July 31, 2021

Helium and hydrogen compressed like on Jupiter

Is the recipe for the dressing the secret to the formation of the largest and oldest planet, Jupiter, three hundred times more massive than Earth? As trivial as it is, this analogy can be deduced from an experiment published in Nature on May 26 by a Franco-American team from CEA, the Californian laboratory Lawrence Livermore and the University of Rochester (United States). On condition of replacing oil with hydrogen and vinegar with helium, the two most abundant elements in the Universe and in Jupiter.

Then, the whole question is to know if this mixture is homogeneous, like a slightly hot vinaigrette, or inhomogeneous, with two separate phases, like oil and vinegar at room temperature. “We must always remain cautious and wait for the experiment to be reproduced, but the result is convincing and confirms that, in Jupiter, a layer where the two elements are separated exists”, notes astrophysicist Gilles Chabrier, CNRS research director at ENS Lyon. This banal phenomenon is not so trivial. For example, thanks to him, another planet, Saturn, appears younger than its age, because it is brighter: the helium in it, separated from the hydrogen, falls in drops and, rubbing against the surrounding environment, gives off more energy, therefore radiation.

Anvils and lasers

“As we showed in 2019, the existence of an inhomogeneous mixing layer in Jupiter is necessary to explain the recent measurements of the Galileo probes (1995-2003) and especially Juno, still in orbit, which is sensitive to the gravity “, specifies Gilles Chabrier. But no one had made this sauce until the publication of Nature. The researchers reproduced the Dantesque temperature and pressure conditions of Jupiter to observe the evolution of a mixture of about 90% hydrogen and 10% helium. First, they made samples in France, at room temperature, but compressed to 40,000 times atmospheric pressure using two diamond or sapphire anvils. Then the set was shipped to Rochester to expose it to the fires of one of the most powerful lasers in the world, Omega, which delivers its energy in a nanosecond. Under the shock, the target compresses to two million atmospheres and its temperature rises to more than 10,000 ° C. At the same time, two much weaker lasers come to probe its properties. One measures the speed of the shock wave to find out the pressure and temperature. The second observes the reflectivity of the mixture.

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