Scientists from the second Greenland Ice Sheet Project (GISP2) may have found the first chemical record of a meteorite's impact in ice. On the basis of preliminary evidence of particulate iridium deposits in two separate ice cores from Greenland's Summit Region that are 4–20 times the background concentrations, Robert Sherrell of Rutgers University and his colleagues believe that they may have identified trace material from the asteroid that exploded in the atmosphere over Siberia's Tunguska River region in 1908. If confirmed, the technique might be used to document other asteroid impacts that date back hundreds of thousands of years. Asteroids of Tunguska-size are estimated to hit the Earth every few hundred years, but scientists have had no direct evidence of these events. The major advantage of using ice cores is that they have great temporal resolution, allowing relatively short-term deposits of extraterrestrial material, which is much richer in iridium, to be detected, Sherrell explains.