New research paves the way for scientists to better understand what happens at a sub-atomic scale when ice melts.
The work has implications including improving predictions of the release of greenhouse gases from thawing permafrost.
When ultraviolet light hits ice—whether in Earth’s polar regions or on distant planets—it triggers a cascade of chemical reactions that have puzzled scientists for decades.
Now, researchers at the University of Chicago Pritzker School of Molecular Engineering and the Abdus Salam International Centre for Theoretical Physics (ICTP) in Italy have used quantum mechanical simulations to reveal how tiny imperfections in ice’s crystal structure dramatically alter how ice absorbs and emits light.
The findings appear in Proceedings of the National Academy of Sciences.
“No one has been able to model what happens when UV light hits ice with this level of accuracy before,” says Giulia Galli, UChicago professor of molecular engineering and one of the senior authors of the new work. “Our paper provides an important starting point to understand the interaction of light with ice.”
“Together, we could start to unravel a problem that has been very challenging to tackle,” adds Ali Hassanali, an ITCP senior scientist who collaborated with Galli on the new research.
The mystery about ice and light goes back to experiments in the 1980s, when researchers discovered something puzzling: Ice samples exposed to UV light for just a few minutes absorbed certain wavelengths of light, but samples exposed to UV for hours absorbed different wavelengths. This suggested the ice chemistry had changed over time.
Since then, scientists proposed various chemical products that might form in the ice to explain these observations, but lacked the tools to test their theories.
“Ice is deceptively difficult to study. When light interacts with ice, chemical bonds break, forming new molecules and charged ions that, in turn, fundamentally alter its properties,” explained ITCP scientist Marta Monti, the first author of the study.
In the new work, the team turned to advanced modeling approaches that the Galli lab developed to study materials for quantum technologies. The methods let them study ice at a level which was not possible before.
“Ice is extremely hard to study experimentally, but computationally we can study a sample and isolate the effect of specific chemistry in ways that can’t be done in experiments, thanks to the sophisticated computational methods we have developed to study the properties of defects in complex materials,” says second author Yu Jin, formerly a UChicago graduate student, now at the Flatiron Institute.
The research team simulated four types of ice: defect-free ice arranged in a perfect crystal lattice, and ice with three different imperfections in its structure.
In one case, water molecules were missing from the water crystal, leaving a gap called a vacancy. In other instances, charged hydroxide ions were introduced into the structure. For the third set of computational experiments, ice’s strict hydrogen bonding rules were violated in what’s known as a “Bjerrum defect”—either two hydrogen atoms end up between the same pair of oxygen atoms, or none, disrupting the normally orderly structure.
The researchers could add these defects one at a time and observe how each type changed the way ice absorbed and emitted light. This type of precise control is impossible in physical ice samples but can be attained computationally.
The team showed that the onset of absorption of UV light occurs at different energies in defect-free ice and when hydroxide ions are inserted in the sample—explaining, at least qualitatively, decades-old experiments.
Bjerrum defects produced even more extreme changes in light absorption, potentially explaining the unexplained absorption features that appear in ice exposed to UV light for extended periods. Each type of defect created a unique optical signature, like a fingerprint that experimentalists can now look for in real ice samples.
The simulations also revealed what happens at the molecular level—when UV light hits ice, water molecules can break apart to form hydronium ions, hydroxyl radicals and free electrons. Depending on the defects present, these electrons can either spread through the ice or become trapped in tiny cavities.
“This is the foundation for understanding much more complex scenarios,” says Monti. “Now that we know how individual defects behave, we can start modeling ice with multiple defects, surfaces and eventually the messiness of real natural samples.”
For now, the work addresses the tip of the iceberg when it comes to fundamental questions about ice photochemistry.
But eventually, deeper studies of the interactions of UV light and ice could extend our understanding of environmental challenges and astrochemistry. Permafrost—permanently frozen ground in polar regions—traps greenhouse gases. As global temperatures rise and sunlight hits this ice, understanding how it releases those gases becomes critical for predicting climate change.
“There is ice in certain parts of the Earth that contains gases, and when it’s hit by light or when you raise the temperature just a little bit, these gases are released,” Galli says. “Better knowledge about how ice melts and what it releases under illumination could have incredible impacts on understanding these gases.”
The findings also may have implications for understanding the conditions on icy moons such as Jupiter’s Europa and Saturn’s Enceladus, where UV radiation constantly bombards ice-covered surfaces and may drive the formation of complex molecules.
The team is now working with experimentalists to design measurements that can validate their computational predictions. They’re also extending the work to study more complex collections of defects in ice and probe the impact of melted water as it accumulates on the surface of ice.
Funding came from the European Commission, CINECA supercomputing, MareNostrum5, MICCoM (through Argonne National Laboratory, via the Department of Energy).
Source: University of Chicago
