Planetary Science

To Bounce or Not To Bounce

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Science  08 Oct 2004:
Vol. 306, Issue 5694, pp. 199
DOI: 10.1126/science.306.5694.199b

The terrestrial planets formed by the accumulation of rocky fragments. First, kilometer-sized planetesimals accumulated over a period of about 100 to 10,000 years, then megameter-sized embryos accumulated over 100,000 to one million years. Finally, the embryos either collided in giant impacts or became isolated on stable orbits to produce the inner solar system.

Agnor and Asphaug modeled the accretional efficiency of megameter embryos by simulating collisions between two one-10th-Earth-mass bodies at different velocities and collision angles. Low-velocity head-on collisions produced one larger aggregate, whereas low-velocity glancing collisions forced the bodies to become bound gravitationally and eventually to merge. Higher-velocity glancing collisions allowed the two bodies to bounce and escape the local system. If these collisional scenarios for same-sized bodies are put into the collision dynamics assumed for solar system formation, then only about 50% of collisions will lead to merging, which is a relatively inefficient accretion rate that leaves a lot of bouncers exiting the solar system at high speed on random paths. — LR

Astrophys. J. 613, L157 (2004).

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