To determine the date of the impact, Gary R. Byerly of Louisiana State University and his colleagues studied tiny grains called spherules within the recovered rocks. A meteorite impact leaves its mark with these droplets, which result from the condensation of the vaporized asteroid, and with a trail of metals that are rare on Earth. By measuring the radioactive decay of uranium and lead within mineral grains known as zircons--which are created by Earth processes such as volcanoes and are extremely resistant to change--the team dated the impact to 3.47 billion years ago, plus or minus two million years.
Despite the catastrophic nature of the collision, no impact crater remains, and the scientists don't know where the crash took place. It most likely occurred in water, they say, because most of the planet was submerged at the time and the resulting destruction would help to erase signs of the crash. "It would take only a second or two for a meteor that's 20 kilometers in diameter to pass through the ocean and impact the rock beneath," study co-author Donald R. Lowe of Stanford University says. "That would generate enormous waves kilometers high that would spread out from the impact site, sweep across the ocean and produce just incredible tsunamis--causing a tremendous amount of erosions on the microcontinents and tearing up the bottom of the ocean."