The problem of density measurement under extreme conditions

In geosciences, the density of minerals, rocks, and melts at pressures up to several million atmospheres and temperatures of several thousand degrees is of critical importance because it governs the long-term planetary evolution as well as volcanic processes. But how can the density of a material be measured at such extreme conditions? To answer this question for a crystalline mineral or a rock, scientists use X-ray diffraction with which one measures the spacing between the periodically arranged atoms. There is, however, a problem if the material has a disordered structure, i.e. is non-crystalline, like glasses or molten rocks. In this case, the volume of the sample has to be measured directly — the density of a material equals its mass divided by volume. However, such measurements are extremely difficult because of the tiny volume of the sample brought to high pressure. Previously, these measurements required large scale X-ray facilities and highly specialized equipment, thus being very expensive. Now, a team led by scientist Sergey Lobanov of the GFZ German Research Centre for Geosciences is introducing a new method in which a laser the size of a shoebox allows them to measure the volume of samples brought to pressures similar to that at the depth of more than 2000 km in the Earth.

Inside the Earth, the rock is under unimaginably high pressure, up to several million times higher than normal atmospheric pressure. However, contrary to widespread belief, the Earth’s mantle is not liquid, but solid. The rock behaves in a viscoplastic fashion: It moves centimeter by centimeter per year, but it would burst under a hammer blow. Nevertheless, the slow movements drive the Earth’s crustal plates and tectonics, which in turn trigger volcanism. Chemical changes, for example, caused by water squeezed out of subducted crustal plates, can change the melting point of the rock in such a way that suddenly molten magma is formed. When this magma makes its way to the Earth’s crust and to the surface, volcanic eruptions occur.

Density of disordered materials

No instrument in the world can penetrate the Earth’s mantle to study such processes in detail. Therefore, one must rely on calculations, seismic signals and laboratory experiments to learn more about the Earth’s interior. A diamond anvil cell can be used to generate the extremely high pressures and temperatures that prevail there. The samples explored in it are smaller than the tip of a pin. Their volume is in the sub nanoliter range (e.g. at least 10 million times smaller than 1 milliliter). When material is compressed under such high pressures, the internal structure changes. To analyze this precisely, X-rays are used on crystals to generate diffraction patterns. This allows conclusions to be drawn about the volume of the crystal lattice and thus also the density of the material. Non-crystalline materials, such as glasses or molten rocks, have so far kept their innermost secrets to themselves. This is because for disordered materials X-ray diffraction does not provide direct information on their volume and density.

Simple trick: measurement with laser instead of X-ray beam

Using a simple trick, researchers led by Sergey Lobanov have now succeeded in measuring the refractive index and density of silicon dioxide (SiO₂) glass, one of the most important materials in industry and geology, at pressures of up to 110 gigapascals. This is a pressure that prevails at a depth of more than 2,000 kilometers in the Earth’s interior and is 1.1 million times higher than normal atmospheric pressure. The researchers used a multicolor laser to measure the brightness of its reflection from the pressurized sample. The brightness of the laser reflection contained information on the refractive index, a fundamental material property that describes how light slows down and bends as it travels through the material, but also the path length of the laser inside the sample. Materials with a high refractive index and density, such as diamonds and metals, typically appear bright and shiny to our eye. Instead of looking at the tiny samples with a naked eye, Lobanov and his colleagues used a powerful spectrometer to record changes in brightness at high pressure. These measurements yielded the refractive index of SiO₂ glass and provided key information to quantify its density.

Significance of the density measurement of glasses for the geosciences

“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. Previous measurements of glass density at high pressure required large and expensive synchrotron facilities that produce a tightly focused beam of X-rays that can be used to view the tiny sample in a diamond anvil cell. These were challenging experiments and only the densities of very few glasses have been measured to a pressure of 1 million atmospheres. 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,” Lobanov says. “This can be done outside of synchrotron facilities and is therefore much easier and less costly. Our work thus paves the way to future studies of glasses that approximate Earth’s present-day and long-gone melts. These future studies will provide new quantitative answers about the evolution of the early Earth as well as the driving forces behind volcanic eruptions.”

New possibilities for the investigation of non-crystalline, initially non-transparent solids

Because the samples are extremely small and therefore ultra-thin, even materials that look like a lump of rock in large pieces become translucent. According to the researchers, these developments open up new possibilities for studying the mechanical and electronic properties of non-crystalline solids that appear nontransparent in larger volumes. According to the authors of the study, their findings have far-reaching implications for materials science and geophysics. In addition, this information could serve as a benchmark for computational studies of the transport properties of glasses and melts under extreme conditions.

Lobanov emphasizes that this kind of study was only made possible by the collegial environment at the GFZ. He heads a Helmholtz Young Investigator Group called CLEAR in the “Chemistry and Physics of Geomaterials” section. “Our experimental capabilities to probe samples at high pressure is only one thing,” says Lobanov, “at least as important were the discussions with colleagues in other sections, which helped me develop the ideas and implement them.”

The Sea Level Rise Technical Report provides the most up-to-date sea level rise projections for all U.S. states and territories by decade for the next 100 years and beyond, based on a combination of tide gauge and satellite observations and all the model ensembles from the Sixth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC). The report projects sea levels along the coastline will rise an additional 10-12 inches by 2050 with specific amounts varying regionally, mainly due to land height changes.

The report updates the federal government’s 2017 sea level rise projections, and provides additional information on tide, wind, and storm-driven extreme water levels affecting current and future coastal flood risk. A suite of federal tools are using this data, including the NOAA Sea Level Rise Viewer, which are critical to the Administration’s commitment to tackle the climate crisis by making actionable climate data accessible to those who need it.

“For businesses along the coast, knowing what to expect and how to plan for the future is critical,” said U.S. Secretary of Commerce Gina M. Raimondo. “These updated projections will help businesses, and the communities they support, understand risks and make smart investments in the years ahead.”

“This new data on sea rise is the latest reconfirmation that our climate crisis ⁠ — as the President has said ⁠ — is blinking ‘code red,'” said Gina McCarthy, National Climate Advisor. “We must redouble our efforts to cut the greenhouse gases that cause climate change while, at the same time, help our coastal communities become more resilient in the face of rising seas.”

“This is a global wake-up call and gives Americans the information needed to act now to best position ourselves for the future,” said Rick Spinrad, Ph.D., NOAA Administrator. “As we build a Climate Ready Nation, these updated data can inform coastal communities and others about current and future vulnerabilities in the face of climate change and help them make smart decisions to keep people and property safe over the long run.”

The report also finds that the sea level rise expected by 2050 will create a profound increase in the frequency of coastal flooding, even in the absence of storms or heavy rainfall.

“By 2050, moderate flooding ⁠ — which is typically disruptive and damaging by today’s weather, sea level and infrastructure standards ⁠ — is expected to occur more than 10 times as often as it does today,” said Nicole LeBoeuf, NOAA National Ocean Service Director. “These numbers mean a change from a single event every 2-5 years to multiple events each year, in some places.”

“This report supports previous studies and confirms what we have long known: Sea levels are continuing to rise at an alarming rate, endangering communities around the world. Science is indisputable and urgent action is required to mitigate a climate crisis that is well underway,” said Bill Nelson, NASA Administrator. “NASA is steadfast in our commitment to protecting our home planet by expanding our monitoring capabilities and continuing to ensure our climate data is not only accessible but understandable.”

This multi-agency effort is a product of the Interagency Sea Level Rise and Coastal Flood Hazard and Tool Task Force, composed of NOAA, NASA, EPA, USGS, DoD, FEMA and the U.S. Army Corps of Engineers, as well as several academic institutes. The report leverages methods and insights from both the United Nations Intergovernmental Panel on Climate Change (IPCC) 6th Assessment Report and supporting research for the U.S. DoD Defense Regional Sea Level database offsite link.

Read the Sea Level Rise Technical Report: https://oceanservice.noaa.gov/hazards/sealevelrise/sealevelrise-tech-report.html

“Despite how different Venus fly traps are from humans, studying the structure and function of these mechanosensitive channels gives us a broader framework for understanding the ways that cells and organisms respond to touch and pressure,” says co-senior author and Scripps Research professor Andrew Ward, PhD.

“Every new mechanosensitive channel that we study helps us make progress in understanding how these proteins can sense force and translate that to action and ultimately reveal more about human biology and health,” adds co-senior author Ardem Patapoutian, PhD, a Scripps Research professor who won the Nobel Prize in Physiology or Medicine for research on the mechanosensitive channels that allow the body to sense touch and temperature.

Mechanosensitive ion channels are like tunnels that span the membranes of cells. When jostled by movement, the channels open, letting charged molecules rush across. In response, cells then alter their behavior — a neuron might signal its neighbor, for instance. The ability for cells to sense pressure and movement is important for people’s senses of touch and hearing, but also for many internal body processes — from the ability of the bladder to sense that it’s full to the ability of lungs to sense how much air is being breathed.

Previously, scientists had homed in on three ion channels in Venus fly traps thought to be related to the ability of the carnivorous plant to snap its leaves shut when its sensitive trigger hairs get touched. One, Flycatcher1, caught researchers’ attention because its genetic sequence looked similar to a family of mechanosensitive channels, MscS, found in bacteria.

“The fact that variants of this channel are found throughout evolution tells us that it must have some fundamental, important functions that have been maintained in different types of organisms,” says co-first author Sebastian Jojoa-Cruz, a graduate student at Scripps Research.

In the new study, the researchers used cryo-electron microscopy — a cutting-edge technique that reveals the locations of atoms within a frozen protein sample — to analyze the precise arrangement of molecules that form the Flycatcher1 protein channel in Venus fly trap plants. They found that Flycatcher1 is, in many ways, similar to bacterial MscS proteins — seven groups of identical helices surrounding a central channel. But, unlike other MscS channels, Flycatcher1 has an unusual linker region extending outward from each group of helices. Like a switch, each linker can be flipped up or down. When the team determined the structure of Flycatcher1, they found six linkers in the down position, and just one flipped up.

“The architecture of Flycatcher1’s channel core was similar to other channels that have been studied for years, but these linker regions were surprising,” says Kei Saotome, PhD, a former postdoctoral research associate at Scripps Research and co-first author of the new paper.

To help elucidate the function of these switches, the researchers altered the linker to disrupt the up position. Flycatcher1, they found, no longer functioned as usual in response to pressure; the channel remained open for a longer duration when it would normally close upon removal of pressure.

“The profound effect of this mutation tells us that the conformations of these seven linkers is likely relevant for how the channel works,” says co-senior author Swetha Murthy, PhD, of Vollum Institute at Oregon Health and Science University, a former postdoctoral research associate at Scripps Research.

Now that they solved the molecular structure, the research team is planning future studies on the function of Flycatcher1 to understand how different conformations affect its function. More work is also needed to determine whether Flycatcher1 is solely responsible for the snapping shut of Venus fly trap leaves, or whether other suspected channels play complementary roles.

In addition to Jojoa-Cruz, Saotome, Murthy, Patapoutian and Ward, authors of the study, “Structural insights into the Venus flytrap mechanosensitive ion channel Flycatcher1,” are Che Chun Alex Tsui and Wen-Hsin Lee of Scripps Research, and Mark Sansom of University of Oxford.

This work and the researchers involved were supported by funding from the National Institutes of Health (R01 HL143297, R01 HL143297), a Ray Thomas Edwards Foundation grant, the Wellcome Trust (grant 208361/Z/17/Z), the Biotechnology and Biological Sciences Research Council (grants BB/N000145/1 and BB/R00126X/1), the Engineering and Physical Sciences Research Council (grant EP/R004722/1), a postdoctoral fellowship from the Jane Coffin Childs Memorial Fund for Medical Research, the Skaggs-Oxford Scholarship, the Croucher Foundation, and the Howard Hughes Medical Institute.