for National Geographic News
Scientists turned planetary detectives say they may have solved a solar system whodunit: What caused a cataclysmic asteroid assault on Earth and neighboring planets some 3.9 billion years ago?
Researchers suspect that the devastating bombardment, which lasted between 20 and 200 million years, originated in the main asteroid belt between Jupiter and Mars.
The U.S.-led team based their conclusion on evidence found in the unhealed wounds of ancient asteroid collisions left on the moon and the inner planets Mars, Mercury, and Venus.
Writing in tomorrow's issue of the journal Science, the scientists note that the intense period of crater-making impacts, known as the late heavy bombardment, was likely caused by repositioning of the solar system's giant outer planets.
During their study, the team investigated the most pockmarked celestial surfaces in the solar system, such as those visible on the moon and Mars.
The researchers compared the number and size of craters on older terrains with newer surface regions, such as the volcanic plains on Mars.
Younger planetary surfaces revealed impacts by relatively fewer, smaller projectiles. Craters in heavily scarred, older regions closely matched the size range of objects in the main asteroid belt.
Renu Malhotra, a planetary sciences professor at the University of Arizona, Tucson, co-authored the study. She says that the more recent impacts were likely caused by much smaller, less numerous near-Earth asteroids. Such impacts still occur today.
Malhotra notes that in the main asteroid belt small asteroids greatly outnumber large ones: There are about ten times as many asteroids measuring 300 meters (1,000 feet) in diameter than those measuring 3 kilometers (2 miles).
The scientist says that while the surface of the Earth would once have shown similar evidence of a 3.9-billion-year-old pummeling by asteroids, the scars have long since healed.
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