. Now, nearly 40 years later, the archive of data collected by the spacecraft is still providing unexpected results.
In an article published today in PNASAstronomer Burkhard Militzer of the University of California, Berkeley, wanted to explore why Voyager 2 data shows something unexpected: The magnetic fields of both planets, which are not that far apart in mass, are not dipolar. Earth, Jupiter and Saturn all have dipolar magnetic fields, meaning they have a north and south pole – the kind of configuration we’re obviously used to.
But Uranus and Neptune are not. Instead, their magnetic fields are more of a confusing maze than an orderly set of lines.
Oil and water
Earth’s rotating, partially molten core generates much of our planet’s magnetic field. But on Uranus and Neptune this is not the case. Instead, their magnetic field is generated in the mantle.
And Militzer discovered that the composition of the upper mantle of planets is very different from that further down. The planets are both considered “ice giants” rather than gas giants because beneath their atmospheres is an icy mantle, composed of compressed water, methane and ammonia, beneath which lies the planet’s rocky core.
Inside the mantle, Militzer says, he found that layers of water, methane and ammonia stratify, like oil and water. “They stay separated into an oxygen layer and a carbon-nitrogen layer and that’s when I realized that was probably a good answer to pursue and that’s how I started,” Militzer says Astronomy.
He made this discovery by modeling the known compositions of the ice giants – a difficult proposition given that they have only been visited once. Militzer says he relied on a number of previous studies that used Voyager data to build a simulation of the interiors of both planets.
He found that the magnetic field is generated in the aqueous layer, while the layers containing carbon, nitrogen and hydrogen do not produce magnetism. And it is because of this behavior that the magnetic fields of ice giants are messier.
Militzer’s simulations took all the chemicals involved in these interactions and determined how they would act in each layer. He found that the water layer experiences convection, which could drive the planets’ magnetic fields. The deeper layers essentially don’t form many chemical bonds, or mostly form them with each other, which doesn’t really allow for convection in the simulation.
Far-reaching implications
The only way to truly test the results would be through in situ measurements. There are numerous proposals for spacecraft to visit both worlds, and the furthest is a mission concept called Uranus Orbiter and Probe. The mission would launch a Galileo-like probe into Uranus’ atmosphere to take measurements. And the right suite of instruments could provide data from Militzer and others with confirmation of whether a layer of convective water is driving the disordered magnetic field. However, that mission is also still in development, with the green light far from certain.
However, the implications of this discovery extend far beyond our solar system. Neptune- and Uranus-type exoplanets are more abundant than Jupiter-like planets, and worlds called mini-Neptunes, with masses between that of Earth and Neptune, are even more abundant than those. So by understanding Neptune and Uranus more deeply, we may be able to understand more about those other planets as well, which helps us build a more complete view of the planets that populate our galaxy.
