Extreme pressure has "a completely different atomic table," vital to understanding space.
The Institute for Advanced Study, which has played host to such luminaries as Albert Einstein and Kurt Gödel, is holding a series of talks to celebrate the birthday of another one of its famous faculty: Freeman Dyson. Dyson made important contributions to a huge variety of fields and gave us the concept of the Dyson Sphere. The talks in his honor covered many of the fields that he influenced, and here, we'll describe the talk by chemist Russel Hemley.
Pretty much everyone agrees that the Universe is run by physics. But for a lot of science, there are so many complicating factors and abstractions that stand between the physics and phenomena we see. Most people would put the boundaries somewhere in fields like biology and geology. Hemley would place a clear boundary at chemistry.
Hemley focuses on extreme conditions, saying that "we have a completely different atomic table at extreme pressures." The elements we're familiar with under normal conditions tend to see their behaviors shift down and to the right as pressures increase. This leads to unexpected results. At 20 GigaPascals (each GigaPascal is about 10,000 atmospheres of pressure), O2 breaks down and forms an eight-atom box—which happens to be a brilliant scarlet in color. Add another 10GPa, and it turns into a superconductor, as do sulfur, boron, and lithium.
Lithium and other metals also undergo multiple phase transitions, gradually opening up into a complex lattice with open spaces internally. Rather than circulating freely, the electrons often get stuck in these spaces. Hemley referred to this as "electret bonding" and described it as being entirely new.
And it isn't just the elemental properties that change; their reactivity changes. "There is no such thing as a noble element from the perspective of hydrogen," Hemley said, going on to describe a Xenon-hydrogen compound that exists at high temperatures. Every Xenon atom gets surrounded by eight molecules of hydrogen, each of which takes electrons from it.
Hemley said the reason for all these changes comes back to the orbitals occupied by electrons. We tend to think of atoms as solid spheres defined by their outermost electrons (he illustrated this with a photo of a stack of cannon balls). But the reality is that these orbitals can change based on the environment the atom finds itself in. Under pressure, these orbits gradually distort and undergo further changes, some of which may lead to the electrons being booted from orbitals entirely.
The nature of those changes can be difficult to predict from physical principles, and our models of the changes often fail to anticipate the things we see in experiments. (Which is why Hemley suggested that there's a big gap between physics and chemistry.) So, although we have many models of some of the bizarre things that happen at high pressures, we're not sure how likely they are to reflect reality (this is especially true for hydrogen).
Why does any of this matter? A lot of the material in the natural world is held at extreme pressures: the core of the Earth, the atmosphere of Jupiter, the environments of what now appear to be countless exoplanets like the Super Earths we're discovering. If we're going to understand the environments on these planets, then we're may have to understand how minerals we think are familiar behave in a very different pressure regime.
More real-world data may be on the way shortly. Hemley said that the Sloan Foundation is now funding a Deep Carbon Observatory to understand what's happening to carbon compounds that end up at depth below the Earth's crust. And he's excited to see the Juno probe arrive at Jupiter. Some models of hydrogen at pressure suggest that its conducting phase would occur relatively high in Jupiter's atmosphere, which may mean that the giant planet's magnetic field will be generated outside its core.
Juno might be able to determine whether Jupiter has a core as well.
Courtesy: arstechnica
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