Water not so squishy under pressure

In planets' cores, molecules may not compress tightly

When squeezed to pressures and temperatures like those inside giant planets, water molecules are less squeezable than anticipated, defying a set of decades-old equations used to describe watery behavior over a range of conditions.

Studying how molecules behave in such environments will help scientists better understand the formation and composition of ice giants like Uranus and Neptune, as well as those being spotted in swarms by planet hunters. The new work, which appears in the March 2 Physical Review Letters, also suggests that textbooks about planetary interiors and magnetic fields may need reworking.

“At this point, it’s worth putting together an accurate equation of state over the entire pressure range for planetary modelers to use,” says Bill Nellis, a physicist at Harvard University. Nellis notes that while the new study has generated reliable data for the conditions in question, more work is needed to determine how the new numbers will tweak existing theories.

In the lab, scientists generated pressures reaching 700 gigapascals — almost 7 million times the atmospheric pressure at the Earth’s surface — using the Z machine, an accelerator at Sandia National Laboratories in Albuquerque, N.M. “It really is a regime that we don’t experience in our lives,” says planetary scientist Jonathan Fortney of the University of California, Santa Cruz. “Even at the bottom of the ocean.”

The scientists, from Sandia and Germany’s University of Rostock, did this by punching a dime-sized amount of water with small metal plates that had been magnetically accelerated to speeds of 27 kilometers per second. The resulting shock wave induced planetary core-like pressures; the scientists could then watch how the molecules in the sample reacted.

The water was less compressible than expected, the researchers found, refusing to collapse into denser configurations. Instead, the molecules broke into charged fragments, forming a fluid capable of conducting electricity that behaves more like a weak metal, says study coauthor Marcus Knudson, a physicist at Sandia.

That flow of charged water might be able to generate a magnetic field — a factor that could help explain the enigmatic measurements around Uranus and Neptune, and would suggest that such fields are common. “Water cannot be ignored as a source of magnetic fields,” Knudson says.

Increased rigidity means that scientists might have to rethink how much water lives in planet interiors. Current calculations of planet compositions in the solar system and beyond are based on a planet’s density, as well as on assumptions of interior architecture, which in ice giants includes a water-rich layer. But if planets can’t pack in as much water as expected, then the rest of the ingredient list might need rejiggering.

This could mean adding more elements to a planet’s ingredient list that are heavier than water, such as iron, or subtracting lighter-than-water components such as hydrogen, Fortney says.

But Fortney and study coauthor Nadine Nettelmann, a planetary physicist at the University of Rostock, both note that the effects of increased water stiffness could be counteracted by colder temperatures, which help pack molecules more tightly together. Studying how such high pressures affect mixtures of water and compounds like ammonia or methane — which probably coexist in a planet’s interior — will help scientists draw a more complete picture of a planet’s interior. 

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