In order to understand planets effectively, we have to understand how materials behave at the extreme temperatures and pressures that exist inside of worlds like ours. Consider just the Earth’s mantle. The slow-moving ooze that carries the planet’s crust is generally more of a solid than a liquid, but if the temperature is changed just a small amount, for instance from interactions with water, the oozing material can phase change to a liquid mantle and become the stuff of volcanic eruptions.

In a perfect experiment, we’d send equipment burrowing into the Earth to record all the things that are happening. We don’t live in that reality, however, and the kinds of equipment we can build today can neither get that deep nor survive what they would encounter. Instead, researchers have to replicate the conditions found deep in the Earth inside a lab and then see how different materials act – what their density and configuration is – under those conditions.

Getting small samples to hot, dense conditions is something we know how to do, and for materials with a regular structure, we can use X-ray diffraction to see how the crystals in a material compress and measure a material’s density as the external pressure changes.

Unfortunately, not everything inside our world has a nice regular structure, and measuring the densities of more chaotic materials hasn’t been possible until now.

New research published in Physical Review Letters and led by Sergey Lobanov uses a laser to measure the volume of tiny samples, and then, as students in a freshmen physics lab would, the density can be calculated from the known mass of the sample. But unlike freshmen physics students, they didn’t just measure the outer dimensions of their sample.

Any material sliced thin enough becomes semi-transparent, and the samples in this research were just that kind of small; using multicolored lasers, they were able to measure the path of light through the inside of the material and measure its size with light passing through it. This has to do with another freshmen physics topic: refraction. Which is kind of cool and means that for the first time, we can measure the sizes of things too small to measure with anything other than light and too disorganized to measure with X-ray diffraction.

And this will eventually help us better understand the inside of our planet. As Lebonov explains: Earth was a giant ball of molten rock 4.5 billion years ago. To understand how Earth has cooled and produced a solid mantle and crust, we need to know the physical properties of molten rocks at extreme pressure. However, studying melts at high pressure is extremely challenging, and to circumvent some of these challenges geologists choose to study glasses instead of melts. Glasses are produced by quickly cooling hot but viscous melts. As a result, the structure of glasses often represents the structure of melts they were formed from. We have now shown that the evolution of the sample volume and density of any transparent glass can be accurately measured up to pressures of at least 110 GPa using optical techniques.

This just goes to show that sometimes big machines in boring labs, using freshmen physics principles, can do truly amazing things with tiny samples of our world.

More Information

How to look thousands of kilometers deep into the Earth? (EurekAlert)

“Electronic, Structural, and Mechanical Properties of SiO2 Glass at High Pressure Inferred from its Refractive Index,” Sergey S. Lobanov et al., 2022 February 17, Physical Review Letters