An eighty-year-old prediction for the behavior of hydrogen under enormous pressure has been confirmed for the first time. The observations may help explain why Saturn is strangely hot.
In 1935, Eugene Wigner and Hilliard Huntington proposed that molecular hydrogen squeezed sufficiently would release electrons to become an electricity-carrying metal. The idea has shaped our thinking about the interiors of gas giants, where such pressures are thought to exist.
Nevertheless, models of the solar system's gas giants have a problem. These planets start out very hot, and cool with time. Sometimes external events reverse this, but we're not aware of anything like that within the solar system.
The rate of cooling depends on planetary size. Consequently, temperatures, combined with mass, should indicate a planet's age. Models based on these theories show Jupiter to be the same age as the Earth and Sun, but produce an estimate for Saturn of 2.5 billion years, 2 billion years too young. The first observations of metallic hydrogen may explain why.
“When hydrogen metallizes and mixes with helium in a dense liquid, it can release helium rain," said Dr Mike Desjarlais of Sandia National Laboratories.
Phase transitions, such as when water vapor turns to rain can release, or absorb, a lot of latent heat. Although helium has only about 1% of the latent heat of vaporization of water per kilogram, Saturn has a lot of it. Essentially, helium rain would keep Saturn warmer than calculations of planetary age alone would predict," said Dr Marcus Knudson, co-lead author with Desjarlais of a paper in Science reporting the first experimental observations of transitions to metal in liquid deuterium.
The two used the Sandia Z machine to observe what happens when hydrogen turns from an (electrical) insulator to a metal.
"We started at 20° Kelvin, where hydrogen is a liquid, and sent a few hundred kilobar shock -- a tiny flyer plate pushed by Z's magnetic field into the hydrogen -- to warm the liquid," said Knudson. "Then we used Z's magnetic field to further compress the hydrogen shocklessly, which kept it right above the liquid-solid line at about 1,000° K."
To prove the hydrogen had become metallic the researchers used metals' high reflectivity. They shone a spectrum of light onto the pressurized hydrogen and found it 45% reflective, very close to their prediction.
The findings didn't just demonstrate that Wigner and Huntington were right about hydrogen's metalization, they revealed the conditions under which it occurs. Theoretical estimates of the pressures required occurred over a very wide range, Knudsen pointed out.
The observations will be fed into models to see if it makes sense for Saturn to be experiencing a warming helium rain under all its clouds, while Jupiter and the smaller gas planets are not. In successfully explaining observations of the four gas giants we can study easily, an essential step is taken towards understanding the thousands of gas exoplanets now being discovered.