The giant gas planets of our solar system — Saturn and Jupiter — have deep atmospheres without a solid or liquid base like Earth. Jupiter is our solar system’s largest planet, an immense spinning colorful sphere of methane and ammonia so large it could easily swallow all the other planets. Jupiter harbors a dense extreme atmosphere with constant storms and thunderheads reaching 40 miles from base to top, five times taller than those on Earth, with powerful lightning flashes up to three times more powerful than Earth’s largest “superbolts”.
Nearly 40 years ago, scientists predicted the existence of helium rain inside these planets, which are composed primarily of hydrogen and helium. Helium is a colorless, odorless, tasteless, non-toxic, inert gas with the lowest boiling point among all the elements. Helium is also the second lightest and second most abundant element in the observable universe (hydrogen is the lightest and most abundant). However, achieving the experimental conditions necessary to test the hypothesis of helium rain has not been possible.
Confirmation –Helium Rain is Possible
That is, until now: scientists at the University of Rochester, together with an international collaboration, revealed experimental evidence showing that helium rain is possible over a range of pressure and temperature conditions that mirror those expected to occur inside planets such as Jupiter and Saturn. At such pressures and temperatures, helium condenses and separates from the surrounding hydrogen, which itself is compressed into a slightly lighter liquid and behaves like a metal. The discovery will help scientists determine how such planets form and will provide key insight into the evolution of Earth and the solar system.
Falling Through a Sea of Metallic Hydrogen
“Our experiments suggest that deep inside 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.
“That is a pretty amazing thing to think about next time you look up at Jupiter in the night sky,” says Collins. “This work will help us better understand the nature and evolution of Jupiter, which is particularly important as Jupiter has long been thought to have been somewhat of a space trash collector—protecting our planet in the solar system.”
The international research team included scientists from Lawrence Livermore National Laboratory, the French Alternative Energies and Atomic Energy Commission (CEA), and the University of California, Berkeley, conducted their experiments at the LLE’s Omega Laser Facility.
The Omega Laser
To achieve the pressure and temperature conditions that are expected inside planets like Saturn and Jupiter, the researchers precompressed helium and hydrogen mixtures in a diamond anvil cell to pressures approximately 40,000 times the pressure of 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 ultrafast diagnostic tools, the team measured the shock velocity, the optical reflectivity of the shock-compressed sample, and its thermal emission, and found that the reflectivity of the sample did not increase smoothly with increasing shock pressure, as is the case in most samples the researchers studied with similar measurements.
Instead, they found discontinuities in the observed reflectivity signal, which indicate that the electrical conductivity of the sample was changing abruptly, a signature that the helium and hydrogen mixture was separating. When the helium separates from the hydrogen, it forms droplets—much like droplets of oil forming in a mixture of oil and water—and the helium has the potential to precipitate into helium rain.
Numerically simulating the demixing process is challenging because of subtle quantum effects, but the experiments conducted by the researchers will provide a critical benchmark for future theory and numerical simulations. The team will continue to refine their measurements in order to improve an understanding of materials at extreme conditions.
Image credit: A view of the Cassini spacecraft during one of the last dives toward Saturn it made before the end of the mission. NASA/JPL-Caltech