Ask anyone what their favorite planet in the solar system is besides Earth, and very few will say Uranus or Neptune. The ice giants in the outer solar system are rather dull-looking. Earlier this year, the revelation that their surfaces are a similar shade of greenish blue hasn’t done much to move them up the pecking order.
However, a new theory about what’s inside the seventh and eighth planets from the sun threatens to make it a little more interesting, not least because it involves water — and a lot of it.
Since gas giants like Uranus and Neptune are the most commonly found planets in the broader Milky Way, the discovery could have massive relevance for the search for life.
Deep Ocean
Published this week in Proceedings of the National Academy of Sciences, a new study relying on computer simulations proposes that inside Uranus and Neptune — far below their thick, bluish, hydrogen-and-helium atmospheres — are layers of material that, like oil and water, don’t mix.
Over the years, planetary scientists have suggested that the ice giants contain diamond rain within them. The new theory suggests that instead, a deep ocean of water lies just below layers of clouds in the hydrogen-helium atmosphere. Below the water, goes the theory, is a layer of hydrocarbons — a highly compressed fluid of carbon, nitrogen and hydrogen. The layers are about 5,000 miles (8,000 kilometers) thick.
Little Mixing
According to computer simulations, the two layers exist separately because the temperatures and pressures of the planets’ interiors squeeze hydrogen out of the methane and ammonia. This implies that there is little mixing of particles in a thick layer of the planet’s deep interiors, as there is on Earth, so no global dipolar magnetic field exists. Earth’s dipole field is created by its liquid outer iron core.
It’s a critical theory because it may explain why the ice giants have disorganized and distinctly un-Earth-like magnetic fields. This is a key finding of NASA’s Voyager 2 probe when it conducted flybys of Uranus and Neptune in 1986 and 1989, respectively. No spacecraft has visited since.
Machine Learning
“We now have, I would say, a good theory why Uranus and Neptune have different fields, and it’s very different from Earth, Jupiter and Saturn,” said Burkhard Militzer, a professor of Earth and Planetary Science at the University of California, Berkeley. “”We didn’t know this before. It’s like oil and water, except the oil goes below because hydrogen is lost.”
Militzer developed the new theory thanks to advances in machine learning that allowed him to run a computer model that simulates the behavior of 540 atoms as they are heated and compressed. “One day, I looked at the model, and the water had separated from the carbon and nitrogen. What I couldn’t do 10 years ago was now happening,” he said.
Crucially, the gravity fields produced by his model matched those measured by Voyager 2 nearly 40 years ago.
NASA Mission?
A NASA mission to Uranus, proposed in a paper published in 2021, could confirm the new theory. The flagship mission would send a Uranus orbiter to study the planet. However, a Doppler imager onboard would be needed to measure the planet’s vibrations because a layered planet would vibrate at different frequencies, according to Militzere.
Another reason for NASA to visit Uranus is to explore the five most significant of its 27 moons, which may be ocean worlds. A study published earlier this month that reevaluated images from Voyager 2 suggested that its moon, Miranda, may have an ocean below its icy crust. If that’s true, Miranda has the potential for life alongside other ocean worlds in the solar system, such as Europa at Jupiter and Enceladus at Saturn.
However, any mission to Uranus must leave Earth by 2034 to take advantage of a rare planetary alignment of Neptune, Uranus and Jupiter. A gravity-assist “slingshot” around Jupiter would shorten the journey to Uranus to just 11 years.
Wishing you clear skies and wide eyes.
