Teams from the University of Chicago Pritzker School of Molecular Engineering and the Abdus Salam International Centre for Theoretical Physics performed quantum mechanical simulations to examine how ultraviolet (UV) light alters ice chemistry. Their findings, reported in Proceedings of the National Academy of Sciences, address experiments dating to the 1980s that showed unexplained changes in UV absorption after short versus long exposure.
Using advanced methods from the Galli lab, the researchers isolated the chemical effects of different structural defects. The simulations compared several cases:
- Defect-free ice in a perfect crystal lattice.
- Ice with a vacancy, where a water molecule is missing.
- Ice with charged hydroxide ions introduced into the structure.
- Ice containing a Bjerrum defect, where hydrogen-bonding rules are violated.
Results show that these defects alter the onset and spectrum of UV absorption. Hydroxide ions shift the absorption onset, while Bjerrum defects cause larger, more extreme changes that may explain absorption features after long UV exposure. The work also shows that UV can break water into hydronium ions, hydroxyl radicals and free electrons; electrons either delocalize through the lattice or become trapped in tiny cavities depending on defects.
The team plans experimental tests and further simulations of multiple defects, surfaces and meltwater. The research could affect predictions of greenhouse gas release from thawing permafrost and improve understanding of surface chemistry on icy moons. Funding came from the European Commission, CINECA supercomputing, MareNostrum5 and MICCoM through Argonne National Laboratory via the Department of Energy.
Difficult words
- defect — an imperfection in a solid material's structuredefects
- vacancy — a missing atom or molecule in a crystal
- hydroxide — a negatively charged ion made of oxygen and hydrogen
- hydronium — a positively charged water ion with an extra proton
- hydroxyl — a reactive group consisting of one oxygen and hydrogen
- electron — a small negatively charged particle in atoms or solidselectrons
- delocalize — to spread out an electron's position across many atoms
- lattice — a regular repeated arrangement of atoms in a solid
Tip: hover, focus or tap highlighted words in the article to see quick definitions while you read or listen.
Discussion questions
- How might these findings change predictions about greenhouse gas release from thawing permafrost?
- What experimental tests would you propose to confirm the simulation results about defects in ice?
- Why is it important to study different defects and surfaces in ice for understanding icy moons and planetary surfaces?
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