The high-pressure world of Uranus.

The high-pressure world of Uranus

Experiments involving intense shock waves generated by a metal plate slamming into a liquid-filled container are providing important clues about the interior of the distant, large planet Uranus. These experiments shed light on the origin of the planet's magnetic field, one of the more startling discoveries made when Voyager 2 flew past Uranus in 1986 (SN: 7/5/86, p.4).

Data from Voyager 2 indicate that the planet's magnetic field is sharply tilted -- as if the planet contains a bar magnet that points not along the planet's axis of rotation but toward a spot just 30 [deg.] from its equator. Moreover, the magnetic field at Uranus' surface can be as much as twice that found on earth. These observations suggest the magnetic field is caused, not by an iron core, which would be too deep and too small to create such a field, but by large-scale movements of electrically conducting fluid in the middle reaches of the planet's interior.

To test this idea, William J. Nellis and his colleagues at the Lawrence Livermore (Calif.) National Laboratory recently conducted a series of laboratory experiments useng shock waves to produce the extreme temperature and pressure conditions likely to be found in the interior of Uranus. They experimented on materials thought to make up the planet -- ammonia and methane "ices" -- and on a special, blended, hydrogen-rich liquid, called "synthetic Uranus," consisting of water, ammonia and isopropyl alcohol.

The experimental results, reported in the May 6 SCIENCE, suggest that strong shock waves cause the materials to break apart into elctrically charged fragments, or ions. At certain temperatures and pressures, the electrical conductivity of the dissociated material becomes large enough to account for the existence of a planetary magnetic field.

"Although there is still a question about the details of the chemical composition," Nellis says, "we do have a handle on the conductivity of the material that's there because [the conductivity] seems only weakly dependent on the exact chemical composition."

The results also show that materials deep within the planet are likely to be stiff and dense. There, the pressures and temperatures are high enough to break down the molecules and ions into individual atoms of oxygen, hydrogen, nitrogen and carbon. These extremely light elements end up being packed into a small space, becoming hard materials resistant to further pressure increases. In fact, the conditions may be right for producing the diamond phase of carbon and equivalent phases of oxygen, nitrogen and perhaps hydrogen.