It's raining on Jupiter. And probably on Saturn too.
But it isn't raining rain, you know. It's raining . . . helium.
Yes, droplets of that inert gas that keeps the Goodyear blimp aloft and that powers the runaway house in the movie "Up" are falling like a soft rain from the upper atmosphere of the planet into the gas giant's high-pressure interior. In the process, they're washing away the neon that should also be in the upper atmosphere, researchers from UC Berkeley reported Monday in the online version of the journal Physical Review Letters.
That's neon, as in the ubiquitous neon signs that light up Earth's night. It's one of the so-called noble gases (along with helium, argon, krypton, xenon and radon), which don't undergo chemical reactions with other elements under normal circumstances. The noble gases were present in trace amounts in the massive gas cloud from which the solar system was formed and were expected to still be present in the same concentrations in the gas giants, where there are no thermonuclear forces to convert them into other elements.
But the Galileo probe that plunged into Jupiter's upper atmosphere Dec. 7, 1995, found only about one-ninth the amount of neon that should have been there. There was also less helium than expected, even though helium and hydrogen are the two main constituents of the planet.
Shortly before Galileo struck Jupiter, planetary scientist David J. Stevenson of Caltech published a one-paragraph abstract suggesting that the neon was dissolving in droplets of condensed helium and being lost to the deeper layers. "But there has always been lingering dissatisfaction [with this explanation], since no details of this suggestion were ever published," astrophysicist Jonathan Fortney of UC Santa Cruz wrote in a commentary accompanying the new paper.
The quantum-mechanical calculations necessary to support this thesis were simply not possible at the time, Stevenson said Tuesday. Refinements to the theory were necessary first, and computational power had to increase.
Now, in a detailed theoretical calculation, planetary scientist Burkhard Militzer of UC Berkeley and post-doctoral fellow Hugh F. Wilson have managed to support the explanation. "I'm delighted that they were able to do the calculation and delighted that it came out the way it did," Stevenson said in an interview.
Such calculations are the only way to approach the problem, Militzer said, because the pressures on Jupiter "are so high that they have not been reproduced in a laboratory on Earth."
In simplest terms, their calculations show that helium condenses as a mist, something like a cloud, about 6,000 to 8,000 miles below the tops of Jupiter's hydrogen clouds. As the droplets grow, neon dissolves in them. When the droplets become big enough, they fall to the interior. The calculations show that none of the other noble gases are able to dissolve in the helium.
But the analogy to rain is imperfect. The pressures and temperatures are so high that "you can't tell if hydrogen and helium are a gas or a liquid," Militzer said. They are both fluids, so the "rain" is actually droplets of fluid helium mixed with neon falling through a fluid of metallic hydrogen.
The calculations show that the internal temperature is at least 5,000 degrees Kelvin (8,500 degrees Fahrenheit). "That's an interesting number," Stevenson said. "Neon is providing a thermometer for conditions deep inside Jupiter."
The same process is probably happening on Saturn as well, even though it has only one-third the mass of Jupiter, Militzer said.
And why should we care? "Jupiter is the 800-pound gorilla in the solar system," Stevenson said. "Understanding how Jupiter formed and what it is made of are essential to understanding . . . how the Earth formed. Jupiter is massive, and its gravity influenced the architecture of the solar system, how material aggregated, the placement of the rest of the planets, and the amount of material that hit the Earth," especially water.
Understanding Jupiter, he concluded, will help us better understand our own home.