Internal structure

Uranus does not have a solid surface. Beneath the planet’s atmosphere lies a slushy mix of water, methane, and ammonia. A rocky core probably lies at the planet’s center.

What happens within the planet’s icy layers is a mystery, said Heidi Hammel, vice president for science at the Association of Universities for Research in Astronomy, and vice president of The Planetary Society’s board of directors.

“Does Uranus have an ocean layer and then an atmosphere layer? Does it have a solid core with a transition region, or does everything kind of blur from one layer to the next? That's a fundamental question that we just don't know.”

A Uranus orbiter would help answer this question. As the spacecraft traveled around the planet, Uranus’ gravity would tug on it, causing Doppler shifts in the spacecraft’s radio signal. This is the same phenomenon that makes an ambulance siren change pitch as it passes. By measuring these shifts, scientists could map out the planet’s gravity field, allowing them to reverse-engineer what’s happening inside the planet.

Magnetic field

When Voyager 2 flew past Uranus and Neptune, it found that the ice giants’ magnetic fields were offset from their rotation axes by large amounts. Uranus’ magnetic field is offset by 59 degrees, more than any other planet.

“It's a very complex, weird magnetic field,” said Amy Simon, a senior scientist for planetary atmospheres research in the Solar System Exploration Division at NASA’s Goddard Space Flight Center. “Uranus is tilted over on its side, so the solar wind is hitting it very differently than it would on the other planets.”

It’s not clear whether the magnetic field originates from the planet’s core the way it does on many other worlds like Earth, Jupiter, and Saturn. We also don’t know how the magnetic field has changed since the Voyager 2 flyby.

Magnetic fields are linked to auroras, which can be observed by space telescopes like JWST and Hubble. But this only shows us a small portion of a planet’s magnetic field. The only way to directly measure Uranus’ magnetic field is to fly through it with an orbiting spacecraft.