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The extreme environments of the atmospheres of gas giants are believed to cause droplets of liquid helium to form in the upper atmosphere and rain down upon lower levels. And now, we may have the first experimental confirmation of this phenomenon.
Astronomers and planetary scientists have been aware for at least four decades of the possibility that liquid helium rains from higher atmospheric levels to lower regions in the atmospheres of gas giant planets like Jupiter and Saturn. Unfortunately, an experiment to validate this hypothesis has, thus far, not been viable. That is, until now.
Researchers, including Marius Millot, a physicist at Lawrence Livermore National Laboratory (LLNL), have obtained experimental evidence that the long-standing prediction of helium rains within gas giants — which have atmospheres comprised predominantly of helium and hydrogen — is correct. The team found that these rains could exist over a range of pressures and temperatures commonly found within these giant planets.
We discovered that helium rain is real, and can occur both in Jupiter and Saturn. This is important to help planetary scientists decipher how these planets formed and evolved, which is critical to understanding how the solar system formed.
Marius Millot, Physicist, Lawrence Livermore National Laboratory (LLNL)
Alongside Raymond Jeanloz, a professor of Earth and planetary science and astronomy at the University of California, Berkeley, Millot is one of the authors of a paper documenting the team’s findings published in the latest edition of the journal Nature¹.
The team’s research will be of particular interest to scientists studying the solar system’s largest planet, Jupiter. As well as explaining why its fellow gas giant, Saturn, is brighter than it should be, the existence of helium rains could also explain a long-standing quirk detected in Jupiter’s atmosphere.
Explaining Jupiter’s Neon Deficiency
In 1995 the Galileo probe plunged through the atmosphere of Jupiter, beaming results about the chemical composition back to Earth before the craft was obliterated by the atmosphere’s tremendous pressure.
This data revealed that the upper atmosphere was depleted of both neon and helium, a deficiency that could be explained by helium rains. This is because, as helium forms droplets in the upper atmosphere, it would leech away neon — which mixes easily with helium — causing it to ‘rain down’ to lower levels.
“Helium condenses initially as a mist in the upper layer, like a cloud, and as the droplets get larger, they fall toward the deeper interior,” explains Hugh Wilson, not involved in the Nature paper.
Obtaining experimental evidence of these rains has been hard to achieve on Earth as the helium droplets are believed to form at around 10,000 to 13,000 km below the tops of Jupiter’s helium clouds under extreme conditions.
The pressure at this point high in Jupiter’s atmosphere is up to 2 million times the pressure on Earth, while the temperature can reach around 5000 ⁰C.
Under Pressure: Replicating Jupiter’s Extreme Atmosphere on Earth
To obtain experimental evidence of helium rains on Jupiter the international team conducted tests at the University of Rochester’s Laboratory for Laser Energetics (LLE), the first step of which was to replicate the gas giant’s extreme atmospheric conditions.
Coupling static compression and laser-driven shocks is the key that allows us to reach the conditions comparable to the interior of Jupiter and Saturn, but it is very challenging. We really had to work on the technique to obtain convincing evidence. It took many years and lots of creativity from the team.
Marius Millot, Physicist, Lawrence Livermore National Laboratory (LLNL)
The scientists compressed a mixture of hydrogen and helium to obtain a pressure about 40 thousand times that of Earth’s atmosphere. They then launched strong shockwaves through this sample using LLE’s Omega Laser to compress it further.
This resulted in final pressures of around 60–180 gigapascals (gPa) compared to Earth’s average barometric pressure of around 101 kPa. The final step in the preparation of the sample was to heat the sample to several thousand degrees.
By using ultrafast diagnostic tools, the team was able to measure the velocity of shocks traveling through the sample, its thermal emission, and how much light was reflected from the material.
What they found was rather than a smooth increase in the optical reflectivity as shock pressure increased, there were several ‘jumps’ in the relationship.
These discontinuities signaled to the team that the electrical conductivity of the sample was charging abruptly. This is a sign that the helium and hydrogen in the mixture are separating, something that was suggested in a paper² dating back a decade by LNLL scientists Sebastien Hamel, Miguel Morales, and Eric Schwegler.
“Our experiments reveal experimental evidence for a long-standing prediction: There is a range of pressures and temperatures at which this mixture becomes unstable and demixes,” Millot said. “This transition occurs at pressure and temperature conditions close to that needed to transform hydrogen into a metallic fluid, and the intuitive picture is that the hydrogen metallization triggers the demixing.”
The team will now seek to better refine their model and produce a more precise numerical simulation of the helium/hydrogen demixing process and its subtle quantum effects.
The end result will be a better understanding of how materials behave in extreme environments like those encountered in the atmospheres of gas giants — something that is vital in understanding the physics of Earth’s most titanic neighbors.
Jupiter is especially interesting because it’s thought to have helped protect the inner-planet region where Earth formed. We may be here because of Jupiter.
Raymond Jeanloz, Professor of Earth and planetary science and astronomy, the University of California, Berkeley
References
1. Brygoo. S., Loubeyre. P., Millot. M., et al, [2021], ‘Evidence of hydrogen−helium immiscibility at Jupiter-interior conditions,’ Nature, [https://doi.org/10.1038/s41586-021-03516-0]
2. Hamel. S., Morales. M. A., Schwegler. E., [2011], ‘
Signature of helium segregation in hydrogen-helium mixtures,’ Physical Review B, [https://journals.aps.org/prb/abstract/10.1103/PhysRevB.84.165110]
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