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A number of near-Earth asteroids have made headlines lately, but, thankfully, none is likely to hit us anytime soon. That wasn't the case in our planet's early years. Scientists surmise that between three and four billion years ago the central solar system was battered by a cataclysmic event that sent objects careening into the inner planets in their formative years. The moon, Mars and Venus bear scars from these impacts in the form of crater basins, but no such signs are evident on Earth. Now researchers writing in the current issue of the journal Science say they have the first direct evidence--in the form of layers of ancient rock from Australia and South Africa--that our planet experienced a meteorite impact 3.5 billion years ago. According to the report, the offending body was roughly twice the size of the asteroid that killed the dinosaurs 65 million years ago and created a tsunami that raced around the globe at 500 miles per hour.
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."