Rochester laser experiments show ‘helium rain’ likely falling in solar system
New research at the Laser Energy Laboratory provides clues to the evolution of the solar system.
Almost 40 years ago, scientists predicted the existence of helium rain inside planets composed mainly of hydrogen and helium, such as Jupiter and Saturn. But the realization of the experimental conditions necessary to test this hypothesis was not possible. That is, until now.
In an article published in Nature, scientists from the University of Rochester, in conjunction with an international collaboration, reveal experimental evidence showing that helium rain – helium droplets falling through liquid metallic hydrogen, much like raindrops of water falling through the atmosphere on Earth – is possible over a range of pressure and temperature conditions that mirror those expected to occur inside planets such as Jupiter and Saturn. The discovery will help scientists determine how these planets form and provide key information about the evolution of Earth and the solar system.
What is helium rain?
“Our experiments suggest that deep within Jupiter and Saturn, helium droplets are falling through a massive sea of liquid metallic hydrogen,” says Gilbert (Rip) Collins, the Tracy Hyde Harris Professor of Mechanical Engineering; associate director of science, technology and academics at Rochester’s Laboratory for Laser Energetics (LLE); and director of Rochester’s Center for Matter at Atomic Pressures. “It’s a pretty amazing thing to think about the next time you look at Jupiter in the night sky. This work will help us better understand the nature and evolution of Jupiter, which is particularly important because it has long been thought that Jupiter was something of a space junk collector, protecting our planet in the solar system.
The international research team, which also included scientists from the Lawrence Livermore National Laboratory, the French Alternative Energies and Atomic Energy Commission (CEA), and the University of California, Berkeley, conducted their experiments at the Omega Laser Facility at LLE.
How high-powered lasers replicate pressures inside planets
To achieve the pressure and temperature conditions expected inside planets like Saturn and Jupiter, researchers precompressed mixtures of helium and hydrogen in a diamond anvil cell at pressures of around 40,000 times the pressure of the earth’s atmosphere. They then used the Omega laser to launch strong shock waves into the samples to further compress them and heat them to several thousand degrees.
Using a series of ultra-fast diagnostic tools, the team measured the impact velocity, optical reflectivity of the impact-compressed sample, and its thermal emission, and found that the reflectivity of the sample did not increase steadily with increasing shock pressure, as was the case in most samples the researchers studied with similar measurements.
Instead, they found discontinuities in the observed reflectivity signal, indicating that the electrical conductivity of the sample was changing abruptly, a signature that the mixture of helium and hydrogen was separating. When helium separates from hydrogen, it forms droplets – much like oil droplets forming in a mixture of oil and water – and helium has the potential to precipitate as rain. ‘helium.
Numerical simulation of the demixing process is difficult due to subtle quantum effects, but the experiments conducted by the researchers will provide a critical reference for future theory and numerical simulations. The team will continue to refine their measurements to improve understanding of materials under extreme conditions.
The work was funded by the Lawrence Livermore Lab-led Research and Development Program, the National Science Foundation’s Physics Frontier Program, and the Department of Energy’s Office of Science.
Category: Scientific technology