Blasts from one of the world's most powerful lasers are revealing for the first time how deuterium and hydrogen behave in the hot, pressurized interiors of giant planets. Experts say such information, reported in the current Science, could help resolve a number of mysteries, such as why Saturn appears to be so much younger than the rest of the planets in the solar system.
Matter is well understood under earthly conditions. Physicists know at what combinations of temperature and pressure water boils or freezes, for instance. And, by compressing materials with diamond anvils or explosive gas guns, they've mapped out the "equations of state" for many materials that describe how they behave under high pressure. But neither anvils nor gas guns are yet capable of simulating the high temperatures and pressures at the core of large planets.
Gilbert Collins and his colleagues at Lawrence Livermore National Laboratory figured they could recreate some of those conditions with the Nova laser, a warehouse-sized device that was built to compress fuel pellets for fusion research. The physicists placed a drop of deuterium in a small copper cell outfitted with a plunger at one end and thin beryllium windows on the sides. When the laser pulse hits the plunger, it sends a shock wave through the deuterium. By shining a beam of x-rays through the two windows, the team can clock the shock speed and infer the pressure and resulting density of the deuterium over the 5 or 10 billionths of a second before the whole assembly flies apart. The group found that the deuterium compressed surprisingly easily--its density increased by a factor of six at about half a million times atmospheric pressure, instead of the factor of four predicted by traditional models.
The data give physicists their first real look at the interiors of giant planets such as Saturn, says Gilles Chabrier, a physicist at the Ecole Normale Superiéure in Lyon. There are lots of puzzles there, he says. Current calculations that compare Saturn's current relative warmth with the rate at which it should be losing heat imply that it has only existed for about 2 billion years. "Which is crazy," Chabrier says, since it should have formed with the rest of the solar system, 4.5 billion years ago. If hydrogen, which makes up over 90% of Saturn, changes phase and suddenly goes soft at high pressures, it could store up energy in the same way that water vapor has more energy than liquid water. This stored energy might keep Saturn relatively warm in its old age, he says.