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This Week in Science
Seeding life on Earth ow plausible is it that life on Earth got its start from organic materials brought here by comets or asteroids (page 366)? In a relatively thin atmosphere like that of today's Earth, even small comets would have hit Earth with such velocity that a great percentage of their organic constituents would have been lost en route or would have burned up on impact. But in a thick carbon dioxide atmosphere that may have existed some 4.5 to 3.8 billion years ago (when heavydoi:10.1126/science.1990.249.4967.twis fatcat:7lfruj3wvzhh7a4bcmwpoytwyy