Lab mimicry opens a window into the deep interiors of stars and planets

New work by the School of Physics & Astronomy increases our understanding of the atmosphere and chemistry of celestial objects.

The matter that makes up distant planets and even-more-distant stars exists under extreme pressure and temperature conditions. This matter includes a family of seven elements called the noble gases, some of which, such as helium and neon, are household names.

New work from the School of Physics & Astronomy used laboratory techniques to mimic stellar and planetary interiors in order to better understand how noble gases helium and neon control the atmospheres and internal chemistry of these celestial objects. Their work is published by Proceedings of the National Academy of Sciences.

The team used a diamond-anvil cell to bring the noble gases helium, neon, argon, and xenon to more than 500,000 times the pressure of Earth's atmosphere (50 gigapascals), and used a laser to heat them to temperatures ranging up to 28,000 degrees Celcius.

The gases are called “noble” due to a kind of chemical aloofness; they normally do not combine, or “react,” with other elements. Of particular interest were changes in the gases’ ability to conduct electricity as the pressure and temperature changed, because this can provide important information about the ways that the noble gases do actually interact with other materials in the extreme conditions of planetary interiors and stellar atmospheres.

Insulators are materials that are unable to conduct the flow of electrons that make up an electric current. Conductors, or metals, are materials that allow an electric current. Noble gases are not normally conductive at ambient pressures, but this study found that conductivity can be induced under higher pressures.

The researchers found that helium, neon, argon, and xenon transform from visually transparent insulators to visually opaque conductors at extreme conditions that mimic the interiors of different stars and planets.

This has several exciting implications for how noble gases behave in the atmospheres and interiors of planets and stars.

For example, it could help solve the mystery of why Saturn emits more heat from its interior than would be expected given its age. This is tied to the ability, or inability, of the noble gases to be dissolved in liquid metallic hydrogen, the main constituent of gas giant planets such as Saturn and Jupiter. 

In Jupiter and Saturn, helium would be insulating near the surface and turn metal-like at depths close to both planets' cores. This change from insulator to metal is expected to allow helium to dissolve in hydrogen near the planets' rocky cores.

However, a difference was observed in the behaviour of neon between the laboratory conditions mimicking the two gas giants. The team’s results indicate that neon would remain an insulator even in Saturn’s core. As such, an ocean-like envelope of undissolved neon could collect deep within the planet and prevent the erosion of Saturn’s core compared to its neighbour Jupiter, where core materials, such as iron, would be dissolving into the surrounding liquid hydrogen.

This lack of core erosion could potentially explain why Saturn is giving off so much internal heat compared to its neighbour Jupiter. Erosion of a planet's core leads to planetary cooling as dense matter is raised upward, whereas in Saturn denser material is allowed to collect at the centre of the planet, producing hotter conditions. These findings could provide the key to solving the longstanding mystery of Saturn's internal heat.

"A tiny ocean of neon forming inside Saturn could have a surprising influence on this planet's evolution. Another tiny planetary feature that has big effects is Earth's ocean: despite making up just a fraction of a percent of the planet's total diameter, Earth's ocean plays a remarkable role in controlling the Earth system, for example by allowing mixing of the Earth's exterior and interior. In Saturn an ocean composed of noble gas instead of water could instead prevent mixing of the planet's interior and exterior. This may solve an old mystery as to why Saturn and its larger neighbour, Jupiter, look so different." Stewart McWilliams, the study's lead author. 

Another implication of the team’s findings involves white dwarf stars, which are the collapsed remnants of once-larger stars, having about the mass of our Sun. They are very compact, but have faint luminosities as they give off residual heat. Dense helium is known to exist in the atmospheres of white dwarf stars and may form the surface atmosphere of some of these celestial bodies. The conditions simulated by the team’s laser-heated diamond-anvil cell indicate that this stellar helium should be more opaque (and conducting) than previously expected and this opacity could slow the cooling rates of helium-rich white dwarfs, as well as affect their colour.

Paper abstract 

Opacity and conductivity measurements in noble gases at conditions of planetary and stellar interiors