How UV Light Changes Ice Chemistry ^{CEFR B2}

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.