Twin breakthroughs increase our understanding of the earths interior

For the first-time scientists can now measure that compression under high pressure environments, like those deep underground, thanks to breakthroughs in two brand new research papers.

The findings, from academics at the University of Salford and Queen Mary University London (QMUL) could have a huge impact on our understanding of earthquakes in the deep earth interior and even the possible effectiveness of carbon capture solutions. Surprisingly, until now, we have not been able to measure, or use the laws of physics to predict, how the density of liquids is affected by pressure and temperature at extreme conditions such as those found in planetary interiors.

Their work is just published in journals Physics of Fluids and Reports on Progress in Physics.

Chemistry textbooks may state that liquids are not compressible (in contrast to gases). However, in reality they are compressible.  In planetary and lunar interiors they are subjected to extreme pressures (tens of thousands of atmospheres) and under these conditions everyday fluids such as water shrink massively due to the extreme pressure applied.

There are a small number of laboratories in the world where liquids are subjected to the extreme pressures found in planetary interiors, and one of those laboratories is in Manchester at the University of Salford.  Scientists working there, led by Dr John Proctor, have just made two massive breakthroughs in the study of liquids under the extreme conditions found in planetary interiors. 

Their work looks at the most basic property of liquids when subjected to high pressure combined with high temperature, how much the liquid shrinks under high pressure and how much that is counteracted by the expansion due to the high temperatures found in planetary interiors. 

The experimental breakthrough was made by compressing liquids inside a diamond anvil high pressure cell (a small device that creates massively high pressures) and by applying the technique of confocal laser microscopy to this scientific problem for the first time.  In confocal laser microscopy, the density of the sample is measured by shining a laser beam into a diamond anvil high pressure cell and focussing it alternately on the flat surfaces of both diamonds.  This enabled the scientists to obtain accurate measurements of density versus pressure right up to the freezing point of hot fluids.  In a highlight of the study, ice crystals were created inside the diamond anvil cell at +235°c.  Dr. John Proctor, the lead author, says: “It appears that no-one had thought to try this until now because it is very simple compared to other experimental methods used to study liquids.”

The theoretical breakthrough came through applying the phonon theory of liquid thermodynamics, the brainchild of Prof. Kostya Trachenko of QMUL, to this longstanding problem.  Historically scientists have viewed liquids as being similar to gases, based on their ability to flow.  But most liquid properties are far more similar to the properties of solids.  Prof. Trachenko’s theory, developed over the past 15 years, exploits this to use the laws of physics to predict key liquid properties such as heat capacity – and now, the pressure-density-temperature relationship for liquids – for the first time.  Prof. Trachenko describes the new science:

"Understanding liquids has been an over-a-century-old problem, even though this may seem strange in this age of scientific and technological development. It is exciting that we can finally understand key liquid properties such as their ability to change volume under high pressure. This agrees with new pioneering experiments of Dr Proctor's group. I believe this is what the scientific community will find interesting: many groups are after a predictive theory which can be used to understand new data and guide future experiments in new exciting areas where no-one walked before."

Combining theory and experiment allows us to set up computer models that can incorporate the new experimental data as it becomes available, and make predictions where data are not yet available, regarding the behaviour of liquids in planetary and lunar interiors.  These models can help us to understand the role of water in the earth’s interior in enabling deep-focus earthquakes, and the effectiveness of future carbon capture and storage proposals.  Elsewhere in the solar system our models can be used to understand the behaviour of subsurface methane-ethane, water or nitrogen lakes in locations such as Titan.