Scientists predict cool new phase of superionic ice (Nature Communications)

Posted on by Catherine Zandonella

by Tien Nguyen, Department of Chemistry

Uranus as viewed by Voyager 2 in 1986 (NASA/JPL-Caltech)
Uranus (NASA/JPL-Caltech)

Scientists have predicted a new phase of superionic ice, a special form of ice that could exist on Uranus and Neptune, in a theoretical study performed by a team of researchers at Princeton University.

“Superionic ice is this in-between state of matter that we can’t really relate to anything we know of — that’s why it’s interesting,” Salvatore Torquato, a Professor of Chemistry who jointly led the work with Roberto Car, the Ralph W. ‘31 Dornte Professor in Chemistry. Unlike water or regular ice, superionic ice is made up of water molecules that have dissociated into charged atoms called ions, with the oxygen ions locked in a solid lattice and the hydrogen ions moving like the molecules in a liquid.

Published on August 28 in Nature Communications, the research revealed an entirely new type of superionic ice that the investigators call the P21/c-SI phase, which occurs at pressures even higher than those found in the interior of the giant ice planets of our solar system. Two other phases of superionic ice thought to exist on the planets are body-centered cubic superionic ice (BCC-SI) and close-packed superionic ice (CP-SI).

Each phase has a unique arrangement of oxygen ions that gives rise to distinct properties. For example, each of the phases allows hydrogen ions to flow in a characteristic way. The effects of this ionic conductivity may someday be observed by planetary scientists in search of superionic ice. “These unique properties could essentially be used as signatures of superionic ice,” said Torquato. “Now that you know what to look for, you have a better chance of finding it.”

Salvatore Torquato (left) and Roberto Car (right)
Salvatore Torquato (left) and Roberto Car (right)

Unlike Earth, which has two magnetic poles (north and south), ice giants can have many local magnetic poles, which leading theories suggest may be due to superionic ice and ionic water in the mantle of these planets. In ionic water both oxygen and hydrogen ions show liquid-like behavior. Scientists have proposed that heat emanating outward from the planet’s core may pass through an inner layer of superionic ice, and through convection, create vortices on the outer layer of ionic water that give rise to local magnetic fields.

By using theoretical simulations, the researchers were able to model states of superionic ice that would be difficult to study experimentally. They simulated pressures that were beyond the highest possible pressures attainable in the laboratory with instruments called diamond anvil cells. Extreme pressure can be achieved through shockwave experiments but these rely on creating an explosion and are difficult to interpret, Professor Car explained.

The researchers calculated the ionic conductivity of each phase of superionic ice and found unusual behavior at the transition where the low temperature crystal, in which both oxygen and hydrogen ions are locked together, transforms into superionic ice. In known superionic materials, generally the conductivity can change either abruptly (type I) or gradually (type II), but the type of change will be specific to the material. However, superionic ice breaks from convention, as the conductivity changes abruptly with temperature across the crystal to close-packed superionic transition, and continuously at the crystal to P21/c-SI transition.

As a foundational study, the research team investigated superionic ice treating the ions as if they were classical particles, but in future studies they plan to take quantum effects into account to further understand the properties of the material.

Read the article here:

Sun, J.; Clark, B. K.; Torquato, S.; Car, R. “The phase diagram of high pressure superionic ice.” Nature Communications, Published online August 28, 2015.

This work was supported by the National Science Foundation (DMS-1065894) and the US Department of Energy (DE-SC0008626 and DE-SC0005180).

 

 

More rain leads to fewer trees in the African savanna (PNAS)

Posted on by Catherine Zandonella
Lone tree on savanna
More rain on African savanna leads to fewer trees, a Princeton study found. (Credit PEI)

by Angela Page for the Princeton Environmental Institute

In 2011, an influx of remote sensing data from satellites scanning the African savannas revealed a mystery: these rolling grasslands, with their heavy rainfalls and spells of drought, were home to significantly fewer trees than researchers had previously expected given the biome’s high annual precipitation. In fact, the 2011 study found that the more instances of heavy rainfall a savanna received, the fewer trees it had.

This paradox may finally have a solution due to new work from Princeton University recently published in the Proceeding of the National Academy of Sciences. In the study, researchers use mathematical equations to show that physiological differences between trees and grasses are enough to explain the curious phenomenon.

“A simple way to view this is to think of rainfall as annual income,” said Xiangtao Xu, a doctoral candidate in David Medvigy’s lab and first author on the paper. “Trees and grasses are competing over the amount of money the savanna gets every year and it matters how they use their funds.” Xu explained that when the bank is full and there is a lot of rain, the grasses, which build relatively cheap structures, thrive. When there is a deficit, the trees suffer less than grasses and therefore win out.

To establish these findings, Xu and his Princeton collaborators Medvigy, assistant professor in geosciences, and Ignacio Rodriguez-Iturbe, professor of civil and environmental engineering, created a numerical model that mimics the actual mechanistic functions of the trees and grasses. “We put in equations for how they photosynthesize, how they absorb water, how they steal water from each other—and then we coupled it all with a stochastic rainfall generator,” said Xu.

Whereas former analyses only considered total annual or monthly rainfall, understanding how rainfall is distributed across the days is critical here, Xu said, because it determines who will win in a competition between grasses and trees for the finite resource of water availability.

The stochastic rainfall generator draws on rainfall parameters derived from station observations across the savanna. By coupling it with the mechanistic equations describing how the trees and grasses function, the team was able to observe how the plants would respond under different local climate conditions.

The research team found that under very wet conditions, grasses have an advantage because they can quickly absorb water and support high photosynthesis rates. Trees, with their tougher leaves and roots, are able to survive better in dry periods because of their ability to withstand water stress. But this amounts to a disadvantage for trees in periods of intense rainfall, as they are comparatively less effective at utilizing the newly abundant water.

“We put realistic rainfall schemes into the model, then generated corresponding grass or tree abundance, and compared the numerical results with real-world observations,” Xu said. If the model looked like the real-world data, then they could say it offered a viable explanation for the unexpected phenomenon, which is not supported by traditional models—and that is exactly what they found. They tested the model using both field measurements from a well-studied savanna in Nylsvley, South Africa and nine other sites along the Kalahari Transect, as well as remote sensing data across the whole continent. With each site, the model accurately predicted observed tree abundances in those locations.

The work rejects the long held theory of root niche separation, which predicts that trees will outcompete grasses under intense rainfall when the soil becomes saturated, because their heavy roots penetrate deeper into the ground. “But this ignores the fact that grasses and trees have different abilities for absorbing and utilizing water,” Xu said. “And that’s one of the most important parts of what we found. Grasses are more efficient at absorbing water, so in a big rainfall event, grasses win.”

“Models are developed to understand and predict the past and present state — they offer a perspective on future states given the shift in climatic conditions,” said Gaby Katul, a Professor of Hydrology and Micrometeorology in the Nicholas School of the Environment at Duke University, who was not involved in the research. “This work offers evidence of how shifts in rainfall affect the tree-grass interaction because rainfall variations are large. The approach can be used not only to ‘diagnose’ the present state where rainfall pattern variations dominate but also offers a ‘prognosis’ as to what may happen in the future.”

Several high profile papers over the last decade predict that periods of intense rainfall like those described in the paper will become more frequent around the globe, especially in tropical areas, Xu said. His work suggests that these global climate changes will eventually lead to diminished tree abundance on the savannas.

“Because the savanna takes up a large area, which is home to an abundance of both wild animals and livestock, this will influence many people who live in those areas,” Xu said. “It’s important to understand how the biome would change under global climate change.”

Furthermore, the study highlights the importance of understanding the structure and pattern of rainfall, not just the total annual precipitation—which is where most research in this area has traditionally focused. Fifty years from now, a region may still experience the same overall depth of precipitation, but if the intensity has changed, that will induce changes to the abundance of grasses and trees. This, in turn, will influence the herbivores that subsist on them, and other animals in the biome — essentially, affecting the entire complex ecosystem.

Xu said it would be difficult to predict whether such changes would have positive or negative impacts. But he did say that more grasses mean more support for cows and horses and other herbivores. On the other hand, fewer trees mean less CO2 is captured out of the atmosphere, as well as diminished habitat for birds and other animals that rely on the trees for survival.

What the model does offer is an entry point for better policies and decisions to help communities adapt to future changes. “It’s just like with the weather,” Xu said. “If you don’t read the weather report, you have to take what nature gives you. But if you know in advance that it will rain tomorrow, you know to bring an umbrella.”

This work was supported by the Princeton Environmental Institute and the Andlinger Center for Energy and the Environment at Princeton University.

Read the abstract.

Xiangtao Xua, David Medvigy, and Ignacio Rodriguez-Iturbe. Relation between rainfall intensity and savanna tree abundance explained by water use strategies. Published online September 29, 2015, doi: 10.1073/pnas.1517382112. PNAS October 5, 2015.