Michael D. Wheeler
Scientists have wondered why Saturn appears younger than the rest of the planets in the solar system. Now they may be closer to understanding the structure and evolution of far-off planets, thanks to Lawrence Livermore National Laboratory's soon-to-be-decommissioned Nova laser and the hydrogen isotope deuterium.
Scientists have theorized that hydrogen undergoes a transition of state at pressures exceeding a megabar (equivalent to 1 million atmospheres of pressure). At those pressures, they say, hydrogen ceases to behave as an insulator and acts as a conductor. Researchers have tried to re-create high-pressure conditions to see if the theory holds true, first using explosive gas guns that forced a piston to collide violently with hydrogen at the end of a barrel, then squeezing hydrogen with a diamond anvil. Although both of these methods created high pressures, neither could mimic the conditions thought to exist at the core of giant planets.
Reaching a megabar
In 1996, scientists working on the National Ignition Facility sought a way to squeeze deuterium and tritium fuel pellets to trigger fusion. They needed to know the maximum density that the pellets could attain after a megabar shock. A group of Livermore physicists, including Robert Cauble, set out to reach what others had not: the megabar mark. Cauble, familiar with the shortcomings of the diamond anvil and gas gun explosion experiments, decided on a very different approach, one that centered on the Nova laser. "The problem here was no one had ever done these experiments to a megabar," Cauble said.
The physicists placed a deuterium sample in a small copper chamber outfitted with an aluminum cap. When they fired Nova, the beam struck the aluminum cap, heating it so quickly that the surface vaporized, sending a shock through the cap and into the deuterium. Cauble's team set up an x-ray source next to the chamber, shining the beams through two thin beryllium windows on either side of the chamber. The x-rays radiographed the deuterium in the chamber, allowing the researchers to infer the shocked density and pressure. A laser probe shone through a sapphire window tested whether the isotope displayed reflectivity -- indicating that a phase change occurred during the brief interval when it was shocked by Nova.
Cauble said the experiments have led some prominent astronomers to rethink the physics behind planets in the solar system.