When it was announced last month that the Mars Odyssey satellite had found water ice beneath the planet’s frozen carbon dioxide south polar ice cap, at least one scientist was thrilled. “I felt excited!” says Dr. Lidija Siller, a physicist from the University of Newcastle. “I believe that the data I have explains how this water got trapped underneath the surface.” Dr. Siller presented the results of her research — which involves studying photochemical reactions in ice — at the Condensed Matter physics conference on Monday, part of the Institute of Physics Congress in Brighton, England. Photochemical reactions are changes in the chemistry of a substance that occur when light is shined at it. On Mars, both ultraviolet (UV) light from the Sun and low energy electrons can cause photochemical reactions in the carbon dioxide ice caps. The electrons are produced when high energy X-rays from the Sun fall on the ice.