No. 61: Jan-Feb 1989
Astronomers have long wondered about Mercury. Its density (5.44) is unusually high for such a small planet, and its orbit's inclination (7°) and eccentricity (0.206) are also anomalously high. In one blow. W. Benz, A.G.W. Cameron, and W. Slattery may have solved all three problems.
Four frames from a computer simulation of proto-Mercury being stripped of its lighter, outer crust by a collision. Frame times are -1, +2.3, +7.7, and +41.7 minutes after impact. The dark molten sheet of iron in Frame #4 will collapse into a sphere, while the silicates will escape Mercury's gravitational pull.
They think Mercury's original, lighter, silicate outer layers were stripped off during the impact of one of the small protoplanets that are thought to have swirled around the inner solar system shortly after its formation. Computations on a supercomputer revealed to these three researchers that, if the protoplanet had hit Mercury at between 20 and 30 kilometers/second, then its dense iron core would have survived pretty much intact. A lower velocity would not have stripped off the lighter outer layers; anything higher would have blasted the whole planet into smithereens.
Calculations of this type also suggest that if a protoplanet the size of Mars had hit protoearth, it likewise would have stripped off its light silicate mantle. After this material that had been torn off gravitationally sphericized itself in orbit around the earth, it became--you guessed it - our moon.
(Stewart, Glen R.; "A Violent Birth for Mercury," Nature, 335:496, 1988. Also: Anonymous; "Mercury Stripped by Blow from Meteorite," New Scientist, November 5, 1988.)
Comment. It seems that our early solar system was somewhat Velikovskian in character, with many celestial missiles flying about. But that was long ago - or was it?
Reference. Mercury's idosyncracies are cataloged in Chapter AH in our catalog: The Moon and the Planets. For information about this book, visit: here.