Angular Momentum Transfer in Catastrophic Asteroid Impacts

Details Angular Momentum Transfer in Catastrophic Asteroid Impacts Angular Momentum Transfer in Catastrophic Asteroid Impacts

Sep 1996

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996dps....28.1028l&link_type=abstract

American Astronomical Society, DPS meeting #28, #10.28; Bulletin of the American Astronomical Society, Vol. 28, p.1102

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Scientific paper

Incomplete knowledge of angular momentum transfer in asteroid impacts has hampered efforts to deduce asteroid collisional histories from their rotation rates. This problem traditionally has been investigated using impact experiments on cm-scale, strength-dominated targets. Recent evidence, however, indicates that impacts on asteroids of km size and larger may be controlled by gravity rather than strength, and that the analogy to laboratory impacts may not hold. Accordingly, we have modelled catastrophic impacts on gravitating asteroids to better understand angular momentum transfer in such events. We employ a 3--D, strengthless, gravitating SPH computer code. Target bodies are 10 to 1000 km in diameter and do not initially rotate. Impact speeds are 3--7 km/s; impact angles are 15--75(deg) . Each target is composed of 1791 mass elements: spatial resolution is coarse but acceptable for large scale energy transfer. We simulate the hydrodynamic phase of each impact, after which particle motions are ballistic and treated analytically. Escaping particles have kinetic energy greater than the gravitational energy binding them to the rest of the system; the others reaccrete to form a ``rubble pile'' which is assumed spherical. The rubble pile's size, mass, and angular momentum define its rotation rate. Spin rates for ejected fragments cannot be determined. The target's final spin period depends on the impact angle and the fraction of target mass ejected, but not on impact speed or target size in the ranges tested. The lack of size dependence cannot explain the observed excess of slowly rotating asteroids of ~ 100 km diameter. The fraction of projectile angular momentum retained by the target varies dramatically with impact speed and angle and with target size and fraction of mass removed, complicating its use in models where collision geometry varies. The final spin period of an asteroid losing 50% of its mass is 6--10 hours, comparable to the asteroidal mean of 8 hours. More destructive impacts yield spin rates approaching the 2-hour rotational breakup limit for strengthless stone bodies. We speculate that the observed decreasing trend in spin rate from C to S to M class asteroids results from an increasing trend in density.

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