Manicouagan, Gosses Bluff, Barringer, Clearwater Lakes, Aorounga: exotic yet familiar names to astronauts orbiting our planet. The oldest continental rocks preserve the distinctive pockmarks of past impacts by asteroids and comets. Geologists have identified roughly 180 impacts scars around the globe, some recognized from the ground, others from exploratory oil drilling, and still more from orbiting satellite imagery, Our basic knowledge of the cratering process came from studying Earth's youngest examples, like Arizona's Meteor Crater. That understanding has enabled ur enterpret on the rocky inner planets and the icy moons of the outer solar systems . The timing and frequency of alien impacts deepens in turn our knowledge of how past catastrophes have shaped the history of Earth and lide itself.
