Saturn and Metallic Hydrogen
July 27, 2015
The periodic table is a good guide to estimating the behavior of elements. Elements in a column have similar properties, since they have the same configuration of valence electrons. It's not surprising that aluminum, gallium, and indium are all metals; that carbon, silicon, and germanium are semiconductors; and that helium, neon, argon, krypton, and xenon are all noble gases.
The elements of the first column of the periodic table are known as the alkali metals, principally because they form strong bases; e.g., sodium hydroxide, NaOH. That's how they're characterized by chemists, but physicists are more interested in their electrical properties; so, in those circles, they're known as "simple metals." These metals are easy to understand, since they have just a single valence electron. Gold and silver are also classed as simple metals.
The alkali elements include both hydrogen and francium. Hydrogen is a gas, and francium is a radioactive element that's too unstable to exist as an observable solid. All the solid alkali elements crystallize in the body-centered cubic (BCC) crystal form (see figure). Francium is theoretically predicted to also have this crystal structure. As metals, all the observed alkali elements have high electrical conductivity and low resistivity, as shown in the following table.
Since it's often possible to crystallize gases with application of high pressure and low temperature, would hydrogen behave as a metal when crystallized? This question was considered by Eugene Wigner, and his colleague, Hillard Bell Huntington, in 1935. Their conclusion was that BCC hydrogen would be a metal, but only at a "density many times higher than that of the ordinary, molecular lattice of solid hydrogen."
The pressure at the center of the Earth, about 365 GPa, can nearly be created in the laboratory with an apparatus known as the diamond anvil cell. Momentary high pressures around 100,000 GPa can be created in controlled explosions. So, how high a pressure is needed to produce metallic hydrogen? Wigner and Huntington predicted just 25 GPa, but this estimate was found to be too low. Later estimates placed the pressure possibly within reach of a laboratory experiment, a few hundred GPa, provided that the temperature was low enough.
While metallic hydrogen is unlikely to exist at the center of the Earth, the internal pressure of the gas giant planets is much higher. It's conjectured that a large volume of the interiors of these planets is metallic hydrogen. The gas giants, Jupiter and Saturn are composed primarily of hydrogen, with some helium and other elements. The other gas giants, Uranus and Neptune, have less hydrogen and helium and more of the heavier elements. They are known also as ice giants.
Although Jupiter and Saturn are compositionally similar, there's always been a discrepancy in models that describe their physical state. Planets cool as they age, but Saturn is warmer than it should be. Computer models predict Jupiter's age correctly at about 4.5 billion years, but the prediction for Saturn is only 2.5 billion years. The transition point of solid, molecular hydrogen to metallic hydrogen is an important part of these models, but it's never been experimentally determined.
Scientists at Sandia National Laboratories (Albuquerque, New Mexico) and the University of Rostock (Rostock, Germany) have just observed the insulator-to-metal transition in a dense liquid of the hydrogen isotope, deuterium.[3-4] This measurement refines the conditions for which hydrogen and helium become immiscibile, an important factor in models of the internal structure of these planets.[3-4] This hydrogen transition had never been physically observed.
The experiment was done using Sandia's Z machine, an X-ray generator that produces an intense electromagnetic pulse as part of its operation. The magnetic field associated with the sub-microsecond pulse was used to shocklessly squeeze deuterium at low temperature. This experiment overcame the side-effect of shock compression experiments that the hydrogen temperature is also increased.
Says Marcus Knudson, one of the two lead authors of the paper in Science presenting this experiment,
"We started at 20 degrees 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... 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 degrees K."
When the liquid hydrogen was compressed to more than twelve times its initial density, at a pressure of three megabars (300 GPa) instruments detected that the hydrogen was in an atomic, rather than a molecular, state. Since the electromagnetic pulse prohibited electrical measurements, the detection of metallic hydrogen was done optically. A 45% reflectivity was detected, and this could only have been achieved by a metal.
In the planetary model of Saturn, metallic hydrogen mixed with helium at Saturn's temperature will release helium rain, an energy distribution mechanism that alters the planet's evolution. The helium rain would keep Saturn warmer than expected. The Z-machine is funded by the US National Nuclear Security Administration.
- Electrical resistivities of the elements (data page), Wikipedia.
- E. Wigner and H. B. Huntington, "On the Possibility of a Metallic Modification of Hydrogen," J. Chem. Phys., vol. 3, no. 12 (December 1, 1935), p. 764ff..
- M. D. Knudson, M. P. Desjarlais, A. Becker, R. W. Lemke, K. R. Cochrane, M. E. Savage, D. E. Bliss, T. R. Mattsson, and R. Redmer, "Direct observation of an abrupt insulator-to-metal transition in dense liquid deuterium," Science, vol. 348 no. 6242 (June 26, 2015), pp. 1455-1460.
- Sandia's Z machine helps solve Saturn's 2-billion-year age gap, Sandia National Laboratory Press Releases, June 26, 2015.
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