Collisions in the Solar System
Man Hoi Lee, The University of Hong Kong
Collision is a fundamental process of the formation and evolution of planetary systems such as our Solar System. The planets of our Solar System formed in a disk of gas and dust around the young Sun 4.6 billion years ago. Micron-sized dust particles collided and grew into km-sized bodies called planetesimals, and planetesimals continued to grow by collision into planetary embryos. Giant impacts among planetary embryos are an essential part of the final assembly of planets. A giant impact on Earth resulted in the formation of the Moon, and another on Mars may explain the Martian crustal dichotomy (northern lowlands and southern highlands). Debris left over from planet formation produces impact craters on planetary bodies. Some data suggest that there was a peak in the impact rate about 700 million years after the planets formed, which could be triggered by the migration and instability of the giant planets’ orbits. Collisions have continued to the present time, and ongoing surveys of near-Earth objects are essential for understanding the present-day impact hazard on Earth.
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