CATASTROPHIC IMPACTS ON GRAVITY DOMINATED ASTEROIDS

We use the smoothed particle hydrodynamics method to simulate catastro phic collisions on silicate bodies whose impact response is dominated by gravity rather than material strength. Encounter speeds of 3, 5, an d 7 km sec(-1), impact angles of 15 degrees, 45 degrees, and 75 degree s, and target diameters of 10 to 1000 km are investigated. The project ile and target materials are modelled using the Tillotson equation of state for granite. Our model treats gravity rigorously, but neglects s trength and fracture effects. We calculate the initial hydrodynamic ph ase of each event; after the impact shock wave crosses the target, par ticle motions are nearly ballistic and can be treated analytically. Ma terial that does not escape may reaccrete similar to 1 hr to similar t o 1 yr after the impact. The partitioning of impact energy into heat a nd motion of projectile and target material favors kinetic energy at h igher speeds and larger projectile:target diameter ratios, but does no t depend on the absolute size scale of the event. After the impact, mo st of the kinetic energy is carried by a small amount of fast ejecta. Particle velocity distributions are not sensitive to size scale and ha ve complex, evolving shapes that are poorly represented by simple appr oximations. The catastrophic threshold (impact energy per unit target mass required to permanently eject 50% of the target against gravity) ranges from 8 x 10(3) J kg(-1) at 10 km diameter to 1.5 x 10(6) J kg(- 1) at 1000 km, varying as target diameter to the 1.13 +/- 0.01 power. Extrapolating these results suggests that gravity dominance extends to stony bodies as small 250 +/- 150 m in diameter, smaller than previou sly believed. This result implies that asteroids as small as a few hun dred meters across may be ''rubble piles.'' Nearly catastrophic impact s can exhume target core material and catapult surface rocks to the an tipodes (''scrambling'' the target), but selective removal of the oute r layers is inefficient. Most material strongly heated in these impact s escapes, limiting globally averaged heating from a single collision to less than or equal to 50 degrees C for asteroids less than or equal to 1000 km in diameter. (C) 1996 Academic Press, Inc.