"Binary Asteroids and the Formation of Doublet Craters"

W. F. Bottke, Jr. and H. Jay Melosh

(1996) Icarus 124, 372-391.

At least 10% (3 out of 28) of the largest known impact craters on Earth and a similar fraction of all impact structures on Venus are doublets (i.e. have a companion crater nearby), formed by the nearly simultaneous impact of objects of comparable size. Mars also has doublet craters, though the fraction found there is smaller (2-3%). These craters are too large and too far separated to have been formed by the tidal disruption of an asteroid prior to impact or from asteroid fragments dispersed by aerodynamic forces during entry. We propose that some fast rotating rubble-pile asteroids (e.g. 4769 Castalia), after experiencing a close approach with a planet, undergo tidal breakup and split into multiple co-orbiting fragments. In some cases these fragments evolve into stable binary systems, which re-encounter and impact the planet during a later pass, creating two distinct craters.

To test this idea, we modeled close encounters between between fast-rotating contact-binary asteroids, our first-order approximation for rubble-pile asteroids, and a chosen planet. Our results show that Earth's tidal forces frequently create binary asteroids, but that the separation distance between the binary's components is almost always too small to produce a doublet crater at impact. However, once the components are orbiting one another, small perturbations from repeated distant Earth encounters, along with mutual tidal forces between the components, frequently increase the separation distance between the components in a random-walk fashion. To model these effects, we combined our numerical model of planetary encounters with a Monte-Carlo code that computes the frequency and characteristics of repeated Earth encounters as well as mutual tidal effects occurring between Earth encounters. Our results show that ~15% of all Earth-crossing asteroids evolve into co-orbiting binary asteroids with well-separated components. Asteroids on solely Mars-crossing orbits produce a smaller fraction of binaries (< 5%).

Folding these results into another model treating impact encounters between binary asteroids and a chosen planet, we found we could duplicate the observed fraction of doublet craters found on Earth, Venus, and Mars. Our results suggest that any search for asteroid satellites should place emphasis on km-sized Earth-crossing asteroids with short-rotation periods.

For more information, contact Bill Bottke:bottke@astrosun.tn.cornell.edu