Nasa Cassini: Liquid Hydrogen provides important data for planetary models

The gas giant Jupiter, captured by the US-European space ship Cassini-Huygens. 

Credit: NASA/JPL/Space Science Institute

Using DESY's X-ray laser FLASH, researchers took a sneak peek deep into the lower atmospheric layers of giant gas planets such as Jupiter or Saturn.

The observations of the team around lead author Dr. Ulf Zastrau from the University of Jena reveal how liquid hydrogen becomes a plasma, and provide information on the material's thermal conductivity and its internal energy exchange, which are important ingredients for planetary models.

The hydrogen jet inside the sample chamber. 

Credit: Sven Toleikis/DESY

The scientists present their experiments in Friday's issue of the scientific journal Physical Review Letters.

The atmosphere of gas giants consists mainly of hydrogen, which is the most abundant chemical element in the universe.

"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."

The researchers therefore 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.

The scientists used DESY's X-ray laser FLASH to heat liquid hydrogen, almost instantaneously, from minus 253 to around 12,000 degrees Celsius and simultaneously observed the properties of the element during the heating process.

Hydrogen is the simplest atom of the periodic table, consisting of a single proton in the atomic nucleus, which is orbited by a single electron. Normally, hydrogen occurs as a molecule consisting of two atoms.

Ulf Zastrau

The X-ray laser pulse initially heats only the electrons. These slowly transfer their energy to the protons, which are around 2,000 times heavier, until a thermal equilibrium is reached.

The molecular bonds break during this process, and a plasma of electrons and protons is formed.

Although this process takes many thousands of collisions between electrons and protons, the studies showed that the thermal equilibrium is attained in just under a trillionth of a second (a picosecond).

Thomas Tschentscher

"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."

More Information: “Resolving ultra-fast heating of dense cryogenic hydrogen”; U. Zastrau et al.; Physical Review Letters, 2014; DOI: 10.1103/PhysRevLett.