 Instead, scientists have used the debris from meteor impacts, such as the Meteor Crater in Arizona, knowing that the material from such meteors coming from the asteroid belt would have been made at the same time as the Earth.Tyson then outlines the work Patterson did as a graduate under his adviser Harrison Brown to provide an accurate count of lead in zircon particles from Meteor Crater, and to work with similar results being collected by George Tilton on uranium counts; with the established half-life of uranium's radioactive decay to lead, this would be used to estimate the age of the Earth.Patterson found that his results were contaminated by lead from the ambient environment, compared to Tilton's results, and required the construction of the first ultra-high cleanroom to remove all traces of environmental lead.