Researchers at the University of Chicago Pritzker School of Molecular Engineering (UChicago PME) and the Abdus Salam International Centre for Theoretical Physics (ICTP) have usedquantum mechanical simulations to reveal how subtle imperfections within ice's crystal structure critically alter its light absorption and emission properties. The study was published in the Proceedings of the National Academy of Sciences.
New research paves the way for scientists to better understand what happens at a sub-atomic scale when ice melts, which has implications including improving predictions of the release of greenhouse gases from thawing permafrost. Image Credit: Galli Group
The interaction of ultraviolet light with ice, a process observed from Earth's polar regions to distant planets, initiates a cascade of chemical reactions that has presented a long-standing scientific puzzle.
The study establishes a foundational understanding of sub-atomic processes during ice melting, which is crucial for improving predictions related to the release of greenhouse gases from thawing permafrost.
No one has been able to model what happens when UV light hits ice with this level of accuracy before. Our paper provides an important starting point to understand the interaction of light with ice.
Giulia Galli, Study Senior Author and Liew Family Professor, Molecular Engineering, University of Chicago Pritzker School of Molecular Engineering
“The Trieste-Chicago collaboration brought together our expertise in water and ice physics with advanced computational methods for studying light-matter interactions. Together, we could start to unravel a problem that has been very challenging to tackle,” added Ali Hassanali, Senior Scientist, ICTP.
Trieste collaborated with Galli on the new research.
A Decades-Old Puzzle
The interaction between ice and ultraviolet (UV) radiation has presented a long-standing scientific enigma, originating from experiments conducted in the 1980s. These early investigations revealed a perplexing phenomenon: ice samples subjected to brief UV exposure (minutes) exhibited distinct light absorption characteristics compared to samples exposed for extended durations (hours).
This differential absorption strongly indicated a time-dependent alteration in the ice's chemical composition. While various chemical products were hypothesized to account for these observed changes, the requisite analytical tools to empirically validate these theories were unavailable at the time.
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.
Marta Monti, Study First Author, ICTP
The team's recent investigation successfully deployed advanced modeling methodologies, initially conceived by the Galli lab for quantum technology materials, to achieve an unprecedented resolution in the analysis of ice.
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.
Yu Jin, Study Second Author and Postdoctoral Research Fellow, Flatiron Institute
The Fingerprints of Imperfections
This study utilized computational simulations to meticulously investigate the optical properties of four distinct ice structures. The simulated samples included a perfectly ordered, defect-free ice lattice, alongside three variations incorporating specific structural imperfections: ice with molecular vacancies (missing water molecules), ice with introduced charged hydroxide ions, and ice exhibiting Bjerrum defects, which involve violations of the strict hydrogen bonding rules (either two hydrogen atoms or none between oxygen atoms).
Unlike physical ice samples, where introducing controlled defects is impossible, the researchers could computationally add defects individually and observe their specific impact on ice's light absorption and emission properties.
The research team demonstrated that the initiation of UV light absorption occurs at distinct energy levels in defect-free ice compared to ice containing hydroxide ions. This finding offers a qualitative explanation for experimental observations that have puzzled scientists for decades. Bjerrum defects were shown to produce even more extreme shifts in light absorption, potentially clarifying previously unexplained absorption features observed in ice exposed to UV light over extended periods.
Significantly, each type of defect generated a unique optical signature, functioning as a distinct "fingerprint" that experimentalists can now actively seek in real-world ice samples. Beyond these practical applications, the simulations also unveiled the molecular-level dynamics: when UV light interacts with ice, water molecules can dissociate, forming hydronium ions, hydroxyl radicals, and free electrons.
The presence and specific nature of the defects then determine whether these liberated electrons disperse throughout the ice structure or become trapped within microscopic cavities.
“This is the foundation for understanding much more complex scenarios. 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,” says Monti.
From Fundamental Physics to Melting Permafrost
Current investigations into ice photochemistry represent an initial step in addressing fundamental questions. The comprehensive studies of UV light-ice interactions are crucial for advancing the understanding of critical environmental challenges and astrochemistry. Permafrost, defined as permanently frozen ground in polar regions, sequesters significant greenhouse gas volumes. Therefore, as global temperatures escalate and solar radiation affects these ice formations, deciphering the release mechanisms of these gases is indispensable for robust climate change forecasting.
“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. Better knowledge about how ice melts and what it releases under illumination could have incredible impacts on understanding these gases,” said Galli.
These results could also provide valuable insights into the chemical processes occurring on icy moons, such as Europa and Enceladus. On these moons, constant exposure to UV radiation may trigger the creation of complex molecules within their ice-covered surfaces.
To validate their computational predictions, the team is collaborating with experimentalists to develop targeted measurement strategies. Furthermore, they are expanding their research to investigate more intricate defect structures in ice and analyze the effects of accumulating meltwater on the ice surface.
The study was supported by funding from the European Commission, CINECA supercomputing, MareNostrum5, and MICCoM (through Argonne National Laboratory, under Contract No. DE-AC02-06CH1135 from the Department of Energy).
Journal Reference:
Monti, M., et al. (2025) Defects at play: Shaping the photophysics and photochemistry of ice. Proceedings of the National Academy of Sciences. DOI:10.1073/pnas.2516805122. https://www.pnas.org/doi/10.1073/pnas.2516805122.