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Researchers develop technique to recreate pressures inside supergiant planets

Our interest in other planets expands every time a new discovery is made, like the recent news that an Earth-like planet - Gliese 581c - seems to be the most promising place for the presence of life so far of the 200-plus planets astronomers have found beyond our solar system.



New Mexico State University physicist Kanani K.M. Lee helped develop a technique that can reproduce the intense pressures found inside supergiant planets. (NMSU photo)

But how do we really learn the details about other planets and stars: What are they made of? Is water likely to be present? What is happening inside these planets' interiors?

A New Mexico State University researcher and her colleagues have developed a new technique to reproduce the intense pressures found in the interiors of planets like Earth, giants like Jupiter and even supergiant planets and stars many times the size of Jupiter.

In the current online edition of the Proceedings of the National Academy of Sciences, NMSU physicist Kanani K.M. Lee and others report the development of a technique that combines diamond-anvil cells and high-power lasers that can create pressure on a sample that is 100 to 1,000 times as strong as previously possible. Under such pressure, even familiar substances like water take on bizarre and unexpected characteristics.

"Water becomes electronically conducting, metal-like under very high pressures and temperatures," said Lee, an assistant professor of physics. "The water, initially transparent, becomes opaque, then it reflects, almost like a metal."

Lee's part of the research was done when she was a student at the University of California-Berkeley. Her adviser, Raymond Jeanloz, a professor of astronomy and of earth and planetary science at UC Berkeley, is the paper's principal author. Other colleagues on the paper are from Lawrence Livermore National Laboratory and France's Atomic Energy Commission.

Lee was part of a team that developed the coupling method between diamond-anvil cells and lasers, which were both existing methods of creating high pressure.

"By coupling the two methods, we can get the best of both worlds," Lee said.

Pressure is measured in "atmospheres," with one atmosphere being the atmospheric pressure a person would experience at sea level on a warm day. As a person moves higher in elevation, the atmospheric pressure decreases.

Inside the Earth, the pressure increases until it reaches about 3.6 million atmospheres at the center. Inside Jupiter, the pressure reaches about 70 million atmospheres.

Diamond anvil cell experiments, a long-established method, use two opposing cone-shaped diamonds to exert extreme pressures on a sample, creating pressures up to a few million atmospheres. The temperature and the amount of pressure can be varied, and the compressed sample can be studied for a long period of time.

In contrast, laser-induced shock waves can create much higher pressures, but the process generates high temperatures and the sample is squeezed for only a split second. By pre-compressing a sample inside a diamond-anvil cell before sending a laser-induced shock wave through it, the new technique combines the best of both methods, with pressures of a billion atmospheres soon to be possible.

So far, pressures of tens of millions of atmospheres have been reached using a laser at the University of Rochester's Laboratory for Laser Energetics in New York. The next step will be at LLNL's National Ignition Facility, where nearly 200 lasers will be focused on one sample. The goal will be to reach 100 million atmospheres.

"It's like the sky's the limit," said Lee, who incorporates discussion of these techniques into her teaching at NMSU.

High-pressure experiments lead to interesting findings that tell researchers a lot about what may be found inside planets. Learning how water behaves under intense pressures and temperatures led researchers to conclude that planetary interiors may hold much more water than is present on the surface.

Neptune, for example, contains far more water than is found on Earth.

"We think there might be one to 10 oceans of water within the Earth," Lee said. While that's a lot of water, it's still a small fraction of the total mass of this planet. Neptune, in comparison, may be composed of 50 percent water.

"Although we are the so-called water planet," Lee said, "in comparison, Neptune has much, much more water."

Researchers also expect to learn a lot about basic chemistry as they submit samples to these high pressures.