The Injection of Asteroid Fragments into Resonances

Abstract

Asteroidal fragments, coming from the main asteroid belt through chaotic dynamical routes, represent the likely source of most meteorites and near-Earth asteroids (NEAs). Many numerical experiments carried out over the past decade point to two specific source locations: the 3:1 mean motion resonance with Jupiter near 2.5 AU and the inner edge of the belt near 2.1 AU, where the dynamics is dominated by the g = g 6 (or v 6 ) secular resonance. We have tried to model and assess in a quantitative way the first part of the meteorite/NEA delivery process, namely the ejection of fragments from cratering and breakup events undergone by the existing asteroids as a consequence of impacts and the chance insertion of the escaping fragments into the "dangerous" strips of the phase space close to the resonances. For every parent asteroid, the efficiency of this process depends on several factors: (i) the mass vs ejection velocity distribution of the fragments, (ii) the escape velocity of the parent body, (iii) the Δ V required to approach a resonance surface, (iv) the width of the strip surrounding the resonance surfaces where chaotic eccentricity increases are possible, and (v) the amount of ejected material per unit time. The assumed number vs size distribution of asteroids at diameters <30 km is also critical in determining the overall flux. The results show that: (1) a large fraction of meteorites and NEAs could be generated by a small (≈1%) and possibly nonrepresentative fraction of the known asteroid population, mostly made of relatively large bodies located in the neighborhood of the two resonances; (2) both the 3:1 and the g = g 6 resonance are potentially effective channels for fragment collection and delivery, although they sample in a different way the orbital elements and the physical properties (size and taxonomic type) of the parent objects; and (3) the g = g 6 resonance is an efficient fragment collector not only near the inner edge of the belt, but also for several moderate-inclination (15° to 20°) asteroids at semimajor axes of about 2.4 and 2.7 AU. As for meteorite origin, within the existing uncertainties our model is consistent with the observed meteoroid flux on the Earth and suggests that a few large and possibly peculiar S-type asteroids and Vesta are the parent bodies of ordinary chondrites and basaltic achondrites, respectively. However, a poor knowledge of several key parameters and a lack of understanding of how a significant fraction of the asteroid fragments are launched at velocities of hundreds of meters per second prevent us from making claims stronger than those for the consistency of our model.