Scientists at the University of Chicago Pritzker School of Molecular Engineering and at the Abdus Salam International Centre for Theoretical Physics used quantum mechanical simulations to study how ultraviolet (UV) light changes ice chemistry. Their results, published in Proceedings of the National Academy of Sciences, aim to explain experiments from the 1980s that showed different absorption after minutes versus hours of UV exposure.
The team, led in part by Giulia Galli, used advanced computational methods developed in her lab. They compared defect-free ice with ice that had a vacancy, added hydroxide ions, or contained a Bjerrum defect. The simulations showed these defects shift UV absorption and change emission behaviour; Bjerrum defects produced larger changes that may explain long-exposure features.
At the molecular level, UV can break water to form hydronium ions, hydroxyl radicals and free electrons. Depending on defects, electrons may spread through the ice or become trapped in tiny cavities. The researchers are now working with experimentalists and plan to model surfaces, meltwater and multiple defects to test their predictions. The work could inform studies of permafrost gas release and chemistry on icy moons such as Europa and Enceladus.
Difficult words
- simulation — computer model of a physical processsimulations
- vacancy — a missing atom or molecule position
- hydroxide ion — a negatively charged oxygen and hydrogen ionhydroxide ions
- hydronium ion — a positively charged water molecule ionhydronium ions
- absorption — when a substance takes in light energy
- permafrost — soil or rock frozen for many years
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Discussion questions
- How could better simulations of surfaces, meltwater and multiple defects help laboratory experiments?
- Why might these results be important for studies of permafrost gas release or icy moons?
- If you were an experimentalist, what simple test would you try to check the simulations' predictions?
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