Frontiers Physics World  December 2017

Has liquid metallic hydrogen on Jupiter been recreated?

Under pressure Researchers used diamond-anvil cells to try to make liquid metallic hydrogen at conditions found on Jupiter (Mohamed Zaghoo / Harvard Seas)

Two researchers at Harvard University in the US claim to have created liquid metallic hydrogen in the lab at conditions that exist inside gas-giant planets such as Jupiter and Saturn. Mohamed Zaghoo and Isaac Silvera came to this conclusion after squeezing liquid hydrogen to pressures of 140–170 gigapascals (1.4 million atmospheres) in a diamond-anvil cell and laser-heating it to 1800–2700 K (PNAS 114 11873).

They noticed the reflectance of their sample increased until – at a certain transition temperature – it abruptly rose to 50–55%, which Zaghoo says are “values typical of metallic behaviour”. Assuming that their liquid hydrogen is a simple metal with free electrons, Zaghoo and Silvera calculated the conductivity from this reflectance, finding it to be six to eight times higher than a previous value measured by William Nellis, also at Harvard, for shock-compressed liquid hydrogen in 1996.

The existence of such a metallic state inside Jupiter is the reason why the planet has a magnetic field. However, Zaghoo says his results imply that Jupiter’s field is stronger than previously thought and that it originates much nearer to the surface than assumed, given that his experiment took place at conditions found at just 84% of the planet’s total radius. In fact, he reckons Jupiter’s dynamo layer extends out to at least around 91% of the radius, claiming that new results from the Juno space mission support those conclusions.

Not everyone is convinced. “Their experiments are not for the conditions of relevance to the outer region of Jupiter,” warns David Stevenson, a planetary scientist from the California Institute of Technology. “What they did is extrapolate outwards to lower pressure, and this is debatable.”

Nellis, meanwhile, who works at Harvard independently from Silvera’s group, says his 1996 measurement is reliable because he measured the conductivity directly by electrical probing, whereas “to derive conductivity from a free-electron theory [as Zaghoo and Silvera have done] can introduce unknown systematic errors”. Moreover, Nellis measured a different phase of metallic hydrogen to those seen in the current experiment and it is unclear which might exist on Jupiter.

Philip Ball