BINARY ASTEROIDS AND THE FORMATION OF DOUBLET CRATERS

At least 10% (3 out of 28) of the largest known impact craters on Eart h and a similar fraction of all impact structures on Venus are doublet s (i.e., have a companion crater nearby), formed by the nearly simulta neous impact of objects of comparable size. Mars also has doublet crat ers, though the fraction found there is smaller (2%). These craters ar e too large and too far separated to have been formed by the tidal dis ruption of an asteroid prior to impact, or from asteroid fragments dis persed by aerodynamic forces during entry. We propose that some fast r otating rubble-pile asteroids (e.g., 4769 Castalia), after experiencin g 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 d uring a later pass, creating two distinct craters. To test this idea, we modeled close encounters between fast-rotating contact-binary aster oids, our first-order approximation for rubble-pile asteroids, and a c hosen planet. Our results show that Earth's tidal forces frequently cr eate binary asteroids, but that the separation distance between the bi nary's components is almost always too small to produce a doublet crat er at impact. However, once the components are orbiting one another, s mall perturbations from repeated distant Earth encounters, along with mutual tidal forces between the components, frequently increase the se paration distance between the components in a random-walk fashion. To model these effects, we combined our numerical model of planetary enco unters with a Monte Carlo code that computes the frequency and charact eristics of repeated Earth encounters as well as mutual tidal effects occurring between Earth encounters. Our results show that similar to 1 5% of all Earth-crossing asteroids evolve into co-orbiting binary aste roids with well-separated components. Asteroids on solely Mars-crossin g orbits produce a smaller fraction of binaries (<5%). Folding these r esults into another model treating impact encounters between binary as teroids and a chosen planet, we found we could duplicate the observed fraction of doublet craters found on Earth, Venus, and Mars. Our resul ts suggest that any search for asteroid satellites should place emphas is on km-sized Earth-crossing asteroids with short-rotation periods. ( C) 1996 Academic Press, Inc.