Gas giant Jupiter
March 12, 2014

FLASH Laser Used To Peer Deep Inside Gas Giant Planets

April Flowers for - Your Universe Online

An international team of researchers recently got a sneak peek deep into the lower atmospheric layers of gas giant planets such as Jupiter or Saturn using Deutsches Elektronen-Synchrotron's (DESY) X-ray laser FLASH. The team's observations, led by Dr. Ulf Zastrau of the University of Jena, demonstrate how liquid hydrogen becomes a plasma. The findings, published in Physical Review Letters, also provide information on the material's thermal conductivity and its internal energy exchange, both important ingredients for planetary models.

Hydrogen, the most abundant chemical element in the universe, is the main ingredient in the atmosphere of gas giants. “We have very little experimental knowledge about the hydrogen in the interior of such planets,” says Zastrau. “This is despite our very good theoretical models.” Because of this, the team decided to use cold liquid hydrogen as a sample of the planetary atmosphere. “Liquid hydrogen has a density that corresponds to that of the lower atmosphere of such giant gas planets,” explains Zastrau. DESY's X-ray laser FLASH was used to heat the liquid hydrogen, almost instantaneously, from minus 253 to approximately 12,000 degrees Celsius. This allowed the team to observe the properties of the element during the heating process.

Along with being the most abundant chemical element, hydrogen is also the simplest atom of the periodic table. It consists of a single proton in the atomic nucleus, orbited by a single electron. Generally, hydrogen occurs as a molecule consisting of two atoms. At first the X-ray laser pulse from FLASH heats only the electrons, which slowly transfer their energy to the protons — about 2,000 times heavier than the electrons — until a thermal equilibrium is reached. During this process, the molecular bonds break and a plasma of electrons and protons is formed. The observations reveal that although the process might take many thousands of collisions between electrons and protons, the thermal equilibrium is attained in just under a trillionth of a second — called a picosecond.

“We are carrying out experimental laboratory astrophysics,” explains Zastrau. Previously, research teams have relied on mathematical models to describe the interior of gas giants like Jupiter. The dialectric properties of hydrogen — the thermal and electrical conductivities, for example — which are required to correctly simulate the massive, outward-directed heat flows in giant gas planets have been important model parameters.

“The study has revealed the dielectric properties of the liquid hydrogen,” reports Dr. Philipp Sperling from the University of Rostock. “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.” The team's experiments, which will have to be repeated at other temperatures and pressures in order to create a detailed picture of the entire planetary atmosphere, allowed them to locate a first point in the phase diagram of hydrogen.

Because hydrogen does not naturally exist in a liquid form on Earth, this study requires a great deal of effort. First, the hydrogen must be cooled to minus 253 degrees Celsius to liquefy it. “We use extremely pure hydrogen gas and force it through a copper block that is cooled by liquid helium,” explains DESY researcher Dr. Sven Toleikis, a member of the team. “The temperature must be very precisely controlled during this process. If the hydrogen gets too cold, it freezes and blocks the line,” says Toleikis. If this occurs, a small heater is used to re-liquefy the hydrogen. A nozzle, much like a finger, projects into the experimental chamber at the end of the copper block. A fine jet of liquid hydrogen, with a diameter of just one fiftieth of a millimeter (20 micrometers), flows from the tip of the nozzle. Many years of cooperation between DESY and the University of Rostock were necessary to create this experimental setup.

Intense pulses of DESY’s FLASH soft X-ray laser were shot at the fine jet of hydrogen in order to study the properties of liquid hydrogen as it vaporizes. “For the experiment, we used FLASH’s unique ability to split up the individual flashes,” explains Toleikis. “The first half of the flash heats up the hydrogen, and we use the second half to investigate its properties.” The researchers used the Split-and-Delay Unit, which was developed in cooperation with the University of Münster and the Helmholtz-Zentrum Berlin, to delay the second half of the flash by a tiny fraction of a second — up to 15 picoseconds or a trillionth of a second. The team was able to use this process of slightly different delay times to observe the way in which a thermal equilibrium is established between the electrons and protons in the hydrogen, similar to a super-slow motion camera.

Interpreting the data from those observations was not simple, however. “It took us a long time to understand what was actually happening in the experiment,” says Prof. Ronald Redmer, who leads the Rostock working group. To model the process, the scientists used density functional theory, which is a standard of quantum physics which is used to describe systems with many electrons. For systems with two different temperatures, such as this one, this standard procedure does not work. “Before we were able to correctly describe the observations, we had to extend density functional theory with a two-temperature model,” reported Redmer.

"Our experiment showed us the way of how to investigate dense plasmas with X-ray lasers,” says Dr. Thomas Tschentscher, scientific director of the European XFEL X-ray laser, at which experiments will be possible in 2017. “This method opens up the road for further studies, e.g. of denser plasmas of heavier elements and mixtures, as they occur in the interior of planets. Hopefully, the results will provide us among others with an experimentally based answer to the question, why the planets discovered outside our solar system do not exist in all imaginable combinations of properties as age, mass, size or elemental composition, but may be allocated to certain groups.”

DESY is the leading German particle accelerator center, and one of the premiere accelerators in the world. The research team included scientists from the US research centers SLAC National Accelerator Laboratory and Lawrence Livermore National Laboratory, the Helmholtz Institute Jena, the University of Oxford, the GSI Helmholtz Centre for Heavy Ion Research, the Hamburg Centre for Ultrafast Imaging (CUI), the University of Münster and the European XFEL.