Planets tend to cool as they get older, but Saturn is hotter than astrophysicists say it should be without some additional energy source. The unexplained heat has caused a two-billion-year discrepancy for computer models estimating Saturn’s age. “Models that correctly predict Jupiter to be 4.5 billion years old find Saturn to be only 2.5 billion years old,” says Thomas Mattsson, manager of Sandia’s High-Energy-Density Physics Theory Group.

Results from Sandia’s Z Machine provides hard data for an 85-year-old theory that could correct mistaken estimates of the planet Saturn’s age. (Image courtesy of NASA/JPL/Space Science Institute)

Results from Sandia’s Z Machine provides hard data for an 85-year-old theory that could correct mistaken estimates of the planet Saturn’s age. (Image courtesy of NASA/JPL/Space Science Institute)

In 1935, physicists Eugene Wigner and Hilliard Huntington predicted that molecular hydrogen, normally an insulator, becomes metallic if squeezed by enough pressure. “That long-ago prediction would explain Saturn’s temperature because when hydrogen metallizes and mixes with helium in a dense liquid, it can release helium rain,” said Mike Desjarlais (in Sandia’s Pulsed-Power Sciences Center). “Essentially, helium rain would keep Saturn warmer than calculations of planetary age alone would predict,” said Sandia researcher Marcus Knudson (in Sandia’s Dynamic Materials Properties Dept.). Knudson and Desjarlais are the lead authors of a June 26 Science article, “Direct observation of an abrupt insulator–to-metal transition in dense liquid deuterium.”

This proposed density-driven hydrogen transition had never been physically observed until Sandia’s recent experiments. The tests ran on Sandia’s Z Machine, which can send a huge but precisely tuned sub-microsecond pulse of electricity at a target. The correspondingly strong magnetic field surrounding the pulse was used to shocklessly squeeze deuterium—a heavier variant of hydrogen—at relatively low temperatures. (Previous experiments elsewhere used gas guns to shock the gas. This increased its pressure but at the same time raised its temperature beyond the range of interest for the density-driven phase transition.)

Said Desjarlais, “When the liquid was compressed to over 12 times its starting density, we saw the signs that it became atomic rather than molecular. The transition, at three megabars of pressure, gives theorists a solid figure to use in their calculations and helps identify the best theoretical framework for modeling these extreme conditions.” The results need to be plugged into astrophysical models to see whether the now-confirmed transition to atomic hydrogen significantly decreases the age gap between the two huge planets.

“The Sandia work shows that dense hydrogen can be metallic, which in turn changes the coexistence of hydrogen and helium in the planet,” said Mattsson. “The mechanism of helium rain that has been proposed is therefore very plausible, given our results, but the scientific discussion will continue over the next few years in establishing a new consensus.”

This research was performed in collaboration with professor Ronald Redmer’s research group at University of Rostock in Germany and is a part of the Z Fundamental Science Program at Sandia. The multidisciplinary team included researchers with expertise in innovative experimental design, diagnostics, and pulse-shaping capabilities, matched with theoretical analysis using methods based on quantum mechanics. Other authors besides Knudson, Desjarlais and Mattsson include Redmer and Andreas Becker at University of Rostock and Ray Lemke, Kyle Cochrane, Mark Savage, and Dave Bliss at Sandia.

The Z Machine is a National High Energy Density Science Facility supported by the National Nuclear Security Administration.

Read the Sandia news release.