Ice giants

New tool unpacks the mysteries of the Solar System's biggest weirdos

Scientists are working to unravel the mysteries of Uranus and Neptune.

Historical/Corbis Historical/Getty Images

Uranus and Neptune are cold, icy mysteries. They’re so radically different from the rest of the planets in the Solar System, that scientists feel that getting a better understanding of these two giants is crucial to understanding how planets form. Now, a new modeling tool offers a chance for what scientists have wanted for decades: a glimpse inside.

They’re starting with looking at the thermal and electrical patterns on these planets that would be impossible to reproduce in a lab.

On their respective surfaces, Uranus and Neptune might be the two most forgettable planets in the Solar System. They lack neither the flair of Jupiter and Saturn nor the striking colors of Mars and Mercury. They’re varying shades of blue — both of them.

Neptune as viewed through the Hubble Space Telescope.NASA/JPL/STScI

But look a little harder, and you’ll see two outliers. The designation as “ice giants” means that each of the planets is made of hot, dense fluids. These fluids are made up of "icy" matter– in this case, water, methane, and ammonia, all of which are surrounded by a rocky core.

This has led some scientists to suggest that the two are “naked” cores of gas giants like Saturn and Jupiter, uncovered for reasons unclear.

Federico Grasselli and Stefano Baroni, first and last author of the paper published in July in Nature Communications detailing the model, offer another compelling reason: “Neptune and Uranus are also probably composed primarily of water." While neither are suspected of sustaining life, merely having water exist in such odd circumstances could reveal crucial facts about how planets form, especially exoplanets.

"In the last years, they have been also receiving more attention as testbench to study classes of water-based exoplanets, which may be common in the extra-solar planetary systems: the study of water abundant planets and moons is currently of great interest in the quest for extraterrestrial life," Grasselli tells Inverse over email.

And learning about a planet’s interior, the pair say, can be a crucial first step to learning everything else.

Composite images of Uranus. Its sideways poles are a mystery to scientists.Lawrence Sromovsky, University of Wisconsin-Madison/W.W. Keck Observatory

"Our knowledge of planetary interiors is based on the features of the planet's surface and magnetic field, which are themselves influenced by the physical characteristics of their internal structure, like the transport of energy, mass, and charge through the internal intermediate layers," Grasselli explains.

Diving in — Measurements precise enough to imitate a planet's interior level was a complex endeavor, using a quantum-mechanically grounded theory of heat and charge transport as the basis for the study.

Calculations, Grasselli says, were "performed on high-performance supercomputers" for months. The study required them to created new data analysis tools that were capable of obtaining reliable values for thermal and electrical conductivity.

"All in all, a tough challenge on several fronts," Grassellie says, "but we made it."

The team studied water in three forms, liquid, gas, and superionic. One might expect “solid” to be the third in that list, but Grasselli and Baroni explain that ice on these planets isn't the same as the ice we think of. On Uranus and Neptune, the water is different — denser, and with an electrical charge.

"All in all, a tough challenge on several fronts... but we made it."

Superionic water is best viewed as between liquid and solid phases.

”The oxygen atoms of the H2O molecule are organized in a crystalline lattice, while hydrogen atoms diffuse freely like in a charged fluid,” the pair explain.

All three forms of water on Uranus and Neptune form thermal and electrical currents. These can get quite powerful: Neptune has the strongest winds in the Solar System, going faster than 2,000 km/h (1,200 mph).

Thermal and electrical output also offer crucial data on planetary history. “The thermal and electrical transport coefficients dictate the planet's history, how and when it was formed, how it cooled down,” the authors say.

Here, the researchers found that the superionic water has a far greater conductivity than previously imagined, especially in terms of creating magnetic fields. Superionic water mainly exists in the dense layers underneath the convective fluid region where planets generate their magnetic fields.

Especially in the case of Uranus, which NASA dubs the “Sideways Planet” because of its odd magnetic fields and how it rotates on its side, this could be a crucial piece of the puzzle of figuring out how it came to exist.

And it’s not just for these two. Grasselli tells Inverse that his team's method could be used to "study the transport properties of saline water in the global oceans below the icy crust, say, of the moons of Jupiter and Saturn." Few places in the Solar System have been as closely observed as the moons of these two giants, especially the ones with the potential for watery life like Europa.

Abstract: The impact of the inner structure and thermal history of planets on their observable features, such as luminosity or magnetic field, crucially depends on the poorly known heat and charge transport properties of their internal layers. The thermal and electric conductivities of different phases of water (liquid, solid, and super-ionic) occurring in the interior of ice giant planets, such as Uranus or Neptune, are evaluated from equilibrium ab initio molecular dynamics, leveraging recent progresses in the theory and data analysis of transport in extended systems. The implications of our findings on the evolution models of the ice giants are briefly discussed.
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