An effort to mimic the conditions of planetary gas-giant interiors in the lab might have solved the mystery of why Saturn is so much hotter than its larger next-door neighbor, Jupiter. Both are pretty similar, as planets go: They are both composed of mostly hydrogen and helium and are roughly the same size. But given Saturn and Jupiter’s initial temperatures and how these planets are expected to radiate energy into space, Jupiter appears to have cooled off normally whereas Saturn has held onto its heat. A study published on June 15 in Proceedings of the National Academy of Sciences suggests a new model that could explain this enigmatic difference between the two gas giants.
Most scientists agree that the key to understanding Jupiter and Saturn’s temperature disparity lies deep below the planets’ surfaces, says lead author Stewart McWilliams, an Earth and planetary scientist at the University of Edinburgh School of Physics and Astronomy. The matter that makes up these planetary interiors is extremely compressed and extremely hot but we cannot observe this material directly because it is masked by the planets’ opaque surfaces. To gain a better understanding of what is actually going on inside gas giants, scientists have to imitate such exotic conditions in the lab.
Because noble gases are primary constituents of planetary atmospheres, a group of scientists led by Alexander Goncharov, a physicist at the Carnegie Institution for Science, subjected these gases to extreme temperatures and pressures to see how the gases would behave. The noble gases are the shy guys of the atomic world—the electron shells around these atoms are filled, so they are chemically inert and do not usually exchange or share electrons with their neighbors. But when these noble gases are squeezed by high pressure or heated as they rain down through the two gas giants’ hydrogen envelopes into extreme interior conditions, their electrons start to pop off, making them more willing to interact with other elements.
Scientists are still working to establish a solid understanding of this behavioral change, called the insulator-to-conductor transition. Goncharov’s team decided to experiment on four noble gases (helium, neon, argon and xenon), using a diamond anvil cell to compress them with more than 100,000 times the pressure of Earth’s atmosphere and heat them upward of 27,750 degrees Celsius.
The scientists observed a curious difference in the way neon behaved under the conditions mimicking the interiors of Saturn and Jupiter. Their experiment indicated that neon would remain an insulator even in the extreme heat and pressure of Saturn’s core, so it would refuse to interact with the surrounding hydrogen. Consequently, Goncharov’s team suggested that Saturn’s neon rain might pool into a protective ocean around the planet’s core, preventing core materials from interacting with the surrounding hydrogen and dissolving away.
On the other hand, at the temperature of Jupiter’s core, Goncharov’s team observed that neon would undergo the insulator-to-conductor transformation. Given its increased willingness to interact with the surrounding matter, the neon at Jupiter’s core would not isolate itself into a protective layer as it would in Saturn. Instead, the hydrogen shell would gradually eat away at the materials that make up the Jovian core. As the dense matter at the heart of Jupiter dissolved into the surrounding hydrogen, it would float up toward the planet’s surface and cool. Cloaked in its neon shield, Saturn’s core would stay hotter for longer, even if that shield were very thin.
“The proposition of the authors is definitely very interesting in that it opens new possibilities for Saturn’s interior models,” says Tristan Guillot, an astrophysicist at the Côte d’Azur Observatory, who was not involved with the study. “Also, the technique that they are using, laser shocks on precompressed material, is really promising for better understanding the behavior of matter in the conditions of planetary interiors. I look forward to further work in that direction.”
Nadine Nettelmann, a physicist at the University of California, Santa Cruz, who also was not part of the study, issues a word of caution about the neon shield model. “This is an interesting idea but more of speculative character,” she says, “as the mixing behavior of neon might depend on pressure, too.” According to Nettelmann, the pressures Goncharov’s team probed in their experiment were much lower than those found at the edges of Saturn’s and Jupiter’s cores.
According to Guillot, the neon shield model is something that could feasibly be tested, given observational data from the Cassini orbiter. We might soon have a definitive answer to the question of how Saturn has so jealously guarded its heat.