Planetary innards: Gas giant insights

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  • Published: Mar 15, 2014
  • Author: David Bradley
  • Channels: X-ray Spectrometry
thumbnail image: Planetary innards: Gas giant insights

Deep, deep down

A putative gas giant orbiting a red dwarf (Image: NASA) X-ray studies are shedding new light on what might happen deep down in the atmosphere of planets like this and our own solar system's Jupiter and Saturn

A sneak peek at what might lie deep in the lower atmospheric layers of gas giant planets, including our relatively near neighbours Jupiter and Saturn, has been observed by an international team of researchers using DESY's X-ray laser FLASH in Hamburg, Germany.

Ulf Zastrau from the University of Jena and colleagues used Deutsches Elektronen-Synchrotron, DESY, instrumentation to investigate how liquid hydrogen becomes a plasma and so offer intriguing clues as to the material's thermal conductivity and its internal energy exchange; important components in models of the atmospheres of gas giant planets. Details of the experiments were published in the journal Physical Review Letters.

FLASH model

The atmosphere of gas giants consists mainly of hydrogen, however, astronomers have very little experimental knowledge about the hydrogen in the interior of such planets. "This is despite our very good theoretical models," says Zastrau. However, modelling the atmosphere of Jupiter should be possible on Earth. "Liquid hydrogen has a density that corresponds to that of the lower atmosphere of such giant gas planets," he explains. Using DESY's X-ray laser FLASH the team was able to heat a sample of liquid hydrogen in an instant from minus 253 Celsius to approximately 12 000 degrees and observe the structural changes that take place during this extreme transition. The X-ray pulse initially adds energy to the two electrons in the dihydrogen molecule but this is slowly (relatively speaking) transferred to the two protons, which are 2000 times the mass) until a thermal equilibrium is attained. In the process, the molecular bond between hydrogen atoms is cleaved and the molecules torn apart to form a plasma. This process, the team says, occurs after many thousands of collisions between electrons and protons but takes a mere picosecond to happen.

"We are carrying out experimental laboratory astrophysics," enthuses Zastrau. Until now, mathematical models based on dielectric properties of hydrogen have been the only way to describe the interior of a gas giant. This new study reveals experimentally the dielectric properties of liquid hydrogen. "When you know the thermal and electrical conductivities of the individual layers of hydrogen in the atmosphere of a giant gas planet, you can calculate the associated temperature profile," says team member Philipp Sperling of the University of Rostock. This study has now allowed the team to pin down the first point in the phase diagram of dense hydrogen.

Under pressure

The study involves precision cooling of hydrogen gas involving forcing it through a very chilly copper block cooled by liquid helium. Critically, the hydrogen must become liquefied and not frozen in the process. It escapes the copper block as a fine jet through a tiny nozzle projecting finger like from the end into the instrument's experimental vacuum chamber. The unique ability of FLASH to generate very short but intense laser pulses is then exploited to rapidly vaporize the liquid jet of hydrogen with a rapid-fire second pulse grabbing the data. By studying the system in this way with slightly different delay times, the way in which a thermal equilibrium is established between the electrons and the protons in the hydrogen plasma can be observed in "slow motion".

The team confesses that it took a long time to interpret the data they obtained this way. The researchers used one of the standard quantum mechanics tools at their disposal, density functional theory (DFT), although DTF requires a system at a single temperature rather than two. The team therefore had to extend DFT itself to a two-temperature system before they could describe the observations correctly. This pioneering work leads the way to future investigations of plasmas using X-ray lasers. Ultimately, it should be possible to study denser plasmas containing heavier elements or mixtures to offer a more realistic picture of a gas giant atmosphere or indeed the interiors of planets.

The experiments and modelling should help astronomers explain why so far planets observed outside our own Solar System have not been seen in all imaginable combinations of properties such as age, mass, size or elemental composition, but exist as specific planetary types. The next step will be to repeat the experiments at other temperature and pressure extremes to add more points to the diagram and provide a detailed picture of the entire planetary atmosphere, from the highest reaches to deep within where pressures and gravity are strongest.

Related Links

Phys Rev Lett, 2014, 112, 105002: "Resolving ultra-fast heating of dense cryogenic hydrogen"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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