Why is Saturn hotter than it should be?

Chuck Bednar for redOrbit.com – @BednarChuck

In most cases, planets become cooler with age, but Saturn has defied tradition by being hotter than astrophysicists believe it should be without the presence of additional energy sources, and experts from Sandia National Laboratories believe that they’ve discovered why.

According to Thomas Mattsson, manager of Sandia’s high-energy-density physics theory group, the same computer models that correctly put Jupiter’s age at 4.5 billion years old concluded that Saturn is just 2.5 billion years old. Using the laboratory’s experimental Z machine, his team set out to find out the cause of this two-billion-year, heat-related discrepancy.

Mattsson and his colleagues were able to verify a longstanding proposition which suggested that molecular hydrogen, which is typically an insulator, will become metallic if it is subjected to high-enough levels of pressure. This notion, first predicted in 1935 by physicists Eugene Wigner and Hilliard Huntington, claimed that a pressured lattice of hydrogen molecules would decompose into individual atoms, releasing electrons capable of carrying a current.

First-ever observations of density-driven hydrogen transition

That prediction, the authors report in a study published last week in the journal Science, would help explain Saturn’s higher-than-expected temperature. Hydrogen, when it becomes metallic and mixes with helium in a dense liquid, can trigger the release of helium rain, an energy source that could keep Saturn warmer than planetary age calculation would predict.

The researchers said the experiments using the Z machine marks the first time that this proposed density-driven hydrogen transition has ever been physically observed. With the device, they were able to use a massive but precise sub-microsecond pulse of electricity to create a strong magnetic field, which squeezed the heavy hydrogen variant deuterium at low temperatures.

Similar experiments conducted previously by other researchers used gas guns to shock the gas, which increased pressure but caused the temperature to be too high for this density-driven phase transition, they said. Their findings now have to be added to astrophysical models to see if the transition to atomic hydrogen can explain the age differences between Jupiter and Saturn.

Investigating the density functional theory (DFT) methods

To find out more about the research, redOrbit got in touch with Marcus Knudson, a staff scientist at Sandia and a principal author of the new paper, and his colleague Mike Desjarlais, who is also a researcher at the New Mexico-based facility where the work was conducted.

Knudson explained that he and his colleagues “were motivated by the large spread in the predicted pressures and densities for this insulator-to-metal transition for the various density functional theory (DFT) methods. While all of the various methods predict a first-order transition from the mostly molecular, insulating fluid to the mostly atomic, metallic fluid, the density at which this is predicted to occur varies by a factor of two (0.75-1.5 g/cc for hydrogen).”

“We knew that the Z machine could deliver enough energy to a hydrogen (or deuterium) sample to reach the necessary pressures, [but] we just needed to come up with an experimental configuration that would allow us to reach those pressures while maintaining a relatively low temperature,” he added. “If we were able to do that, then we would be able to experimentally verify that such a transition takes place, and determine the pressure and infer the density at which this transition occurs.”

Understanding liquid hydrogen’s behavior under extreme conditions

He added that collaborators from the University of Rostock in Germany, who specialize in planetary modeling (especially the gas giants Jupiter and Saturn), had established a direct link between the metallization of the dense hydrogen liquid and the release of helium rain from a hydrogen-helium mixture, as would be found on those two worlds.

They contacted Sandia and proposed the Z machine experiments through the lab’s Fundamental Science Program in order to gain a better understanding of liquid hydrogen under those extreme conditions. Furthermore, Desjarlais said that this transition is “very sensitive to the theoretical framework used to perform calculations for hydrogen under these conditions.”

“This left the field with an uncomfortable degree of uncertainty as to which framework was best for modeling dense liquid hydrogen,” he explained. “Obtaining good data on this transition narrows down considerably the choice of models to use – and this kind of advance is not necessarily limited to just hydrogen, it potentially helps us do a better job of modeling other materials more accurately.”

An important piece in the Saturn luminosity puzzle

Monitoring the sample using optical diagnostics, Knudson said that he and his colleagues saw that at approximately 1Mbar (or one million times atmospheric pressure), it “went dark,” which means that the band gap had closed to around 2 eV (the energy within the visible spectrum). As the pressure and density continued to increase, they observed “an abrupt jump” in the signal as the sample “became reflective across the visible spectrum,” meaning it had become metallic.

He said that the results “confirmed that an abrupt, density driven, insulator-to-metal transition does occur in the dense fluid, as DFT methods predict, and more importantly provide a pressure and density for this transition in the 1000-2000 K temperature regime,” but noted that this alone will not fully explain why Saturn appears to be younger than it really is.

“Our data provides an important piece of the puzzle in that it helps narrow down considerably the choice of quantum models to use in modeling hydrogen in the gas giant planets,” said Desjarlais, noting that in some models, the helium-rain heat source is predicted for both Jupiter (where it is not needed to explain planetary luminosity) and for Saturn (where it is required).

“A lot of work needs to be done still on the planetary modeling side to push the science forward. The data from Z will constrain the choice of quantum model to use,” he continued, with Knudson adding that their team’s research “will provide more confidence in our understanding of the internal structure of Saturn. Whether this will help to explain the luminosity problem for Saturn remains to be seen, and will be the focus of future studies.”


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