FOCUS ON RESEARCH: GEOPHYSICS & PLANETARY SCIENCES

Planetary Collisions
Erik Asphaug, Associate Professor

Erik Asphaug's group has been working on a number of studies relating to the origin of terrestrial planets and asteroids. One focus is the "late stage" episode of accretion when giant planetary collisions took place, such as the one believed to have formed the Moon. Out of a hundred or more ~1000 to ~5000 km diameter bodies, the four terrestrial planets (Mercury, Venus, Earth and Mars) are believed to have formed in the course of a few ten million years.

But most of these impacts were not accretionary: the impactor "bounces off". The figures below shows a couple of planetary collisions at 30° incidence between a differentiated embryo with mass comparable to Mars (m=5.98*1026g) and an embryo with (a) half and (b) a tenth the target mass, at impact speed of (a) 1.5 and (b) 2.0 times the two-body escape velocity. These are presumably typical. Blue (light-grey) is mantle rock; red (dark-grey) is core iron, and snapshots are before, during and 3 hours after the collision in side-view.

Mantle is stripped from planets in all planetary collisions, and (b) results in a number of very iron-enriched small worlds - aspects of significant relevance to meteoritics and to the origin of asteroids. Widespread production of silicate dust is expected in the aftermath of late stage planet formation, possibly identifiable in infrared observations by Spitzer and other telescopes. The collisional evolution of terrestrial planets is far more complex than previously understood.

Two papers focused on large planet collisions include Canup and Asphaug 2001, and Agnor and Asphaug 2004; these efforts are supported by NASA's Planetary Geology and Geophysics program.