Forming the Flora Family: Implications for the Near-Earth Asteroid Population and Large Terrestrial Planet Impactors

Abstract

Abstract Formed from a catastrophic collision of a parent body larger than 150 km in diameter, the Flora family is located in the innermost part of the main belt near the secular resonance. Objects in this region, when pushed onto planet-crossing orbits, tend to have relatively high probabilities of striking the Earth. These factors suggest that Flora may be a primary source of present-day LL chondrite-like NEOs and Earth/Moon impactors. To investigate this possibility, we used collisional and dynamical models to track the evolution of Flora family members. We created an initial Flora family and followed test asteroids 1 and 3 km in diameter using a numerical code that accounted for both planetary perturbations and nongravitational effects. Our Flora family members reproduce the observed semimajor axis, eccentricity, and inclination distributions of the real family after to 1.4 Gyr. A consistency with the surface age inferred from crater spatial densities found on (951) Gaspra may favor the latter age. Our combined collisional and dynamical runs indicate that the family has lost nearly 90% of its initial kilometer-sized members. At its peak, after the family-forming event, Flora family members filled NEO space with nearly 1000 size bodies before fading to its present contribution of 35–50 such NEOs. Therefore, it is not currently a major source of large NEOs. We also find 700–950 and 35–47 kilometer-sized asteroids struck the Earth and Moon, respectively, most within the first 300 Myr after family formation. These results imply that Flora played a major role in providing impacts to the mid-Proterozoic Earth.